WO2022173720A1

WO2022173720A1 - Adjustable probe for plasma diagnostics

Info

Publication number: WO2022173720A1
Application number: PCT/US2022/015578
Authority: WO
Inventors: Ayan CHOUDHURY; Pierre TOCHON; Paul Harris
Original assignee: Fuse Energy Technologies Corp.
Priority date: 2021-02-09
Filing date: 2022-02-08
Publication date: 2022-08-18

Abstract

An adjustable probe for plasma diagnostics is disclosed that includes a probe body extending along a probe axis, and a probe head adjustably coupled to the probe body and having a probing surface configured to contact a plasma in a plasma chamber. The probe head is configured for adjustable positioning along the probe axis with respect to the probe body to vary an exposed area of the probing surface. The plasma probe can also include a position-adjusting assembly configured to control a degree of insertion of the probe head within the probe body, for example, by removable insertion of a spacer within the probe body in abutting engagement with the probe head. A method of adjusting a plasma probe to vary an exposed area of the probing surface of the probe is also disclosed. The disclosed techniques can be used with various types of plasmas, including fusion plasmas.

Description

ADJUSTABLE PROBE FOR PLASMA DIAGNOSTICS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application No. 63/147,489 filed on February 9, 2021, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The technical field generally relates to plasma technology, and more particularly, to probes for plasma diagnostics.

BACKGROUND

[0003] Nuclear fusion energy is energy produced by a nuclear fusion process in which two or more lighter atomic nuclei are joined to form a heavier nucleus whose mass is less than the sum of the masses of the lighter nuclei. The difference in mass is released as energy, which can be harnessed to produce electricity. Fusion reactors are devices whose function is to make use of fusion energy. Many fusion reactors rely on magnetic plasma confinement to confine high-temperature plasmas to sufficiently high- density with prolonged stability. Electric probes, such as Langmuir and Mach probes, are common plasma diagnostic tools used for the characterization of plasma parameters. In fusion applications, in situ measurements are generally limited to plasma edge regions, since most, if not all, plasma probes are unable to withstand the heat load from fusion plasma cores. While conventional plasma probes have advantages, they also have a number of limitations so that challenges remain in the field of probes for plasma diagnostics.

SUMMARY

[0004] The present description generally relates to a plasma diagnostic probe having an adjustable probing surface area and to a method of adjusting a plasma probe.

[0005] In accordance with an aspect, there is provided a plasma probe for characterizing a plasma in a plasma chamber, the plasma probe comprising: a probe body extending along a probe axis; and a probe head adjustably coupled to the probe body and having a probing surface configured to contact the plasma in the plasma chamber, wherein the probe head is configured for adjustable positioning along the probe axis with respect to the probe body to vary an exposed area of the probing surface.

[0006] In some embodiments, the probe head is coaxially coupled to the probe body.

[0007] In some embodiments, the plasma probe further comprises a position-adjusting assembly configured to control a degree of insertion of the probe head within the probe body along the probe axis. In some embodiments, wherein the position-adjusting assembly comprises at least one spacer configured for removable insertion within the probe body in abutting engagement with the probe head. In some embodiments, the probe head is configured for adjustable retraction into and extension out of the probe body by adjusting a number and/or a length of the at least one spacer inserted within the probe body. In some embodiments, the position-adjusting assembly is configured to control the degree of insertion of the probe head within the probe body by adjusting a number and/or a length of the at least one spacer removably inserted within the probe body. In some embodiments, the at least one spacer has an annular or ring-like cross-sectional shape in a plane transverse to the probe axis. In some embodiments, the at least one spacer is configured or arranged to extend axially and concentrically in an annular gap formed between the probe body and the probe head. In some embodiments, upon insertion of the at least one spacer in the annular gap formed between the probe body and the probe head, the at least one spacer is in sliding frictional engagement with either or both of an inner periphery of the probe body and an outer periphery of the probe head. In some embodiments, a number of the at least one spacer ranges between one and ten or twenty.

[0008] In some embodiments, the probe head is configured for adjustable positioning with respect to the probe body without disconnection of the probe head from the probe body.

[0009] In some embodiments, the probe head is configured for adjustable positioning with respect to the probe body by disconnection of the probe head from the probe body in a first probe head position and reconnection of the probe head to the probe body in a second probe head position different from the first probe head position, wherein the first and second probe positions correspond to different sizes of the exposed area of the probing surface. In some embodiments, the probe head is configured for adjustable positioning within the probe body by insertion of at least one spacer inside the probe body in abutting engagement with the probe head between the disconnection and reconnection operations. In some embodiments, the insertion of the at least one spacer inside the probe body results in a decrease of the size of the exposed area of the probing surface. In other embodiments, the insertion of the at least one spacer inside the probe body results in an increase of the size of the exposed area of the probing surface.

[0010] In some embodiments, the probe head comprises a probe head base coupled to and housed partially within the probe body, and the probing surface extends axially along an outer periphery of the probe head base.

[0011] In some embodiments, the probing surface comprises an array of azimuthally spaced, axially extending side electrodes disposed on the outer periphery of the probe head base. In some embodiments, the array of side electrodes is configured for operation as a Mach probe array. In some embodiments, a number of the side electrodes ranges between two and sixteen side electrodes. In some embodiments, the side electrodes are disposed in a circular arrangement in a plane transverse to the probe axis. In some embodiments, the side electrodes are disposed in a polygonal arrangement in a plane transverse to the probe axis. In some embodiments, the probe head includes an array of azimuthally spaced, axially extending side insulators circumferentially interleaved with the array of side electrodes to provide electrical insulation between the side electrodes.

[0012] In some embodiments, the probe head comprises at least one tip electrode protruding axially from the probe head base. In some embodiments, the at least one tip electrode comprises a reference electrode and one or more Langmuir electrodes. In some embodiments, the plasma probe may be configured for operation as a Langmuir/Mach probe, for example, as a Gundestrup probe.

[0013] In some embodiments, the probe body comprises a probe shaft and a probe head holder axially and releasably coupled between the probe shaft and the probe head, wherein the probe head is configured for adjustable positioning with respect to the probe body by a sequence of operations comprising a disconnection of the probe head holder from the probe shaft, a disconnection of the probe head from the probe head holder, an insertion of at least one spacer inside the probe head holder, a reconnection of the probe head to the probe head holder in abutting engagement with the spacer, and a reconnection of the probe head holder to the probe shaft. In some embodiments, the probe head is configured for removable insertion through a head-receiving aperture formed in a front end wall of the probe head holder.

