JPH07135099A - Ion beam current measuring device and method - Google Patents

Abstract

PURPOSE:To provide a device and method to measure an ion beam current contactlessly at high precision without disconnecting flow of an ion beam accelerated by an accelerator or the like. CONSTITUTION:A magnetic field by an ion beam inside vacuum piping 10 is introduced to a superconductive element 14 by a magnetism collector (permalloy) 16, and the superconductive element 14 cooled by a cooling part 20 detects a magnetic flux of the field to be displayed in an ion beam current meter 26, so an ion beam current is measured. A magnetic shield 18 is provided around the vacuum piping for eliminating effects of an external magnetic field.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for measuring an ion beam current accelerated by an accelerator or the like, and more particularly to an apparatus and method capable of non-contact and continuous measurement of an ion beam current. .

[0002]

2. Description of the Related Art At present, there is a method of using a Faraday cup as a method of accurately measuring an ion beam current accelerated by an accelerator or the like. In this method, the ion beam is received in the cup and the amount of current is directly measured.

Further, a method of indirectly estimating the beam amount from the peak value of the beam profile monitor or measuring the amount of current injected into the target has been adopted.

[0004]

Since the conventional method measures as described above, the ion beam is received in the cup, so that the ion beam irradiation to the product or the like can be performed while the ion beam current is being measured. There wasn't. On the contrary, the ion beam current could not be measured while the product etc. were being irradiated with the ion beam. In addition, a beam profile monitor may be used to measure the beam current during irradiation of products, but in this method, the change in the peak value does not accurately represent the ion beam current, so the ion beam current during accurate irradiation Beam quantity is not measured.
Further, in the measurement by receiving the ion beam on the target, the secondary electrons generated at the moment when the ion collides with the target have a great influence on the amount of ion beam current to be measured. Since the amount of ion beam current was determined by making corrections according to the type and shape, it was not possible to perform accurate measurement.

The present invention has been made in order to solve the above-mentioned problems, and the ion beam current is non-contacted without interrupting the flow of the ion beam accelerated by an accelerator or the like, that is, secondary electrons. It is an object of the present invention to provide an apparatus and method for measuring accurately without the influence of

[0006]

In order to achieve the above object, an ion beam current measuring apparatus of the present invention comprises a magnetic field collecting means for collecting a magnetic field generated by the flow of an ion beam and corresponding to the ion beam current. A magnetic flux measuring means having a superconducting element sensitive to the magnetic field collected by the magnetic field collecting means, for measuring the magnetic flux of the magnetic field, and cooling for cooling the superconducting element to a temperature at which the superconducting element is sensitive to the magnetic field. And a magnetic shielding means for magnetically shielding the space in which the ion beam is flowing, the magnetic field collecting means and the superconducting element from a magnetic field external thereto.

To achieve the above object, the ion beam current measuring method of the present invention comprises a step of collecting a magnetic field generated by the flow of an ion beam and corresponding to the ion beam current, excluding other external magnetic fields. Measuring the magnetic flux of the collected magnetic field excluding the external magnetic field using a superconducting element cooled to a temperature sensitive to the magnetic field.

[0008]

In the ion beam current measuring apparatus of the present invention constructed as described above, the magnetic field collecting means collects a magnetic field corresponding to the ion beam current generated by the flow of the ion beam, and the magnetic field is collected by the cooling means. The magnetic flux measuring means measures the magnetic flux of the magnetic field, as the superconducting element cooled to a temperature sensitive to the magnetic field is sensitive to the collected magnetic field, whereby the ion beam current is measured. Since the space in which the ion beam is flowing, the magnetic field collecting means and the superconducting element are magnetically shielded from the magnetic field external to them by the magnetic shielding means, the magnetic field collecting means and the superconducting element are protected from external magnetic fields which become noise. By collecting only the magnetic field generated by the ion beam except for and sensitive to the magnetic flux of the magnetic field, the magnetic flux measuring means can accurately measure only the magnetic field generated by the ion beam, and therefore the ion beam current is accurate. Well measured.

In the ion beam current measuring method of the present invention configured as described above, the magnetic field generated by the flow of the ion beam and corresponding to the ion beam current is collected except for the external magnetic field, and the magnetic field The magnetic flux of the collected magnetic field, excluding the external magnetic field, is measured by using a superconducting element cooled to a temperature sensitive to, and thereby the ion beam current is measured.

[0010]

EXAMPLES First, the principle of measurement of the ion beam current according to the present invention will be described. The accelerated ion beam can be regarded as a current flowing in a vacuum tube. Then, a magnetic field is always generated where the current flows, depending on the amount of current. The ion beam current can be indirectly measured by accurately measuring this magnetic field. However, the magnetic field generated by the accelerated ion beam is not so large in general, and a superconducting element is used because it cannot be measured by an ordinary magnetometer using a coil or the like in order to measure a small magnetic field with high accuracy. If a superconducting element is used, it is possible to measure a DC magnetic field and an AC magnetic field depending on the purpose.

