JP6203648B2 - Negative ion source device and method for replacing plasma generation unit in negative ion source device - Google Patents

Description

  The present invention relates to a negative ion source device and a plasma generation unit replacement method in the negative ion source device.

  As an ion source device used for an accelerator or the like, for example, a plasma sputtering type negative ion source device using a solid as a raw material (see Patent Document 1) or a negative ion beam generating a negative ion beam using a gas (for example, hydrogen gas) as a raw material. Ion source devices are known. The latter negative ion source device includes, for example, a chamber, a filament for generating plasma in the chamber, a hydrogen gas supply unit for supplying hydrogen gas into the chamber, and a cesium for supplying cesium vapor into the chamber. A supply source. Since an electric current is passed through the filament, thermoelectrons are emitted from the heated filament and collide with hydrogen gas in the chamber to generate plasma. And the negative ion is produced | generated when the slow electron in plasma or the electron of the plasma electrode surface reacts with a hydrogen molecule, a hydrogen atom, or a hydrogen ion.

  Since the work function is reduced on the surface of the substance to which cesium has adhered, cesium has a function of promoting the generation of negative ions. Therefore, when cesium is supplied into the chamber by the cesium supply source, the amount of negative ions extracted from the negative ion source is increased. Thus, the cesium supplied in the vapor state into the chamber gradually accumulates in a solid or liquid state on the wall surface of the chamber as the negative ion source device is operated.

JP 2004-030966 A

  By the way, with the operation of the negative ion source device, the filament that is the plasma generation unit is consumed, so that it is necessary to open the chamber to the atmosphere in order to replace the filament. However, when cesium, which is a highly reactive alkali metal, comes into contact with the atmosphere, an oxidation reaction proceeds with a violent reaction, and there is a risk of ignition. Therefore, air is introduced into the chamber in advance before the chamber is released to the atmosphere, the oxidation reaction of cesium adhering to the wall surface of the chamber is completed, and after the reacted cesium oxide is removed from the chamber, the filament is exchanged. Is called.

  As described above, in the negative ion source device, the maintenance work for removing cesium oxide from the chamber takes time and effort. In addition, the reacted cesium oxide cannot be reused, resulting in an increase in the amount of cesium used.

  Therefore, an object of the present invention is to provide a negative ion source device and a method for exchanging a plasma generation unit in the negative ion source device that can easily and quickly replace the plasma generation unit.

  A negative ion source device according to one aspect of the present invention includes a first chamber provided with a through hole that communicates the inside and the outside, a source gas supply unit that supplies a source gas into the first chamber, A plasma generating unit that is attached to the through hole and generates plasma using the source gas supplied by the source gas supplying unit, and a promoting substance that promotes the generation of negative ions and has reactivity with the atmosphere in the chamber When the promoting substance supply unit to be supplied and the plasma generation unit are removed from the through hole, the inert gas supply unit for supplying an inert gas into the chamber and the plasma generation unit are removed from the first chamber. A pressure adjusting unit that adjusts the pressure of the inert gas supplied from the inert gas supply unit into the first chamber to be higher than the pressure outside the first chamber. .

  In the negative ion source device according to one aspect of the present invention, the pressure of the inert gas supplied from the inert gas supply unit into the first chamber by the pressure adjustment unit is greater than the pressure outside the first chamber. Is adjusted to be higher. Therefore, even when the plasma generation unit is removed from the through hole for exchanging the plasma generation unit, the inert gas is discharged from the inside to the outside of the first chamber through the through hole. Therefore, there is almost no possibility that the atmosphere flows into the first chamber through the through hole, and the reaction between the promoting substance and the atmosphere existing in the first chamber is suppressed. This eliminates the need for work and time for removing the air and the reacted promoting substance from the first chamber, so that the plasma generation unit can be replaced easily and quickly. In addition, since the reaction between the atmosphere and the promoting substance is suppressed, the consumption of the promoting substance can be greatly reduced.

  The pressure adjusting unit may be a safety valve that closes when the pressure in the first chamber is less than a predetermined magnitude and opens when the pressure in the first chamber is greater than or equal to a predetermined magnitude. Good. In this case, when the inert gas is supplied from the inert gas supply unit into the first chamber, the pressure in the first chamber can be easily adjusted.