[0014] In some embodiments, the probing surface is configured for electrical connection to a probe driving and measuring circuit.

[0015] In accordance with another aspect, there is provided a method of adjusting a plasma probe, the plasma probe comprising a probe body extending along a probe axis and a probe head adjustably coupled to the probe body and having a probing surface configured for contacting a plasma in a plasma chamber, the method comprising adjusting a degree of insertion of the probe head within the probe body with respect to the probe axis to control an exposed area of the probing surface.

[0016] In some embodiments, adjusting the degree of insertion of the probe head within the probe body comprises: disconnecting the probe head from the probe body in a first probe head position corresponding to a first size of the exposed area of the probing surface; and reconnecting the probe head to the probe body in a second probe head position different from the first probe head position and corresponding to a second size of the exposed area of the probing surface different from the first size.

[0017] In some embodiments, adjusting the degree of insertion of the probe head within the probe body comprises, between disconnecting the probe head from the probe body and reconnecting the probe head to the probe body, inserting at least one spacer inside the probe body in abutting engagement with the probe head.

[0018] In some embodiments, the probe body comprises a probe shaft and a probe head holder axially and releasably coupled between the probe shaft and the probe head, and adjusting the degree of insertion of the probe head within the probe body comprises a sequence of steps comprising: disconnecting the probe head holder from the probe shaft; disconnecting the probe head from the probe head holder; inserting at least one spacer inside the probe head holder; reconnecting the probe head to the probe head holder in abutting engagement with the at least one spacer; and reconnecting the probe head holder to the probe shaft.

[0019] In accordance with another aspect, there is provided a plasma probe for characterizing a plasma in a plasma chamber, the plasma probe comprising: a probe body extending along a probe axis; and a probe head having a probing surface configured for contacting the plasma in the plasma chamber, wherein the probe head is configured for adjustable positioning along the probe axis with respect to the probe body to vary an exposed area of the probing surface.

[0020] In accordance with another aspect, there is provided a method of adjusting a plasma probe comprising a probe body and a probe head adjustably coupled to the probe body and having a probing surface configured for contacting a plasma in a plasma chamber, the method comprising adjusting a degree of axial insertion of the probe head into the probe body to control an exposed area of the probing surface.

[0021] In accordance with another aspect, there is provided a plasma processing system comprising: a plasma chamber configured to contain a plasma; and a plasma probe configured to characterize the plasma in the plasma chamber, the plasma probe comprising: a probe body extending along a probe axis, and a probe head having a probing surface configured for contacting the plasma in the plasma chamber, wherein the probe head is configured for adjustable positioning with respect to the probe body along the probe axis to vary an exposed area of the probing surface.

[0022] Other method and process steps may be performed prior, during, or after the steps described herein. The order of one or more steps may also differ, and some of the steps may be omitted, repeated, and/or combined, as the case may be. [0023] Other objects, features, and advantages of the present description will become more apparent upon reading of the following non-re strictive description of specific embodiments thereof, given by way of example only with reference to the appended drawings. Although specific features described in the above summary and in the detailed description below may be described with respect to specific embodiments or aspects, it should be noted that these specific features may be combined with one another unless stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Fig. 1 is schematic partial perspective view of a plasma probe, in accordance with an embodiment.

[0025] Fig. 2 is a schematic perspective view of probe coupling assembly in which the plasma probe of Fig. 1 is received for coupling the plasma probe to a plasma chamber of a plasma processing system.

[0026] Fig. 3 is a schematic longitudinal cross-sectional view of the probe coupling assembly of Fig. 2, taken along section line 2-2 in Fig. 2.

[0027] Fig. 4 is a schematic longitudinal cross-sectional view of a plasma processing system including a plasma chamber, depicted with the plasma probe of Fig. 1 coupled thereto by means of the probe coupling assembly of Fig. 2.

[0028] Fig. 5 is a schematic partial longitudinal cross-sectional view of the plasma probe of Fig. 1, taken along section line 5-5 in Fig. 1.

[0029] Figs. 6A to 6C depict a plasma probe in three different operating configurations corresponding to three different sizes for the exposed area of the probing surface, in accordance with an embodiment.

[0030] Figs. 7A to 7F depict six different stages of a method of adjusting the size of an exposed area of the probing surface of a plasma probe, in accordance with another embodiment.

DETAILED DESCRIPTION

[0031] In the present description, similar features in the drawings have been given similar reference numerals. To avoid cluttering certain figures, some elements may not be indicated if they were already identified in a preceding figure. The elements of the drawings are not necessarily depicted to scale, since emphasis is placed on clearly illustrating the elements and structures of the present embodiments. Furthermore, positional descriptors indicating the location and/or orientation of one element with respect to another element are used herein for ease and clarity of description. Unless otherwise indicated, these positional descriptors should be taken in the context of the figures and should not be considered limiting. Such spatially relative terms are intended to encompass different orientations in the use or operation of the present embodiments, in addition to the orientations exemplified in the figures. Furthermore, when a first element is referred to as being “on”, “above”, “below”, “over”, or “under” a second element, the first element can be either directly or indirectly on, above, below, over, or under the second element, respectively, such that one or multiple intervening elements may be disposed between the first element and the second element.

[0032] The terms “a”, “an”, and “one” are defined herein to mean “at least one”, that is, these terms do not exclude a plural number of elements, unless stated otherwise.

[0033] The term “or” is defined herein to mean “and/or”, unless stated otherwise.

[0034] The expressions “at least one of X, Y, and Z” and “one or more of X, Y, and Z”, and variants thereof, are understood to include X alone, Y alone, and Z alone, as well as any combination of X, Y, and Z.

[0035] Terms such as “substantially”, “generally”, and “about”, which modify a value, condition, or characteristic of a feature of an exemplary embodiment, should be understood to mean that the value, condition, or characteristic is defined within tolerances that are acceptable for the proper operation of this exemplary embodiment for its intended application or that fall within an acceptable range of experimental error. In particular, the term “about” generally refers to a range of numbers that one skilled in the art would consider equivalent to the stated value (e.g., having the same or an equivalent function or result). In some instances, the term “about” means a variation of ±10% of the stated value. It is noted that all numeric values used herein are assumed to be modified by the term “about”, unless stated otherwise. The term “between” as used herein to refer to a range of numbers or values defined by endpoints is intended to include both endpoints, unless stated otherwise.

[0036] The term “based on” as used herein is intended to mean “based at least in part on”, whether directly or indirectly, and to encompass both “based solely on” and “based partly on”. In particular, the term “based on” may also be understood as meaning “depending on”, “representative of’, “indicative of’, “associated with”, “relating to”, and the like.