At this time, if an attempt is made to measure a minute magnetic field with high sensitivity, an external magnetic field which causes noise will occur. Therefore, a magnetic shield for preventing the effect is provided so as to surround the ion beam. In addition, in order to measure the minute magnetic field with high sensitivity and to prevent the measured value from changing depending on the position of the ion beam with respect to the superconducting element which is the measuring element,
A magnetic collector made of permalloy or the like that efficiently collects the generated magnetic field and guides it to the superconducting element is provided around the ion beam and inside the magnetic shield. Furthermore, the superconducting element is shielded from the external temperature environment by a heat insulating material or the like in order to operate it stably, and is cooled to an operating temperature by a cooling device.

An embodiment of the ion beam current measuring apparatus and method of the present invention based on the above-described measurement principle will be described below with reference to the drawings. The present invention is not limited to such an embodiment.

FIG. 1 shows, in the form of a partial cross-section, the basic construction of an embodiment of the ion beam current measuring apparatus and method of the present invention. In FIG. 1, 10 indicates a vacuum pipe through which an ion beam 12 accelerated by, for example, an accelerator or the like is flowing. The inside of the vacuum pipe 10 is evacuated, and the ion beam 12 flows therein, that is, in the direction orthogonal to the plane of the paper in the drawing. Since the magnetic field that is generated by the flowing ion beam 12 in the circumferential direction orthogonal to the direction of the ion beam 12 is usually small, it is collected and efficiently collected in the superconducting element 14.
In order to lead to the above, inside the vacuum pipe 10, a magnetic collector 16 made of a magnetic material such as permalloy is arranged in a ring shape so as to surround the ion beam 12. The superconducting element 14 is inserted in one place of the ring-shaped magnetic collector 16. The magnetic collector 16 collects only the magnetic field generated by the ion beam 12 and guides it to the superconducting element 14, and the superconducting element 14 detects only the magnetic flux of the magnetic field. In order to shield the inside of the vacuum pipe 10, a magnetic shield 18 is provided outside the vacuum pipe 10 so as to surround it. Although not shown in the drawing, a magnetic shield is provided before and after this measurement system so as not to obstruct the beam and to hide the superconducting detector including the magnetic collector 16 and the superconducting element 14. . A cooling unit 20 is provided for cooling the superconducting element 14 to a temperature at which it can detect a magnetic field. Further, the cooling part 20 has a heat insulating part, and liquid nitrogen 22 is put into the outer peripheral part of the cooling part 20 for primary cooling, and liquid helium 24 having a function of secondary cooling is put into the inside thereof. Superconducting element 1
The magnetic flux of the magnetic field detected by 4 is converted into a current and displayed by the ion beam ammeter 26 via an amplification control unit (described later). In order to facilitate understanding, a part of FIG. 1 schematically shows the relationship between the minute magnetic field and the current / voltage in the superconducting element 14.

FIG. 2 shows the superconducting element 14 and the ion beam ammeter 26 shown in FIG. 1, and the amplification control section of the superconducting element 14, in which two Josephson elements are arranged in a superconducting loop having a small inductance. An example configured with a known dcSQUID magnetometer is shown. In FIG. 2, 14 is a superconducting device composed of a coil in which two Josephsen devices are connected in parallel, 28 is a direct current source, 30 is an amplifier, 32 is a lock-in amplifier, and 34 is an oscillator. The symbols L, C and R respectively indicate a coil, a capacitor and a resistor.

Next, the operation of the ion beam current measuring apparatus according to one embodiment of the present invention constructed as shown in FIGS. 1 and 2 will be described below. In FIG. 1, the minute magnetic field generated when the ionized beam travels in the vacuum pipe 10 with the magnetic shield 18 is collected by the magnetic collector 16 and guided to the superconducting element 14. At this time, since the inside of the vacuum pipe 10 is shielded from the external magnetic field by the magnetic shield 18, the magnetic collector 16 collects only the magnetic field generated by the ion beam 12 and guides it to the superconducting element 14.
The superconducting element 14 is cooled in advance by the cooling unit 20 to a temperature at which the guided minute magnetic field can be detected.

