US3575598A - Plasma simulation and analysis apparatus and method - Google Patents

Abstract

There is disclosed a method of interacting a plasma beam with a magnetic field and a preferred plasma simulation and analysis apparatus for carrying out said method comprising a source permitting a plasma beam to be controllably synthesized to have known characteristics and which can be directed to a region of interaction with a magnetic field. In order to eliminate boundary effects caused by polarization electric fields developed across the plasma density gradients at the plasma stream boundaries, the magnetic field is generated in a radial geometry. It is found that above a critical value of magnetic field, the electrons in the plasma beam are driven in annular orbit in the radial magnetic field and do not penetrate the field, whereas the positive ions traverse the field and impact on an electrostatic collector. The apparatus is such as to act as an electron deflector and to thus afford the possibility of generating a type of plasma behavior hitherto unavailable for observation and is also such as to be useful for simulating outer space ionosphere phenomena and conditions and for the creation of plasma stream phenomena for purposes of analytical study with a view to fusion techniques and the like.

Description

United States Patent [72] Inventors William Bernstein Los Angeles; John Michael Sellen, J r., Encino; Howard S. Ogawa, Los Angeles, Calif. [21] Appl. No. 707,054 [22] Filed Feb. 21, 1968 [45] Patented Apr. 20, 1971 [73] Assignee TRW Inc.

Redondo Beach, Calif.

[54] PLASMA SIMULATION AND ANALYSIS APPARATUS AND METHOD 6 Claims, 7 Drawing Figs. [52] US. Cl. 250/49.5,

313/63, 313/161, 313/231, 315/111 [51] Int. Cl. I-I0lj l/50, H01j 37/26, 1105b 1/00 [50] Field ofSearch 313/63, 161, 231; 315/] 11;250/49.5 (4),41.9 (3), (Inquired) [56] References Cited UNITED STATES PATENTS 2,760,076 8/1956 Dalton et al. 250/41.9(3) 2,922,905 1/1960 Van De Graaff 313/63 3,288,993 1 H1966 Steinhaus et al. 250/41.9(3) 3,336,475 8/1967 Kilpatrick 313/63X 3,387,131 6/1968 Helmet 3,435,208 3/1969 Hansenetal.

Primary Examiner-James W. Lawrence Assistant Examiner-Palmer C. Demeo Attorneys-Daniel T. Anderson, Edwin A. Oser and Jerry A.

Dinardo ABSTRACT: There is disclosed a method of interacting a plasma beam with a magnetic field and a preferred plasma simulation and analysis apparatus for carrying out said method comprising a source permitting a plasma beam to be controllably synthesized to have known characteristics and which can be directed to a region of interaction with a magnetic field. In order to eliminate boundary effects caused by polarization electric fields developed across the plasma density gradients at the plasma stream boundaries, the magnetic field is generated in a radial geometry. It is found that above a critical value of magnetic field, the electrons in the plasma beam are driven in annular orbit in the radial magnetic field and do not penetrate the field, whereas the positive ions traverse the field and impact on an electrostatic collector. The apparatus is such as to act as an electron deflector and to thus afford the possibility of generating a type of plasma behavior hitherto unavailable for observation and is also such as to be useful for simulating outer space ionosphere phenomena and conditions and for the creation of plasma stream phenomena for purposes of analytical study with a view to fusion techniques and the like.

PAIENTEB APRZO I97! SHEET 1 UF 2 William Bernstein H. S. gowa John M. SeHen Jr INV m5 ATTORNEY PATENTEU AFRZO m:

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n l/Z William Bernsmin HS- Oguwol John M- Sellen Jr:

INVENTORS kTTDRNEY PLASMA SIMULATION AND ANALYSIS APPARATUS AND IVETHIOI) BACKGROU ND OF TH E INVENTION The interactions between a moving diamagnetic plasma and a stationary transverse magnetic field, and conversely, between a stationary diamagnetic plasma and a moving magnetic field have been under extensive theoretical and experimental investigation. Interest in the phenomena results both from a purely scientific motivation and because the knowledge obtainable thereby facilitates the interpretation of a variety of plasma phenomena useful in many applications. These plasma phenomena interpretations include studies of the interaction of collisionless plasma streams with magnetic fields in a manner such as is believed occurs in outer space, and studies of the interactions of colliding plasma streams. Further applications occur in the study of the interaction of plasma streams with material bodies and in the analysis of plasma characteristics under confined and/or partially confined conditions such as are suitable for application to fusion reactions.