[0037] The terms “match”, “matching”, and “matched” refer herein to a condition in which two elements are either the same or within some predetermined tolerance of each other. That is, these terms are meant to encompass not only “exactly” or “identically” matching the two elements, but also “substantially”, “approximately”, or “subjectively” matching the two elements, as well as providing a higher or best match among a plurality of matching possibilities.

[0038] The terms “connected” and “coupled”, and derivatives and variants thereof, refer herein to any connection or coupling, either direct or indirect, between two or more elements, unless stated otherwise. For example, the connection or coupling between elements may be mechanical, optical, electrical, magnetic, thermal, chemical, logical, fluidic, operational, or any combination thereof. [0039] The term “concurrently” refers herein to two or more processes that occur during coincident or overlapping time periods. The term “concurrently” does not necessarily imply complete synchronicity and encompasses various scenarios including time -coincident or simultaneous occurrence of two processes; occurrence of a first process that both begins and ends during the duration of a second process; and occurrence of a first process that begins during the duration of a second process, but ends after the completion of the second process.

[0040] The present description generally relates to a plasma probe used for plasma diagnostics that has an adjustable probing surface area. The techniques disclosed herein may be used in various fields and applications, including, to name a few, nuclear fusion, neutron and high-energy photon generation, materials processing, and space propulsion, and with both cold and hot plasmas, including fusion plasmas.

[0041] Electric or electrostatic plasma probes — such as Langmuir probes, Mach probes, and combined Langmuir/Mach probes (e.g., Gundestrup probes) — are common diagnostic tools for measuring and characterizing plasma parameters and conditions in fusion plasmas, industrial process plasmas, and various other low-temperature and high-temperature plasmas. Non-limiting examples of plasma parameters include, to name a few, the electron density, electron temperature, ion density, ion temperature, plasma potential, floating potential, flow velocities, and the electron energy distribution function. These parameters may be derived from the analysis of the current-voltage (I-V) characteristic of the probe. The I-V characteristic may be generated by varying the voltage bias applied to the probe and recording the current, including both the electron and ion currents, through the probe. Different probe designs can be used to measure different plasma parameters. It is appreciated that the theory, instrumentation, operation, and application of electric probes for plasma diagnostics are generally known in the art and need not be described in detail herein other than to facilitate an understanding of the present techniques.

[0042] Referring to Figs. 1 to 7F, there are illustrated various schematic views of an embodiment of an adjustable plasma probe 100 for characterizing a plasma 102 generated, injected, or otherwise contained in a plasma chamber 104 of a plasma processing system 106. It is appreciated that Figs. 1 to 7F are simplified schematic representations that illustrate a number of features and components of, or coupled to, the plasma probe 100, such that additional components that may be useful or necessary for its practical operation may not be specifically depicted. Non-limiting examples of such additional components may include, to name a few, power supplies, electrical connections, gas sources, gas supply lines (e.g., conduits, such as pipes or tubes), pressure and flow control devices (e.g., pumps, valves, regulators, restrictors), operation monitoring and diagnostic devices (e.g., sensors), processors and controllers, and other standard hardware and equipment. [0043] In some embodiments, the plasma processing system 106 depicted in Fig. 4 can be used for generating fusion reactions. The fusion reactions may produce neutrons for use in various applications, including fusion power generation. Magnetic plasma confinement is one of several approaches to achieving controlled fusion for power generation. Non-limiting examples of magnetic plasma confinement approaches include, to name a few, Z-pinch configurations, magnetic mirror configurations, and toroidal configurations, for example, the tokamak and the stellarator. In other embodiments, the plasma processing system 106 may be used to process plasmas other than fusion plasmas.

[0044] In Fig. 4, the plasma processing system 106 implements a Z-pinch configuration. The term “Z- pinch plasma” broadly refers herein to a plasma that has an electric current flowing substantially along the longitudinal or axial direction Z of a cylindrical coordinate system. The axial electrical current generates an azimuthal magnetic field that radially compresses, or pinches, the plasma by the Lorentz force. It is appreciated that in some instances, terms such as “Z-pinch”, “zeta pinch”, “plasma pinch”, “pinch”, “plasma arc” may be used interchangeably with the term “Z-pinch plasma”. Non-limiting possible examples of Z-pinch-based plasmas processing systems are described in co-assigned International Patent Application No. PCT/US2021/062830, filed on December 10, 2021, and co assigned International Patent Application No. PCT/US2022/012502, filed on January 14, 2022, the contents of both documents being incorporated herein by reference in their entirety. However, it is appreciated that the plasma processing system 106 depicted in Fig. 4 is provided by way of example only, and that the plasma probe 100 disclosed herein can be used with various other types of plasma processing systems.

[0045] Referring still to Fig. 4, the plasma processing system 106 extends along a longitudinal Z-pinch axis 108 and includes an inner electrode 110 and an outer electrode 112 surrounding the inner electrode 110 to define an acceleration region 114 therebetween. The inner electrode 110 and the outer electrode 112 each have an elongated configuration along the Z-pinch axis 108. The outer electrode 112 extends forwardly beyond the inner electrode 110 along the Z-pinch axis 108 to define a Z-pinch assembly region 116 adjacent the acceleration region 114. The volume occupied by the acceleration region 114 and the assembly region 116 defines the plasma chamber 104 of the plasma processing system 100.

[0046] In the illustrated arrangement, the inner electrode 110 and the outer electrode 112 both have a substantially cylindrical configuration, with a circular cross-section transverse to the Z-pinch axis 108. The outer electrode 112 encloses the inner electrode 110 in a coaxial arrangement with respect to the Z-pinch axis 108. However, various other electrode configurations may be used in other embodiments. Non-limiting examples include, to name a few, non-coaxial arrangements, non-circularly symmetric transverse cross-sections, three-electrode arrangements, and the like. Depending on the application, the inner electrode 110 may have a full or hollow configuration. The inner electrode 110 and the outer electrode 112 may each be made of any suitable electrically conductive material, such as various metals and metal alloys. Non-limiting examples include, to name a few, tungsten-coated copper and graphite. It is appreciated that the size, shape, composition, structure, and arrangement of the inner electrode 110 and the outer electrode 112 can be varied depending on the application.

[0047] In some embodiments, the plasma processing system 106 is configured to form the plasma 102 in the acceleration region 114, for example, via injection and ionization of a process gas injected in the acceleration region 114. In other embodiments, the plasma processing system 106 is configured to form the plasma 102 outside the acceleration region 114 (e.g., via injection and ionization of a process gas) and to inject the externally formed plasma 102 in the acceleration region 114.