  The negative ion source device according to one aspect of the present invention further includes a second chamber connected to the first chamber on the downstream side from which negative ions generated in the first chamber are extracted, and the safety valve includes: Two chambers may be provided. In this case, since the second chamber generally has a larger installation space than the first chamber, the degree of freedom in design can be increased when installing the safety valve.

  The safety valve may be provided in the first chamber. In this case, since the direct object of the operating pressure of the safety valve is the pressure of the first chamber, the pressure of the first chamber can be adjusted more accurately by the safety valve.

  The first chamber has a cylindrical main body, a lid that closes the open end of the main body and has a through hole, and the plasma generator is detachably attached to the lid. It may be. In this case, since the area where the first chamber is opened when the plasma generating unit is removed from the lid portion is reduced, the possibility that the atmosphere flows into the first chamber through the through hole is further reduced.

  The inert gas supply unit may supply the inert gas into the first chamber from the side opposite to the through hole in the first chamber. In this case, since the inert gas flows in one direction toward the through hole in the first chamber, the possibility that the atmosphere flows into the first chamber through the through hole is further reduced.

  A method for replacing a plasma generation unit in a negative ion source device according to another aspect of the present invention is a method for replacing a plasma generation unit in the negative ion source device, wherein (A) a source gas supply unit and a plasma generation unit (B) supplying an inert gas into the first chamber by the inert gas supply unit, and (C) inerting the inert gas into the first chamber by the inert gas supply unit. A step of removing the plasma generation unit attached to the through hole and attaching a new plasma generation unit to the through hole while continuing to supply the gas.

  In the method for replacing a plasma generation unit in a negative ion source device according to another aspect of the present invention, the plasma generation unit can be replaced easily and quickly in the same manner as the negative ion source device according to one aspect of the present invention. It becomes possible, and it becomes possible to greatly reduce the consumption of the promoting substance.

  The exchange method of the plasma generation unit in the negative ion source device according to another aspect of the present invention may further include a step of heating the plasma generation unit before step (D). In this case, the plasma generator is preheated before the plasma generator is replaced. Therefore, the promoting substance adhering to the surface of the plasma generation unit evaporates and is removed from the surface before the replacement. Therefore, when the plasma generation unit is taken out of the first chamber by replacement, there is almost no possibility that the accelerating substance attached to the plasma generation unit comes into contact with the atmosphere. As a result, the plasma generation unit can be replaced more easily and quickly.

  ADVANTAGE OF THE INVENTION According to this invention, the replacement | exchange method of the plasma production | generation part in a negative ion source apparatus which can perform replacement | exchange of a plasma production | generation part simply and rapidly can be provided.

FIG. 1 is a diagram schematically showing a neutron capture therapy apparatus. FIG. 2 is a diagram illustrating the negative ion source device according to the present embodiment.

  Embodiments of the present invention will be described with reference to the drawings. However, the following embodiments are exemplifications for explaining the present invention and are not intended to limit the present invention to the following contents. In the description, the same reference numerals are used for the same elements or elements having the same function, and redundant description is omitted.

First, the neutron capture therapy apparatus 1 including the negative ion source apparatus 100 according to one embodiment of the present invention is taken as an example, and an outline of the neutron capture therapy apparatus 1 will be described with reference to FIG. The neutron capture therapy device 1 is a device used to perform cancer treatment using boron neutron capture therapy (BNCT), and to a tumor of a patient 50 to which boron ( ¹⁰ B) is administered. Irradiate neutron beam N.

  The neutron capture therapy apparatus 1 includes a cyclotron 2. The cyclotron 2 is an accelerator that generates a charged particle beam R by accelerating negative ions (also referred to as negative ions) generated by the negative ion source device 100. The cyclotron 2 has a capability of generating a charged particle beam R having a beam radius of 40 mm and 60 kW (= 30 MeV × 2 mA), for example. The neutron capture therapy apparatus 1 is not limited to the cyclotron 2 as an accelerator, and a synchrotron, a synchrocyclotron, a linac, or the like may be used.

  The charged particle beam R emitted from the cyclotron 2 travels toward the target 6 through the beam duct 3. A plurality of quadrupole electromagnets 4 and scanning electromagnets 5 are provided along the beam duct 3. The scanning electromagnet 5 scans the charged particle beam R and controls the irradiation position of the charged particle beam R with respect to the target 6.

  The neutron capture therapy apparatus 1 includes a control unit (calculation means) S. The control unit S is an electronic control unit having a CPU, a ROM, a RAM, and the like, and comprehensively controls the neutron capture therapy apparatus 1.