Earlier experimental work of the inventors herein has been published, for example, in an article entitled, Interaction of Collisionless Plasma Streams With Transverse Magnetic Fields which appeared in the July 1964 issue of The Physics of Fluids Volume 7, No. 7, page 977 and in an article entitled, Generation and Diagnosis of Synthesized Plasma Streams which appeared in the Mar. 1965 issue of The Review of Scientific Instruments Volume 36, No. 3, pages 316 to 322. Reference is also made to the bibliographic citations in these articles for a representative summary of the prior art.

SUMMARY OF THE INVENTION The above-referenced publications describe the penetration of synthesized Cs plasma streams through transverse magnetic fields. In those experiments, the observed penetration of electrons through the field was attributed to polarization electric fields developed across the plasma density gradient at the plasma stream boundaries. In the method and apparatus of the present invention, the penetration of an annular plasma stream through a radial magnetic field can be analyzed. This new geometry eliminates those boundary effects which cause the polarization electric fields observed in the previous configuration. The annular path of electrons passing through the radial field is essentially without a terminating boundary or end since it is closed upon itself. This configuration thus affords a considerable latitude in selection of effective path lengths which is not otherwise obtainable.

The presently measured results indicate that electron penetration of the radial magnetic field ceases as the magnetic field intensity is increased above a critical value where the electron cyclotron frequency equals or exceeds the electron plasma frequency. This nonpenetrating region is believed to be stable. For values of magnetic field below the critical value, high frequency oscillations always occur and low frequency oscillations occasionally are observed. In the relationship stated above, the electron cyclotron frequency (co is defined by the expression eBlmc where e is the charge on a single electron (4.803Xl e.s.u.); B is the magnetic field in gauss; m is the electron mass in grams, and c is the velocity of light in centimeters per second. Further, the electron plasma frequency (m is defined by the expression View? where similar terms have the same meaning and n is the electron density of the plasma, i.e., electrons per cubic centimeter.

It follows from these definitions and observed effects that the critical value of magnetic field for nonpenetration of electrons is at least approximately given by:

It will be noted, of course, that in Equation (3) only B, the magnetic field, and n, the electron density of the plasma are variables, all other terms being conventionally defined constants which can be grouped in a constant K where K w 41rmayc IN THE DRAWINGS FIG. 1 is a perspective view of the general arrangement of the apparatus including a plasma beam source and the annular electron deflector having the plasma beam collimator removed for clarity of illustration.

FIG. 2 is a sectional view taken generally on the line 2-2 of FIG. 1 and also including a showing of a suitable collimator.

FIG 3 is a sectional view taken on the line 3-3 of FIG. 2.

FIG. 4 is a graph showing the dependence of critical magnetic field intensity B on the square root of plasma density n, as measured at the collimator. The field intensity is plotted on the vertical axis and measured in gauss (B), whereas n% is plotted on the horizontal axis and is expressed in units of 10". The numerical value of n is here the same as it is where the context indicates its use as electron density since plasma density is defined as the number of pairs of electrons and positively charged ions per cubic centimeter. The experimental points indicated on the graph by the circles are for a plasma density distribution havingV n/B constant across the beam cross section, whereas the points indicated by rectangles are for a plasma beam having a uniform density.

FIG. 5 is a schematic view showing details of the porous tungsten cesium ionizor used in the plasma generator.

FIG. 6 is a schematic view showing details of the construction of a conventional Faraday cup probe for current density measurements.

FIG. 7 is a schematic view showing details of the construction of a conventional hot wire probe used for Langmuir probe measurements of electron temperature and emissive probe measurements of plasma potential.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the drawings, and in particular to FIG. 1, there is shown a perspective view of the general arrangement of the apparatus including a plasma source 10 and a magnetic field generator 11 which functions as an electron filter deflector. It will be seen from FIGS. 1, 2, and 3 that the field generator 11 is generally cylindrical in shape and has an annular cavity 12 formed axially therein. The annular cavity, of course, does not extend the full length of the cylinder and thus forms a cuplike arrangement having a central magnetic pole piece 13 surrounded by an annular pole piece 14 which forms the outer wall of the cylinder.