[0048] The plasma processing system 106 may include a power supply 178 connected to the inner electrode 110 and the outer electrode 112. Various types of power supplies may be used depending on the application. In some embodiments, the power supply 178 may be a switching pulsed-DC power supply and may include an energy source (e.g., a capacitor bank), a switch (e.g., a spark gap, an ignitron, or a semiconductor switch), and a pulse shaping network (including, e.g., inductors, resistors, diodes, and the like). The power supply is configured to apply a voltage between the inner electrode 110 and the outer electrode 112 to generate an accelerating electric field across the acceleration region 114, creating a Lorentz force that pushes the plasma 102 axially forward along the acceleration region 114 until the plasma 102 reaches the assembly region 116 and the Z-pinch begins to form. In the assembly region 116, the direction of the Lorentz force changes from axially forward to radially inward, which makes the plasma 102 collapse inwardly toward the Z-pinch axis 108 to complete the formation of the Z-pinch. The axial current flowing in the Z-pinch generates an azimuthal magnetic field that exerts an inward magnetic pressure and an inward magnetic tension that radially compress the Z-pinch against the outward plasma pressure until an equilibrium is established. In some embodiments, the plasma processing system 106 is configured to produce the Z-pinch with an embedded radially sheared axial flow (i.e., an axial velocity whose magnitude varies as a function of radius within the Z-pinch). Research has demonstrated that sheared plasma flows may provide a promising stabilization approach to achieving and sustaining fusion conditions in Z-pinch configurations. It is appreciated that the theory, configuration, implementation, and operation of sheared-flow-stabilized and other Z-pinch-based plasma confinement devices in nuclear fusion applications are generally known in the art and need not be described in detail herein other than to facilitate an understanding of the present techniques. Reference is made in this regard to the following doctoral dissertation: Golingo, Raymond, Formation of a Sheared Flow Z-Pinch (University of Washington, 2003), the contents of which are incorporated herein by reference in their entirety. [0049] The plasma processing system 106 may also include a vacuum system 118. The vacuum system 118 includes a vacuum chamber 120, for example, a stainless steel pressure vessel or tank. The vacuum chamber 120 houses at least partially various components of the plasma processing system 106, including the inner electrode 110, the outer electrode 112, and the plasma chamber 104. The vacuum system 118 may also include a pressure control system 180 configured to control the operating pressure inside the vacuum chamber 120. In some embodiments, the pressure inside the vacuum chamber 120 may range from about 10 ⁹ Torr to about 20 Torr, for example, from about 10 ⁸ Torr to about 10 ⁴ Torr, although other ranges of pressure may be used in other embodiments.

[0050] In Fig. 4, the plasma probe 100 is inserted inside the plasma chamber 104 along a probe insertion path extending radially with respect to the Z-pinch axis 108 and passing through a vacuum port 122 of the vacuum chamber 120 and an opening 124 in the outer electrode 112. In the illustrated embodiment, the vacuum port 122 and the electrode opening 124 are axially and circumferentially aligned with each other. The vacuum port 122 may be a flange port, for example, a ConFlat™ (CF) flange port. In Fig. 4, the vacuum port 122 and the electrode opening 124 provide the plasma probe 100 with access to the acceleration region 114 of the plasma chamber 104. However, the position of the vacuum port 122 with respect to the plasma chamber 104 may vary depending on the application, and likewise for the position of any associated electrode opening. In some embodiments, the vacuum chamber 120 may include a plurality of vacuum ports configured for providing the plasma probe 100 with access to the plasma chamber 104 from a plurality of corresponding locations. The plasma probe 100 may be connected to the vacuum port 122 of the vacuum chamber 120 via a vacuum flange 126 of a probe coupling assembly 128 within which the plasma probe 100 is received, as described in greater detail below.

[0051] Referring generally to Figs. 1 to 5, the plasma probe 100 generally includes a probe body 130 extending along a probe axis 132, and a probe head 134 coupled to the probe body 130 and having a probing surface 136 configured for entering in contact with the plasma 102 in the plasma chamber 104. The probe head 134 is configured for adjustable axial positioning with respect to the probe body 130 to vary an area of the probing surface 136 exposed to the plasma 102 in the plasma chamber 102. More detail regarding these and other components of, or coupled to, the plasma probe 100 are provided below.

[0052] The probe body 130 extends along the probe axis 132 from a rear end 138 to a front end 140. At the rear end 138, the probe body 130 may be connected to a linear and/or rotary motion feedthrough 170 terminating into a handle 142 configured for grasping by an operator, for example, to allow the plasma probe 100 to be translated along and/or rotated about the probe axis 132. At the front end 140, the probe body 130 is adjustably coupled to the probe head 134. In the illustrated embodiment, the probe head 134 is coaxially coupled to the probe body. The probe body 130 can be made of any suitable electrically insulating and mechanically rigid material. Non-limiting examples of possible materials include glass, ceramic, and glass-ceramic materials. More specific examples include, to name a few, alumina and boron nitride. In some embodiments, the probe body 130 may have a length ranging from about 50 cm to about 200 cm, and a diameter ranging from about 2 mm to about 30 mm, although other probe body dimensions may be used in other embodiments.

[0053] In the illustrated embodiment, the probe body 130 includes a probe shaft 144 and a probe head holder 146, both of which extending along the probe axis 132. The probe head holder 146 is releasably coupled between the probe shaft 144 (at the rear end of the probe head holder 146) and the probe head 134 (at the front end of the probe head holder 146, which corresponds to the front end 140 of the probe body 130) to allow the degree of axial insertion of the probe head 134 within the probe body 130 to be adjusted, as described in greater detail below. In some embodiments, the probe head holder 146 may have a length ranging from about 10 mm to about 75 mm. Depending on the application, the probe shaft 144 and the probe head holder 146 may or may not have identical compositions and cross- sectional sizes and shapes. The probe head holder 146 has a head-receiving aperture 148 in its front end wall, through which the probe head 134 adjustably projects to vary the area of the probing surface 136 exposed to the plasma 102.