In FIG. 2, a constant current is applied by a direct current source 28 in a state in which a minute magnetic field to be measured is applied to a superconducting element 14 composed of a dcSQUID element,
Put the dcSQUID element into operation. On the other hand, a current is passed through the ion beam ammeter 26 to the electric wire 15 laid so as to cancel the magnetic field to be measured, and dcSQUI
The canceling current is adjusted so that the D element is in the quasi-static state. The amount of current when this quasi-static state is achieved is the amount of ion beam current. In addition, it is determined whether the quasi-static state is 10 or less with the canceling current in the electric wire 15 laid by the oscillator.
It is confirmed by passing a minute oscillation current of 0 kHz and confirming that the current waveform detected by the SQUID element is 100 kHz. When not in the quasi-static state, a value close to 200 kHz is shown. The current detected by the SQUID element is the amplifier 3
It is guided to the lock-in amplifier 32 via 0, where the canceling current is varied so as to be the same as the input frequency, and the canceling current is locked at the point where the same frequency is reached.

By operating as described above, the indication value of the ion beam ammeter 26 becomes proportional to the magnitude of the ion beam current.

Further, the correction to make the above-mentioned indicated value represent the magnitude of the ion beam current is performed by the magnetic collector 16
It is possible to make an accurate correction by passing a known amount of electric current through an electric wire previously attached inside the vacuum pipe 10 along the piping direction of the vacuum pipe 10. The electric wire for rehabilitation may be removed from the vacuum pipe 10 after the rehabilitation.

Therefore, the ion beam current measuring device and method of the embodiment shown in FIGS. 1 and 2 can measure the ion beam current continuously without contact and with high accuracy. Further, since the magnetic collector 16 is arranged so as to surround the ion beam 12, stable measurement can be performed even if the positions of the superconducting element 14 and the ion beam 12 change.

Next, a design example is shown below.

1. Basic formula Magnetic field unit is H (A / m), magnetomotive force unit is NI (A)
And

The magnetic flux density B is

[Expression 1] B = μH = μ ₀ · μ * · H (T) Here, the unit of the magnetic flux density is T (tesla), and 1 (T) = 1 (Wb / m ² ). The magnetic flux Φ
The unit of is Wb (weber).

In the above equation, μ is the magnetic permeability, μ ₀ is the vacuum magnetic permeability, and μ * is the relative magnetic permeability.

Note that μ ₀ = 4π × 10 ^-7 = 1.257 ×
10 ⁻⁶ (H / m), 1 (H) = 1 (Wb / A) = 1 (V / s / A) 1 (Wb) = 1 (V / s) = 1 (H · A) is there.

The strength of the magnetic field generated by the current flowing through the infinitely long wire is calculated by the Biot-Savart equation.

[Expression 2] dH = (I / 4π)  (dlr) / r ³ Here, I is a current, dl is a minute length of a part of I, and r is a distance from a point of dl to a point representing dH.

2. The strength of the magnetic field generated by the infinite linear current is derived from the integral value from positive infinity to negative infinity according to the above Biosavart equation.

[0027]

[Equation 3] H = ∫dH = I / 2πr From this formula, if the ion beam current to be measured is 1 μA and the diameter of the magnetic collector 16 is 11 cm, it is 5 points from the point on the magnetic collector 16, that is, the beam. The strength of the magnetic field at the point of 0.5 cm is as follows.

H _5.5 = 2.895 × 10 ^-6 (A / m) The magnetomotive force is NI = 1 × 10 ^-6 (A).

3. Amount of magnetic flux induction by Permalloy magnetic collector 16 Assuming that an annular Permalloy magnetic collector 16 is provided around the beam, its diameter is 11 cm as described above,
The cross section is a rectangle of 1 cm × 1 cm. However, since the size of the superconducting element 14 is smaller than the cross section, the part that guides the magnetic flux to the superconducting element 14 needs to be narrowed down, and the size of the narrowed part is 0.5 cm × 0.5 c in the cross section.
m, and the total length of the portions on both sides of the superconducting element 14 is 2c.
m. In this case, the magnetic resistance RB is

RB = (L ₁ / μ ₀ · μ * · S ₁ ) + (L ₂ / μ _{0
· Μ * · S 2) =} 0.3254 / (μ 0 · 1 × 10 6 · 1 × 10 -4) + 0.02 / (μ 0 · 1 × 10 6 · 2.5 × 10 -5) = 3225 Where L ₁ is the total length of the magnetic collector 16, S ₁ is its cross-sectional area, L ₂ is the total length of the narrowed portion that guides the magnetic flux to the superconducting element 14, and S ₂ is its cross-sectional area.

Therefore, the magnetic flux Φ is as follows.

[0031]

[Formula 5] Φ = NI / RB = 1 × 10 ⁻⁶ /3225.1 = 3.101 × 10 ⁻¹⁰ (Wb) From the above, superconductivity is passed through the Permalloy magnetic collector 16 by the magnetic field induced by the 1 μA beam. The magnetic flux guided to the element 14 is 3.101 × 10 ⁻¹⁰ (Wb)
Becomes Therefore, for a 1 nA beam, 3.101
It becomes x 10 ^-13 (Wb).