The annular cavity 12 between these pole pieces terminates in a recess in which the magnetic windings 15 are disposed. These magnetic windings are connected by external electrical lines shown schematically in FIG. 1 to a power source 16 and a variable current control device 17.

A plurality of smaller apertures 18 extend through the rear wall of the cylinder in order to accommodate the positioning rods 19 of electrostatic collector plates 20 which are used for a purpose to be described below.

It will be seen in FIG. 2 that a central collimator plate 21 is mounted by an insulating member 22 to the central pole piece 13 so as to be spaced therefrom. An outer annular collimator member 23 is similarly mounted by an insulating member 24 to the outer pole piece 14. Collimator 23 is mounted concentrically with member 21 and is spaced therefrom to form an annular gap 25. The annular gap 25 is positioned in front of the annular cavity or recess 12 in the cylindrical magnetic generator 11 and is of less width than the recess in order to direct a plasma beam into the central area of the recess and onto the electrostatic collector members 20. The collimators 21 and 23 and their insulating mountings have not been shown in FIG. I in order to afford a clearer showing of the internal working area of the magnetic field generator.

The plasma source shown schematically in FIG. I is represented in greater detail in FIG. 5 and is described fully in the above-noted article from The Review of Scientific Instruments." That is to say, for the purposes of the present invention, many plasma sources are suitable and the one actually used in a preferred embodiment is that previously described. Any plasma source used must be such as to produce a plasma which is: (I) quiescent, i.e., without oscillation; (2) collisionless with respect to neutrals and charged particles; (3) magnetic field free; and (4) ultraviolet light free in order to eliminate generation of trapped particles in the interaction region.

In fact, both the plasma source and the associated array of diagnostic devices for ascertaining measuring and demonstrating the properties of the synthesized plasma streams which the source generates may be the same as those described in the article.

For the purposes of this discussion, a synthesized plasma is defined as one in which the ions and electrons are separately generated and subsequently mixed to form a neutral plasma. In such a stream, the ions are electrostatically accelerated prior to the introduction of electrons. Because of the relatively larger ion mass and acceleration energies, the bipolar expansion of the ions away from the axis of the ion stream is, in general, small compared to the divergences that are produced by electrostatic lens effects in the ion acceleration region. By proper design, the divergence angles of high perveance sources may be reduced to the order of one degree. The synthesized plasma stream, P; is thus a column whose cross section expands slowly with increasing distance from the source. The radial confinement of the plasma is produced by the motion of the ions so that an axial magnetic filed is not required.

For the cesium plasma stream, the ion velocities are ordered within one part in IO. This corresponds to an initial ion thermal energy of approximately 0.1 electron volts and an ion acceleration energy of several hundredths of electron volts. The electron motion may be made totally random (no net streaming velocity) or be made to range upward to streaming velocities many times greater than the ion velocity. Plasma density, plasma common dimensions, ion velocity, and electron temperature, are similarly under the control of the operator. The synthesized plasma stream, thus, is a quiescent plasma whose properties may be independently and accurately specified and controlled.

Details of the ion source are shown schematically in FIG. 5. A copper boiler tube 30 is maintained at an operating temperature of 475 K. by a boiler heater 31. A cesium ampule 32 is contained within the boiler tube and may be broken by squeezing indentations in the tube. A molybdenum tube 34 is positioned to conduct cesium vapor from the boiler tube to a cesium plenum 35 having a molybdenum base 36 and containing alumina 37 in which a tungsten wire heater 38 is immersed. Heat shield members 39 are provided at the back and sides. The front of the plenum is faced with a porous tungsten disc 40 which has an operating temperature of 1 ,500 K. The accelerator grid 41 is positioned in front of this porous tungsten disc 40 from which ions are emitted.

Atoms of cesium are supplied by the boiler which is maintained at about 475 K. These atoms diffuse through the 2.5 cm. diameter porous tungsten plug 40 and are subsequently ionized on the forward surface of the plug by contact ionization. The tungsten plug is, as noted, maintained at a temperature of about 1,500 K., a temperature sufficient to evaporate ions at the desired current densities and to prevent the accumulation of an absorbed layer of cesium atoms which act to lower the surface work function and prevent the contact ionization.

The ions are accelerated by the planar grid 41 which is formed of 0.005 centimeter tungsten wires with 25 wires per centimeter in each of two perpendicular directions in the plane of the grid. The grid frame is preferably mounted on a lead-in screw which permits the specification of the acceleration distance within increments of 0.002 cm. The current densities for various spacings and voltages are discussed in greater detail in the article from The Review of Scientific Instruments" noted above.