[0054] Referring still to Figs. 1 to 5, the probe head 134 includes a probe head base 150 coupled to and housed partially within the probe head holder 146. The probing surface 136 extends axially and circumferentially along at least part of an outer periphery of the probe head base 150. The probe head base 150 can be made of any suitable electrically insulating and mechanically rigid material, for example, alumina and boron nitride. In the illustrated embodiment, the probe head base 150 includes a rearward base portion 152 and a forward base portion 154. The rearward base portion 152 is shaped as an annular cylinder, and the forward base portion 154 is shaped as a partially hollow cylinder. The rearward base portion 152 has an outer diameter greater than an outer diameter of the forward base portion 154. The probing surface 136 extends axially and circumferentially along an outer periphery of the forward base portion 154. In order to provide a snug fit of the probe head 134 within the probe head holder 146, the outer diameter of the rearward base portion 152 may be substantially equal (e.g., approximately equal or only slight less) to the diameter of the interior of the probe head holder 146 and the diameter defined by the probing surface 136 disposed on the outer periphery of the forward base portion 154 may be substantially equal (e.g., approximately equal or only slight less) to the diameter of the head-receiving aperture 148. It is appreciated that the probe head base 150 may have a variety of other structures and configurations in other embodiments.

[0055] In the illustrated embodiment, the probing surface 136 is provided by an array of eight axially extending side electrodes 156 shaped as thin curved rectangular plates circumferentially arranged on the outer peripheral surface of the forward base portion 154 and passing axially through the rearward base portion 152 of the probe body 130. The number of side electrodes 156 may differ in other embodiments, and may range, for example, between two and sixteen. The side electrodes 156 may each be made of any suitable electrically conductive material, such as various metals and metal alloys. More specific examples include, to name a few, tungsten, stainless steel, molybdenum, and graphite. In some embodiments, the side electrodes 156 may each have an exposed axial length ranging from about 1 mm to about 75 mm, an azimuthal width ranging from about 1 mm to about 20 mm, and a radial thickness ranging from about 0.1 mm to about 5 mm, although other dimensions may be used in other embodiments. Depending on the application, different side electrodes 156 may or may not have identical compositions and dimensions. In the illustrated embodiment, the side electrodes 156 are curved and disposed in a circular arrangement in a plane transverse to the probe axis 132, which matches the cross-sectional shape of the forward base portion 154. In other embodiments, the cross-sectional shape of the forward base portion 154 may be different, for example, polygonal with a number of sides that matches the number of side electrodes 156. In such case, the side electrodes 156 may be flat and disposed in polygonal arrangement in a plane transverse to the probe axis 132. In order to electrically insulate the side electrodes 156 from one another, the probe head 134 may include an array of side insulators 158 circumferentially interleaved with the array of side electrodes 156.

[0056] The array of side electrodes 156 is configured for operation as a multisided Mach probe array for measuring plasma flow velocities. Each side electrode 156 is configured for operation as a single Langmuir probe that may be biased to measure the ion saturation current, /_{sa t}, in the plasma 102 from a different direction. For each pair of side electrodes 156 facing in opposite directions, if the plasma 102 has a velocity flow in either direction, the side electrode 156 facing upstream will measure a higher value of /_{sa t} than the side electrode 156 facing downstream, since the latter is shielded from the plasma 102. It is appreciated that by using multiple pairs of side electrodes 156 (e.g., four in the illustrated embodiment), both the magnitude and the direction of the plasma flow velocity may be determined from the set of /_sat measurements. Furthermore, if the plasma 102 is magnetized, a multisided Mach probe array such as that depicted in Figs. 1 to 5 can allow for the simultaneous measurement of plasma flow velocity components in directions parallel and perpendicular to the magnetic field, since each pair of oppositely facing side electrodes 156 provides a different relative orientation between the probing surface 136 and the magnetic field. It is appreciated that the theory, instrumentation, operation, and application of Mach probes for plasma velocity measurements are generally known in the art and need not be described in detail herein other than to facilitate an understanding of the present techniques.

[0057] The probe head 134 may also include at least one tip electrode 160 extending parallel to the probe axis 132 and protruding forwardly out of the forward portion 154 of the probe head base 150. In the illustrated embodiment, the probe head 134 includes four tip electrodes 160 disposed in equally angularly spaced relationship one to the other with respect to the probe axis 132. However, the number and arrangement of the at least one tip electrode 160 may differ in other embodiments. The tip electrodes 160 may be shaped as a cylinder having a protruding length ranging from about 1 mm to about 50 mm and a diameter ranging from about 0.1 mm to about 5 mm, although other shapes (e.g., planar or spherical) and dimensions may be used in other embodiments. The tip electrodes 160 may each be made of any suitable electrically conductive material, such as various metals and metal alloys. More specific examples include, to name a few, tungsten, stainless steel, molybdenum, and graphite. In the illustrated embodiment, three of the four tip electrodes 160 are configured for operation as a Langmuir probe array (single, double, or triple configuration) and the fourth tip electrode 160 is configured for operation as a reference electrode for the array of side electrodes 156 forming the Mach probe array. Other tip electrode configurations may be used in other embodiments. The Langmuir probe array may be configured to determine various plasma parameters from measurements of their I-V characteristics. Non-limiting examples include, to name a few, the electron temperature, electron density, ion density, plasma potential, floating potential, and the electron energy distribution function.

[0058] It is noted that the plasma probe 100 depicted in Figs. 1 to 5 is configured for operation as a combined Langmuir/Mach probe known in the art as a Gundestrup probe. Reference is made in this regard to MacLatchy et al. “Gundestrup: A Langmuir/Mach probe array for measuring flows in the scrape-off layer of TdeV”, Rev. Sci. Instrum. 63, 3923 (1992), the contents of which being incorporated herein by reference in their entirety.

[0059] Mach probe arrays in conventional Gundestrup probes are generally designed for a predetermined and limited range of plasma conditions. This means that the determination of the effective probing surface area A of a conventional probe intended for use in the diagnosis of a specific plasma generally involves estimating values of a number of parameters of the specific plasma, such as the ion saturation current (7_{sa t}) and the electron and ion densities (n,. «,). temperatures (Ί,. T,), and masses (m,. mi). As mentioned above, the operation of the Mach probe array involves biasing the different side electrodes 156 to their respective ion saturation currents, /_{sa t}, and comparing the different values of /_sat measured by the different side electrodes 156 to determine plasma flow velocities. In the ion saturation current region, 7_sat is determined by the Bohm sheath criterion and is given by

where 0.61 is the Bohm constant, e is the elementary charge ( 1.602 10 ¹⁹ C), n_e is the electron density, A is the effective probing surface area of a Mach probe array electrode, k_B is the Boltzmann constant (1.381 lxl0 ²³ J/K), T_e is the electron temperature, and m, is the ion mass. It is appreciated that for a given value of A, the value of /_sat can vary significantly between different plasmas characterized by different sets of values for «... T,. and m,. In many applications, however, it may be desirable or even required to keep /_{sa t} within a small current range A/_sat. One reason for this is to avoid having to make changes in the configuration or operation of the electrical circuit and/or the data acquisition system of the probe. This means that when using conventional Mach probe arrays, multiple probe designs with different values of probing surface area A may have to be used if the plasma parameters (e.g., «... Ί,. and mj) cannot be estimated with sufficient accuracy and/or if the probe is intended for use with plasmas having different sets of parameters. However, using multiple probe designs may be undesirable, impractical, or even impossible in some applications.