4. Size of Superconducting Element 14 It is known that the inductance L of a loop-shaped superconducting element is calculated by the following equation.

[0033]

L≈4π × 10 ⁻⁷ × r × ln (r / a) (H) Here, r is the radius of the loop (m), and a is the radius of the strand (m).

The radius of the loop of the superconducting element 14 is 2 mm,
If the radius of the strand is 0.25 mm, the inductance L of the superconducting element 14 is

## EQU7 ## L≈4π × 10 ^-7 × 2 × 10 ^-3 × ln (2 / 0.25) = 5.23 × 10 ^-9 (H).

By the way, in order to operate stably above 4 KHz and in the presence of noise due to thermal current, L≤2
It has to be × 10 ^-8 H, and it is said that ^{L≈10 -9} H in order to operate with higher reliability.

Therefore, since the inductance L of the superconducting element 14 is 5.23 × 10 ^-9 H, it is smaller than the upper limit value required for stable operation, and is on the order of a value required for highly reliable operation. Therefore, it can be seen that the superconducting device 14 of the present embodiment operates stably.

From the above design example, the magnetic field induced when an ion beam of about 1 μA is flown is about 3.101 × 1.
Since it is 0 ⁻¹⁰ (Wb), and a minute magnetic field of about 2.07 × 10 ⁻¹⁵ (Wb) can be measured in principle in the magnetic field measurement by the superconducting element, the configuration of the present invention is also in view of the principle. It can be seen that the current of the ion beam can be measured.

Therefore, since the magnetic field induced when an ion beam of about 1 μA is caused to flow is about five orders of magnitude larger than the magnetic field that can be measured in principle by the superconducting element, the accelerator of a commonly used accelerator of about nA or more is used. It can be easily measured in the beam output range.

Although the magnetic flux measuring system using the dcSQUID element as the superconducting element 14 is used in the above embodiment, the RF SQUID or any other magnetic flux measuring system is used depending on the intended use of the ion beam current to be measured. A magnetic flux measurement system using a superconducting element may be used.

Further, if the apparatus of the present invention can be constructed inexpensively by developing a high temperature superconductor, and if a device having a wide measurement range is manufactured, the performance of the ion accelerator and the quality of the product can be improved, and Its applications will be wide-ranging, such as improving the accuracy of various experiments using ion beams.

[0041]

Since the ion beam current measuring apparatus and method for measuring the ion beam current accelerated by the accelerator of the present invention is configured as described above,
The following effects are achieved.

It is possible to measure the magnetic flux of the magnetic field generated by the ion beam current and corresponding to the magnitude of the current while the ion beam is flowing by the superconducting element and without affecting the flow of the ion beam. Therefore, it becomes possible to measure the ion beam current.

Therefore, since the current can be measured without contacting the ion beam, the ion beam current can be measured under any circumstances. For example, while irradiating an ion beam on a product, the ion beam being irradiated can be measured, and there is no error due to separate irradiation and measurement unlike the method using a conventional Faraday cup, Is improved.

Further, since it is non-contact, the influence of secondary electrons is eliminated unlike the conventional method using a Faraday cup, and the measurement accuracy is improved in this respect as well.

Since the present invention has the above-mentioned effects, it can be widely applied to industrial processes using an accelerator, and by using the apparatus or method of the present invention, the quality of products and the accuracy of experiments can be improved. Alternatively, the apparatus or method of the present invention can be widely used for accelerator control applications.

[Brief description of drawings]

FIG. 1 is a diagram showing, in the form of a partial cross-section, the basic configuration of an embodiment of an ion beam current measuring apparatus and method according to the present invention.

FIG. 2 illustrates the superconducting element 14 and the ion beam ammeter 26 shown in FIG.
It is a figure which shows the example comprised by the cSQUID magnetometer.

[Explanation of symbols]

 10: Vacuum piping 12: Ion beam 14: Superconducting element 16: Magnetic collector 18: Magnetic shield 20: Cooling part 26: Ion beam ammeter

Claims (2)

[Claims] 1. A magnetic field collecting means for collecting a magnetic field generated by a flow of an ion beam and corresponding to the ion beam current; and a superconducting element sensitive to the magnetic field collected by the magnetic field collecting means. Magnetic flux measuring means for measuring magnetic flux, cooling means for cooling the superconducting element to a temperature at which the superconducting element is sensitive to the magnetic field, space in which the ion beam is flowing, the magnetic field collecting means and the superconducting element And a magnetic shielding means for magnetically shielding them from an external magnetic field relative to them. 2. A step of collecting a magnetic field generated by a flow of an ion beam, the magnetic field corresponding to the ion beam current except for an external magnetic field, and a superconducting element cooled to a temperature sensitive to the magnetic field. Measuring the magnetic flux of the collected magnetic field excluding the external magnetic field.