As may be seen more clearly in FIG. 2, the plasma generator 10 is provided not only with the accelerator grid 41 but also with a neutralizer 42 which is nothing more than a wire from which electrons are emitted in a quantity sufficient to neutralize the ions generated and to therefore establish plasma conditions. The number of electrons is, of course, controlled by control of the voltage supplied to the wire 42. Preferably, the wire may be a 0.0l2 cm. diameter tungsten wire directly heated to about 2,500 K. By proper phasing of the heating current (a 60 cycle per second half-wave rectified signal) with the pulsed ion acceleration voltage, the wire is made a unipotential source of neutralizing electrons while the beam is on. The neutralizer wire is directly immersed in the ion stream about 0.3 cm. from the accelerator grid. This immersed wire provides maximum coupling between the plasma column and the electron source. To reduce the coupling, a variable position wire about I centimeter from the accelerator grid is used. The radial withdrawal of the wire from the plasma column increases the injection potential and the electron temperature in the plasma.

In FIG. 6 there is schematically shown the details of the construction of the Faraday cup probes used for current density measurement in the plasma. These probes are conventional and are more fully described in the above-noted article. It will, however, be noted that the probe consists of a cup member 50 having an entrance port with a diameter of approximately 0.3 centimeter across which a cross grid 51 extends. Within the cup 50 a cup-shaped electrode 52 is connected to ground through a current measuring resistor 53 across which signal output may be taken from a terminal 54. The outer cup itself is connected through a bias battery 55 to ground. All connections are made by shielded cable 56 which is itself provided with a grounded outer conductor. This Faraday cup is used in a manner well known in the art to measure ion current densities. The plasma density may be determined from the measured ion current density and the known ion velocity.

In FIG. 7 there is schematically shown the construction details of the hot wire probe which may be used for Langmuir probe measurements of electron temperature and emissive probe measurements of plasma potential. This type of probe is also conventional and well known and is described schematically herein merely for convenience.

The housing of the probe may be thin wall stainless steel tubing 60. A pair of nickel support rods 61 and 62 are mounted in the housing and spaced from it by ceramic insulators such as the insulator 63. A cadmium tungsten wire loop 64 is spot welded to the nickel support rods and protrudes out of one end of the housing. The nickel support rods are in turn connected to a filament transformer 65 which has a rectifier 66 connected between one end of it and one of the nickel rods and from the other end of the primary a probe potential variable bias battery is connected through a current measuring resistor 68 to ground. An output signal terminal 69 is connected between battery 67 and resistor 68. The tungsten wire is normally 0.0025 cm. in diameter and 0.6 to 1 cm. in length. The wire is maintained at about l,500 I(., below the point of a contributing electron emission signal, to preserve a clean and stable surface. BEcause of the high ion kinetic energy, the ion current to the probe wire is constant for small variations in probe potential and the electron collection current is taken as the shift in the probe signal from its reference value at a bias of a few volts negative with respect to the plasma. Instrumentation such as the Langmuir probe illustrated in FIG. 7 and the Faraday cup probes illustrated in FIG. 6 is positioned at various convenient areas of the plasma stream in a manner to be described below.

OPERATION OF THE DEVICE The above-noted article in The Physics of Fluids" described the penetration of synthesized Cs plasma streams through transverse magnetic fields. As noted, in those experiments, the observed penetration was attributed to polarization electric fields developed across the plasma density gradients at the plasma stream boundaries. In the present device, the penetration of an annular plasma stream through a radial magnetic field can be observed. The new geometry is found to eliminate those boundary effects which cause the polarization electric fields observed in the previous configuration. The experimental results indicate that electron penetration of the magnetic field ceases as the magnetic field intensity is increased above a critical value where the electron cyclotron frequency approaches the electron plasma frequency as those terms have been defined above. Of course,

the magnetic field is controlled by the device 17 which varies the current flowing to the magnetic coils 15.

In this nonpenetrating region where the critical field value is exceeded, the operation of the device is found to be stable. For values of magnetic field below the critical value, high frequency oscillations always occur and low frequency oscillations in the plasma stream occasionally are observed.