[0060] In this context, the present techniques provide a plasma probe 100 having an adjustable probing surface area, where the probe head 134 is configured for adjustable positioning with respect to the probe body 130 to vary an exposed area of the probing surface 136. For example, in the embodiment of Figs. 1 to 5, the exposed axial length (and thus the exposed area) of each side electrode 156 can be adjusted by varying the degree of axial insertion of the probe head 134 within the probe body 130. This means that by adjustment of the axial positioning of the probe head 134 within the probe body 130 to vary the exposed area of the probing surface 136, the present techniques can allow for the operating range of the plasma probe 100 to be increased to cover a wider range of plasma parameters (e.g., in terms of plasma densities, temperatures, and flow velocities).

[0061] Referring to Figs. 5 and 6A to 6C, the plasma probe 100 includes a position-adjusting assembly or mechanism 162 configured to control a degree of insertion of the probe head 134 within the probe body 130 along the probe axis 132. In the illustrated embodiment, the position-adjusting assembly 162 includes at least one spacer 164 (i.e., a single spacer or a set of spacers) configured for removable insertion within the probe body 130 in abutting engagement with the probe head 134. In this configuration, the probe head 134 is configured for adjustable retraction into and extension out of the probe body 130 by adjusting a number and/or a length of the at least one spacer 164 inserted within the probe body 130. In the illustrated embodiment, the insertion of a spacer 164 inside the probe body 130 results in a decrease of the exposed area of the probing surface 136, as depicted in Figs. 6A (no spacer), 6B (insertion of two spacers 164, exposed area of probing surface 136 reduced), and 6C (insertion of four spacers 164, exposed area of the probing surface 136 further reduced). However, in other embodiments, the opposite configuration may be used, in which the insertion of a spacer 164 inside the probe body 130 results in an increase of the exposed area of the probing surface 136.

[0062] In the illustrated embodiment, the spacers 164 are shaped as annular cylinders or discs (i.e., with an annular cross-sectional shape in a plane transverse to the probe axis 132) configured to extend axially and circumferentially in a spacer-receiving chamber 166 formed between the probe body 130 and the probe head 134 when the probe head 134 is inserted within the probe body 130. In the illustrated embodiment, the spacer-receiving chamber 166 is provided as an annular gap formed between the probe body 130 and the probe head 134 upon coupling the probe body 130 and the probe head 134 together. In this configuration, the spacers 164 may be in sliding frictional engagement with either or both of the inner periphery of the probe body 130 and the outer periphery of the probe head 134. In particular, in order to provide a snug fit of the spacers 164 in the spacer-receiving chamber 166, each spacer 164 may have an outer diameter that is substantially equal (e.g., approximately equal or only slight less) to the diameter of the interior of the probe head holder 146, so that the outer peripheral surface of the spacer 164 abuts against the inner peripheral surface of the probe head holder 146. Each spacer 164 may also have an inner diameter that is substantially equal to the diameter defined by the probing surface 136 disposed on the outer peripheral surface of the forward base portion 154, so that the inner peripheral surface of the spacer 164 abuts against the probing surface 136. However, in some embodiments, the spacers 164 may not be inserted in the spacer-receiving chamber 166 with a snug radial fit. In some embodiments, each spacer 164 may each have an axial length ranging from about 0.5 mm to about 5 mm and an outer diameter ranging from about 2 mm to about 30 mm. The spacers 164 may be made of any suitable electrically insulating and mechanically rigid material. Non- limiting examples of such possible materials include glass, ceramic, and glass-ceramic materials. More specific examples include, to name a few, alumina, boron nitride, and MACOR® (a machinable boro- aluminosilicate glass-ceramic by Corning Inc.). It is appreciated that the spacers 164 may be of varying sizes, shapes, compositions, and configurations, and that different spacers 164 within a set may or may not be identical. In some embodiments, the number of spacer(s) 164 can range from one to about ten or even twenty.

[0063] Referring to Figs. 7A to 7F, the probe head 134 may be configured for adjustable positioning with respect to the probe body 130 by disconnection of the probe head 134 from the probe body 130 in a first probe head position (Fig. 7A) and reconnection of the probe head 134 to the probe body 130 in a second probe head position (Fig. 7F) different from the first probe head position, wherein the first and second probe positions correspond to different values of the exposed area of the probing surface 136 (i.e., the exposed area of the probing surface 136 is larger in Fig. 7A than it is in Fig. 7F). In this illustrated embodiment, the probe head 134 is configured for adjustable positioning within the probe body 130 by inserting, between the disconnection and reconnection operations, one or more spacers 164 (i.e., two spacers 164 in Figs. 7A to 7F) inside the probe body 130 in abutting engagement with the probe head 134. More particularly, the probe head 134 is configured for adjustable positioning within the probe body 130 by performing a method including the following sequence of operations: disconnecting the probe head holder 146 from the probe shaft 144 (Fig. 7B); disconnecting the probe head 134 from the probe head holder 146 (Fig. 7C); inserting one or more spacers 164 inside the probe head holder 146 through its rear end until the set of spacers 164 abuts against the inner surface of its front end wall (Fig. 7D); reconnecting the probe head 134 to the probe head holder 146 in abutting engagement with the one or more spacers 164 (Fig. 7E); and reconnecting the probe head holder 146 to the probe shaft 144 (Fig. 7F). It is appreciated that, in Fig, 7E, the probe head 134 is inserted within the probe head holder 146 through the rear end of the probe head holder 146 until (i) the front surface of the rearward base portion 152 of the probe head base 150 abuts against the rear surface of the rearward- most of the spacers 164 and (ii) the front surface of the forward-most of the spacers 164 abuts against the inner surface of the front end wall of the probe head holder 146. In such a configuration, the total axial length of the one or more spacers 164 determines the variation in the exposed area of the probing surface 136 between the first probe head position (Fig. 7A) and the second probe head position (Fig. 7F), by controlling how much of the probe head 134 protrudes out of the head-receiving aperture 148 of the probe head holder 146.

[0064] Returning to Figs. 1 to 5, as noted above, the plasma probe 100 may be housed at least partly in a probe coupling assembly 128 to allow the plasma probe 100 to operate and couple with other components under vacuum conditions. The probe coupling assembly 128 may include a plurality of vacuum conduits 168a-168c axially coupled to one another via flanges or other types of connections. In the illustrated embodiment, the plurality of vacuum conduits 168a- 168c includes a rear straight conduit 168a, a middle T-shaped conduit 168b, and a front straight conduit 168c. The vacuum conduits 168a-168c may be embodied by any suitable type of pipes or tubes, and may be made of stainless steel or another material suitable for operation under vacuum conditions.

[0065] In the illustrated embodiment, the front end of the front conduit 168c terminates in the vacuum flange 126 introduced above, which is configured for coupling to the vacuum port 122 of the vacuum chamber 120. This allows the plasma probe 100 to be inserted within the plasma chamber 104 through the electrode opening 124 formed in the outer electrode 112 and to allow the probing surface 136 of the plasma probe 100 to enter in contact with the plasma 102 in the plasma chamber 104. The rear end of the rear conduit 168a is coupled to the linear and/or rotary motion feedthrough 170 introduced above, which is configured to allow the plasma probe 100 to be translated along and/or rotated about the probe axis 132. By way of example, the motion feedthrough 170 may be configured to provide a travel length ranging from about 1 mm to about 1 m along the probe axis 132. This can allow the plasma probe 100 to be moved within a range of radial positions inside the plasma chamber 104 and be used to measure radial profiles of plasma parameters, for example, of flow velocities. The intermediate conduit 168b has a rear end coupled to the front end of the rear conduit 168a, a front end coupled to the rear end of the front conduit 168c, and a transverse end coupled to an electrical feedthrough 172 configured to provide a vacuum-sealed electrical connection 174 between, at one end, the side electrodes 156 and the tip electrodes 160 and, at the other end, a suitable electrical circuit 176 for driving plasma probe 100 and measuring its response when exposed to the plasma 102 in the plasma chamber 104. The electrical connection 174 may be provided by any suitable electrical wiring or cables.

[0066] It is appreciated that although several implementations described above use spacers to provide axial adjustment of the probe head within the probe body and control over the exposed area of the probing surface, other implementations may use other types of position-adjusting mechanisms. Non- limiting examples include, to name a few, mechanisms based on springs, screws and nuts, and the like. It is appreciated that several implementations described above provide axial adjustment of the probe head within the probe body involve steps of disconnecting the probe head from the probe body in a first head position and reconnecting the probe head to the probe body in a second head position different from the first head position. However, in other implementations, the probe head may be configured for adjustable positioning with respect to the probe body without disconnection of the probe head from the probe body.

[0067] Furthermore, although in several implementations described above the probing surface of the adjustable probe is embodied by a Mach probe array including a plurality of azimuthally spaced, axially extending side electrodes disposed on the outer periphery of a probe head base, other types of adjustable probes may be used in other implementations, which may or may not be configured as Mach probes. In particular, it is appreciated that any type or configuration of electric plasma probes having a probe body and a probe head having a probing surface whose exposed area may be adjusted by varying the relative axial positioning of the probe head and the probe body are contemplated for use in the present techniques. Non-limiting examples of such probes include, to name a few, single and multiple Langmuir probes, and ball-pen probes.

[0068] Referring to Fig. 4, it is appreciated that the embodiments disclosed herein may also include a control and processing unit 182 configured for controlling, monitoring, and/or coordinating the functions and operations of various system components, including the plasma probe 100, the electrical circuit and the plasma processing system, as well as various temperature, pressure, flow rate, and power conditions. The control and processing unit 182 can generally include one or more processors 184 and one or more memories 186. The control and processing unit 182 can be implemented in hardware, software, firmware, or any combination thereof, and be connected to various system components via wired and/or wireless communication links to send and/or receive various types of signals, such as timing and control signals, measurement signals, and data signals. The control and processing unit 182 may be controlled by direct user input and/or by programmed instructions, and may include an operating system for controlling and managing various system functions. Depending on the application, the control and processing unit 182 may be fully or partly integrated with, or physically separate from, the other hardware components of the plasma probe 100 and the plasma processing system 106.

[0069] The processor 160 may be able to execute computer programs, also generally known as commands, instructions, functions, processes, software codes, executables, applications, and the like. It should be noted that although the processor 184 in Fig. 4 is depicted as a single entity for illustrative purposes, the term “processor” should not be construed as being limited to a single processor, and accordingly, any known processor architecture may be used. For example, the processor 184 may include or be part of a computer; a microprocessor; a microcontroller; a coprocessor; a central processing unit (CPU); an image signal processor (ISP); a digital signal processor (DSP) running on a system on a chip (SoC); a single-board computer (SBC); a dedicated graphics processing unit (GPU); a special-purpose programmable logic device embodied in hardware device, such as, for example, a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC); and/or other mechanisms configured to electronically process information and to operate collectively as a processor. The memory 186, which may also be referred to as a “computer readable storage medium” is capable of storing computer programs and other data to be retrieved by the processor 184. In the present description, the terms “computer readable storage medium” and “computer readable memory” are intended to refer to a non-transitory and tangible computer product that can store and communicate executable instructions for the implementation of various steps of the methods disclosed herein. The computer readable memory may be any computer data storage device or assembly of such devices, including a random-access memory (RAM) device, a read-only memory (ROM) device, a magnetic storage device (e.g., a hard disk drive), an optical storage device (e.g., an optical disc drive), a solid- state storage device (e.g., a solid-state drive or a flash memory drive), and/or any other non-transitory memory technologies. The computer readable memory may be associated with, coupled to, or included in a computer or processor configured to execute instructions contained in a computer program stored in the computer readable memory and relating to various functions associated with the computer. The embodiments disclosed herein may also include one or more user interface devices (not shown) operatively connected to the control and processing unit 182 to allow the input of commands and queries, as well as present the outcomes of the commands and queries. The user interface devices can include input devices (e.g., a touch screen, a keypad, a keyboard, a mouse, a switch, and the like) and output devices (e.g., a display screen, a printer, visual and audible indicators and alerts, and the like).

[0070] In accordance with another aspect, there is also provided a method of adjusting a plasma probe that includes probe body and a probe head. The method can be implemented in a plasma probe 100 such as the one depicted in Figs. 1 to 7F, or another suitable plasma probe. The probe body extends along a probe axis, and the probe head is adjustably coupled to the probe body. The probe head has a probing surface configured for contacting a plasma in a plasma chamber. The method generally includes an operation of adjusting a degree of insertion of the probe head within the probe body with respect to the probe axis to control an exposed area of the probing surface.