The new apparatus configuration is shown in FIGS. 1, 2, and 3. The operating conditions of the plasma source shown in detail in FIG. 5 and the zero magnetic field plasma characteristics are similar to those previously described in the above-noted article in The Review of Scientific Instruments. As noted, a high impedance collimator 2123 placed in front of the magnetic gap defines the annular plasma stream enteringthe magnetic field. Typical plasma densities at the entrance to the field region range from 10' to 10 cm. For the initial electron temperature of 0.25 ev. the Debye lengths lie in the range 0.1-0.035 cm., which are small compared to the width of the plasma annulus. The highest ion energy employed to date has been 300 electron volts corresponding to an ion velocity of 2X10 cm. sec The magnetic field intensity can be adjusted from 0 to about 1,000 gauss. The radial field geometry results in. a HR fall off in intensity across the gap which for the dimensions chosen represents a 25 percent decrease across the thickness of the plasma stream. Under certain source conditions the azimuthally uniform plasma density is also relatively uniform across the gap; under other conditions a density gradient can be generated such that the ratio Vn/B is relatively constant across the gap.

The diagnostic and measurement means employed to date include: I. a movable Faraday cup of the type shown in FIG. 7 which is used to determine the plasma density profile entering the magnetic gap; 2. a movable Langmuir probe of the type illustrated in FIG. 6 which is used to determine electron temperature in the plasma column; 3. a movable emissive probe also structurally of the type shown in FIG. 6 operated to field. For magnetic fields below the critical value, the

collector potential is relatively constant and is approximately equal to the potential of the plasma stream prior to entrance into the field region. As the magnetic field is increased, there is an abrupt increase in collector potential to a value close to the ion acceleration potential. The rise of collector potential signifies a transition from electron penetration to electron cutoff. Simultaneously, the observed neutralizer emission current decreases by an amount about equal to the ion flux entering the aperture confirming that electrons are no longer accompanying ions to the collector. In the nonpenetrating state both the Langmuir probe measurements in the plasma column and the unchanged beam divergence shown no evidence of an electron temperature increase which could be attributed to the return of electrons heated in the interaction region.

In FIG. 4 the critical value of field intensity B is plotted as a function of w/fi (center plasma density) for both a uniform density beam and one for which x n/B is constant across the gap. The experimental points indic ated by circles are for a density distribution having n/B constant and the experimental points indicated by rectangles are for a more uniform density. The straight line has been fitted to the first set of points and experimentally confirms the relationship set forth in equation (3) above.

The observed dimensions, for the two stream configurations are as yet, not understood. The insertion of a small obstacle in front of the annulus during nonpenetrating conditions facilitates penetration, probably because it removes the symmetry in the plasma stream and permits azimuthal polarization electric fields to occur.

No oscillations are observed once the nonpenetrating condition is achieved. High frequency oscillations in the range of 5-70 me. are always observed during penetration. Upon occasion, low frequency oscillations of 50-100 Kc. are observed for weak magnetic fields of 2--5 gauss. For magnetic fields less than the critical value, the high frequency oscillations appear to bev near the electron cyclotron frequency; as the magnetic field is increased to near the critical value, the frequency appears to limit near the electron plasma frequency. Further increases in the magnetic field result in the abrupt switch to the nonpenetrating condition and the disappearance of all oscillations.

The absolute values of magnetic field and density for each transition shown in FIG. 4 together with the observed frequencies indicate that the transition occurs for conditions where the cyclotron electron frequency is approximately equal to the plasma electron frequency.

It is still not entirely clear whether the electrons are trapped in an annular orbit around the center pole 13 or whether the radial magnetic field acts as a mirror for electrons and clearly them back through the plasma stream to the neutralizer. In either event, the annular device generating a radial magnetic field does act to deflect any electrons in the plasma stream entering the gap and to thereby prevent their penetration through the device. It will of course be realized that when the collector element 20v and its rod 19 are removed from the aperture 18, the ions continue on through the apertures. While a sufficiently large magnetic field is being generated, however, the electrons are deflected or filtered from the plasma beam by the radial magnetic field. The device thus clearly acts as an electron deflector or filter andgenerates a pure ionic beam from a initially available natural or manufactured plasma stream.

While a specific embodiment of the invention has been described by way of illustration only, it will be understood that -the invention is capable of many other specific embodiments and modifications and is defined solely by the following claims.