[0071] In some embodiments, the operation of adjusting the degree of insertion of the probe head within the probe body can include a step of disconnecting the probe head from the probe body in a first probe head position corresponding to a first size of the exposed area of the probing surface, and a step of reconnecting the probe head to the probe body in a second probe head position different from the first probe head position and corresponding to a second size of the exposed area of the probing surface different from the first size. In some embodiments, the operation of adjusting the degree of insertion of the probe head within the probe body can further include, between the steps of disconnecting the probe head from the probe body and reconnecting the probe head to the probe body, a step of inserting at least one spacer inside the probe body in abutting engagement with the probe head. [0072] In some embodiments, the probe body includes comprises a probe shaft and a probe head holder axially and releasably coupled between the probe shaft and the probe head, and the operation of adjusting the degree of insertion of the probe head within the probe body can include a sequence of steps including: disconnecting the probe head holder from the probe shaft; disconnecting the probe head from the probe head holder; inserting at least one spacer inside the probe head holder; reconnecting the probe head to the probe head holder in abutting engagement with the at least one spacer; and reconnecting the probe head holder to the probe shaft.

[0073] Numerous modifications could be made to the embodiments described above without departing from the scope of the appended claims.

Claims

1. A plasma probe for characterizing a plasma in a plasma chamber, the plasma probe comprising: a probe body extending along a probe axis; and a probe head adjustably coupled to the probe body and having a probing surface configured to contact the plasma in the plasma chamber, wherein the probe head is configured for adjustable positioning along the probe axis with respect to the probe body to vary an exposed area of the probing surface.

2. The plasma probe of claim 1, wherein the probe head is coaxially coupled to the probe body.

3. The plasma probe of claim 1 or 2, further comprising a position-adjusting assembly configured to control a degree of insertion of the probe head within the probe body along the probe axis.

4. The plasma probe of claim 3, wherein the position-adjusting assembly comprises at least one spacer configured for removable insertion within the probe body in abutting engagement with the probe head.

5. The plasma probe of claim 4, wherein the position-adjusting assembly is configured to control the degree of insertion of the probe head within the probe body by adjusting a number and/or a length of the at least one spacer removably inserted within the probe body.

6. The plasma probe of claim 4 or 5, wherein the at least one spacer has an annular cross-sectional shape in a plane transverse to the probe axis.

7. The plasma probe of claim 6, wherein the at least one spacer is configured to extend axially and concentrically in an annular gap formed between the probe body and the probe head.

8. The plasma probe of claim 7, wherein, upon insertion of the at least one spacer in the annular gap formed between the probe body and the probe head, the at least one spacer is in sliding frictional engagement with either or both of an inner periphery of the probe body and an outer periphery of the probe head.

9. The plasma probe of any one of claims 4 to 8, wherein a number of the at least one spacer ranges between one and ten.

10. The plasma probe of claim 1 or 2, wherein the probe head is configured for adjustable positioning with respect to the probe body by disconnection of the probe head from the probe body in a first probe head position and reconnection of the probe head to the probe body in a second probe head position different from the first probe head position, wherein the first and second probe positions correspond to different sizes of the exposed area of the probing surface.

11. The plasma probe of claim 10, wherein the probe head is configured for adjustable positioning within the probe body by insertion of at least one spacer inside the probe body in abutting engagement with the probe head between the disconnection and reconnection operations.

12. The plasma probe of claim 11, wherein the insertion of the at least one spacer inside the probe body results in a decrease of the size of the exposed area of the probing surface.

13. The plasma probe of any one of claims 1 to 12, wherein the probe head comprises a probe head base coupled to and housed partially within the probe body, and the probing surface extends axially along an outer periphery of the probe head base.

14. The plasma probe of claim 13, wherein the probing surface comprises an array of azimuthally spaced, axially extending side electrodes disposed on the outer periphery of the probe head base.

15. The plasma probe of claim 14, wherein the array of side electrodes is configured for operation as a Mach probe array.

16. The plasma probe of claim 14 or 15, wherein a number of the side electrodes ranges between two and sixteen side electrodes.

17. The plasma probe of any one of claims 14 to 16, wherein the side electrodes are disposed in a circular or polygonal arrangement in a plane transverse to the probe axis.

18. The plasma probe of any one of claims 13 to 17, wherein the probe head comprises at least one tip electrode protruding axially from the probe head base.

19. The plasma probe of claim 18, wherein the at least one tip electrode comprises a reference electrode and one or more Langmuir electrodes.

20. The plasma probe of any one of claims 1 to 19, wherein the probe body comprises a probe shaft and a probe head holder axially and releasably coupled between the probe shaft and the probe head, wherein the probe head is configured for adjustable positioning with respect to the probe body by a sequence of operations comprising a disconnection of the probe head holder from the probe shaft, a disconnection of the probe head from the probe head holder, an insertion of at least one spacer inside the probe head holder, a reconnection of the probe head to the probe head holder in abutting engagement with the spacer, and a reconnection of the probe head holder to the probe shaft.

21. The plasma probe of claim 20, wherein the probe head is configured for removable insertion through a head-receiving aperture formed in a front end wall of the probe head holder.

22. A method of adjusting a plasma probe, the plasma probe comprising a probe body extending along a probe axis and a probe head adjustably coupled to the probe body and having a probing surface configured for contacting a plasma in a plasma chamber, the method comprising adjusting a degree of insertion of the probe head within the probe body with respect to the probe axis to control an exposed area of the probing surface.

23. The method of claim 21, wherein adjusting the degree of insertion of the probe head within the probe body comprises: disconnecting the probe head from the probe body in a first probe head position corresponding to a first size of the exposed area of the probing surface; and reconnecting the probe head to the probe body in a second probe head position different from the first probe head position and corresponding to a second size of the exposed area of the probing surface different from the first size.

24. The method of claim 23, wherein adjusting the degree of insertion of the probe head within the probe body comprises, between disconnecting the probe head from the probe body and reconnecting the probe head to the probe body, inserting at least one spacer inside the probe body in abutting engagement with the probe head.

25. The method of claim 22, wherein: the probe body comprises a probe shaft and a probe head holder axially and releasably coupled between the probe shaft and the probe head, and adjusting the degree of insertion of the probe head within the probe body comprises a sequence of steps comprising: disconnecting the probe head holder from the probe shaft; disconnecting the probe head from the probe head holder; inserting at least one spacer inside the probe head holder; reconnecting the probe head to the probe head holder in abutting engagement with the at least one spacer; and reconnecting the probe head holder to the probe shaft.