We claim: 1. Plasma apparatus comprising: a. means to generate a radial magnetic field. in a predetermined region; b. means to introduce a substantial annular neutralvplasma beam into said radial magnetic field region;

c. means to maintain said radial magnetic field at a value sufficient to prevent electron penetration through said field region; and

d. electrostatic collector means positioned to receive positive ions from said plasma after they have traversed said field region.

2. Apparatus as in claim 1 and further including:

a. means to measure the value of said magnetic field;

b. means to measure the potential of said collector means;

and

c. means to measure a characteristic of said plasma before it enters said field region.

3. Plasma apparatus comprising:

a. means to generate a radial magnetic field in a predetermined region;

b. means to introduce a neutral plasma forming a closed sheath into said radial magnetic field region; and

c. means to maintain said radial magnetic field at least equal to the value at which the electron cyclotron frequency is equal to the electron plasma frequency for said plasma.

4. A method of plasma treatment comprising the steps of:

a. establishing a radial magnetic field in a predetermined region;

b. introducing a neutral plasma into said radial magnetic field region;

c. forming said plasma into a substantially annular sheath;

and

d. maintaining the value of said magnetic field at a value at least equal to that for which the electron cyclotron frequency is equal to the electron plasma frequency of said plasma to prevent electrons in said plasma from penetrating said field region.

5. A method of treating a plasma comprising the steps of:

a. generating a neutral plasma beam;

b. maintaining an electrostatic field axially along said beam;

c. forming said plasma beam into a substantially annular closed sheath; and

d. introducing said beam into a radial magnetic field region centered about said beam axis.

6. A method of plasma treatment comprising the steps of:

a. establishing a radial magnetic field; and

b. introducing an annular neutral plasma beam into said radial magnetic field, said field having a magnitude sufficient to deflect electrons in said plasma and prevent their penetration through said field.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,572,598 Dated May 17, 1971 Inventor) William Bernstein et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Specification Column 1, line 62 "eBlmc" should be line 63 "(4.803 X 10 e.s.u.)" should be --(4.s03 x 10' e.s.u.).

H Column 2, line 15 "\Nrrrmayc should be r mmc Column 3, line 41 "filed" should be -field.

Column 5, line 41 cm should be -cm line 46 "sec should be -sec Column 6, line 12 "shown" should be -show.

In the Claims Column 6, line 74 "substantial" should be -substantially.

Signed and sealed this 7th day of September 1971.

(SEAL) Attest:

ROBERT GOTTSCIIALK Acting Commissioner of Patem EDWARD M..FLETCHER, JR. Attesting Officer

Claims (6)

1. Plasma apparatus comprising: a. means to generate a radial magnetic field in a predetermined region; b. means to introduce a substantial annular neutral plasma beam into said radial magnetic field region; c. means to maintain said radial magnetic field at a value sufficient to prevent electron penetration through said field region; and d. electrostatic collector means positioned to receive positive ions from said plasma after they have traversed said field region.

2. Apparatus as in claim 1 and further including: a. means to measure the value of said magnetic field; b. means to measure the potential of said collector means; and c. means to measure a characteristic of said plasma before it enters said field region.

3. Plasma apparatus comprising: a. means to generate a radial magnetic field in a predetermined region; b. means to introduce a neutral plasma forming a closed sheath into said radial magnetic field region; and c. means to maintain said radial magnetic field at least equal to the value at which the electron cyclotron frequency is equal to the electron plasma frequency for said plasma.

4. A method of plasma treatment comprising the steps of: a. establishing a radial magnetic field in a predetermined region; b. introducing a neutral plasma into said radial magnetic field region; c. forming said plasma iNto a substantially annular sheath; and d. maintaining the value of said magnetic field at a value at least equal to that for which the electron cyclotron frequency is equal to the electron plasma frequency of said plasma to prevent electrons in said plasma from penetrating said field region.

5. A method of treating a plasma comprising the steps of: a. generating a neutral plasma beam; b. maintaining an electrostatic field axially along said beam; c. forming said plasma beam into a substantially annular closed sheath; and d. introducing said beam into a radial magnetic field region centered about said beam axis.

6. A method of plasma treatment comprising the steps of: a. establishing a radial magnetic field; and b. introducing an annular neutral plasma beam into said radial magnetic field, said field having a magnitude sufficient to deflect electrons in said plasma and prevent their penetration through said field.

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