  The control unit S is connected to a current monitor M that measures in real time the current value of the charged particle beam R irradiated to the target 6 (that is, charge and irradiation dose rate), and neutron capture therapy according to the measurement result. Control of each part of the apparatus 1 is performed. As the current monitor M, for example, a non-destructive DCCT (Direct Current Current Transformer) that can be measured without affecting the charged particle beam R can be used.

  The target 6 receives a charged particle beam R and generates a neutron beam N. The target 6 has a disk shape with a diameter of 160 mm, for example. The target 6 may be formed of, for example, beryllium (Be), lithium (Li), tantalum (Ta), or tungsten (W). The target 6 is not limited to a plate shape (solid) and may be liquid.

  The shield 7 shields the generated neutron beam N and gamma rays generated by the generation of the neutron beam N from being emitted to the outside of the neutron capture therapy apparatus 1. The moderator 8 has a function of decelerating the neutron beam N (attenuating the energy of the neutron beam N). The moderator 8 is configured by laminating first and second moderators 8A and 8B. The first moderator 8A mainly decelerates fast neutrons contained in the neutron beam N. The second moderator 8B mainly decelerates epithermal neutrons contained in the neutron beam N.

  The collimator 9 forms an irradiation field of the neutron beam N (irradiation range in a plane orthogonal to the traveling direction of the neutron beam N), and has an opening 9a through which the neutron beam N passes. The neutron beam N generated by the target 6 passes through the moderator 8 and then partially passes through the opening 9 a of the collimator 9, while the remaining part is shielded by the peripheral part that defines the opening 9 a of the collimator 9. As a result, the neutron beam N that has passed through the collimator 9 is formed into a shape corresponding to the shape of the opening 9a.

  The neutron dose measuring device 10 is a device that measures the dose and dose distribution of the neutron beam N irradiated to the patient 50 on the treatment table 51.

  Next, the configuration of the negative ion source device 100 will be described with reference to FIG. The negative ion source device 100 includes a negative ion source 102 and a vacuum box 104. The negative ion source 102 and the vacuum box 104 are connected by an insulating flange 106.

  The negative ion source 102 includes a chamber 108, a magnet 110, a plasma generation unit 112, a cesium introduction unit 114, and a plasma electrode 116.

  The chamber 108 is connected to a vacuum pump (not shown) and can hold the inside in a vacuum state. The chamber 108 includes a main body 108a having a cylindrical shape, and a lid 108b provided on one end side of the main body 108a. The main body 108 a forms the side wall of the chamber 108. At both ends of the main body portion 108a, flange portions 108c and 108d that protrude outward are provided.

  The lid part 108b is detachably attached to a collar part 108c located on one end side of the main body part 108a, and opens or closes one end (open end) of the main body part 108a. A through hole 108e having a circular shape is formed in a substantially central portion of the lid portion 108b. The through hole 108e communicates the inside and outside of the chamber 108 when the lid portion 108b closes one end of the main body portion 108a.

  The magnet 110 has a function of confining the plasma generated in the chamber 108 in the chamber 108. A plurality of magnets 110 are arranged on the outer peripheral surface side of the main body portion 108a. More specifically, the magnet 110 is positioned on the outer peripheral side of the chamber 108 in a state of being separated from the outer peripheral surface of the main body portion 108a. A cooling path (not shown) is provided between the magnet 110 and the outer peripheral surface of the main body 108a. In order to cool the magnet 110 or the wall portion of the main body 108a, a coolant such as water is circulated in the cooling path.

  The plasma generator 112 includes a main body 112a having a cylindrical shape, and a pair of filaments 112b extending outward from the end surface of the main body 112a (the other end of the chamber 108 and the plasma electrode 116). The main body 112a has an outer shape corresponding to the through hole 108e of the lid 108b, and is detachable from the lid 108b (through hole 108e). When the main body portion 112a is attached to the lid portion 108b (through hole 108e), airtightness in the chamber 108 is maintained by interposing an O-ring or the like between the main body portion 112a and the through hole 108e. A DC power supply (not shown) is connected to the main body 112a. The DC power supply applies a voltage and a current to the filament 112b to generate heat, and causes a potential difference between the filament 112b and the chamber 108 (main body portion 108a).

  The cesium introduction part 114 is provided in the lid part 108b so as to penetrate the lid part 108b. The tip of the cesium introduction part 114 is located in the chamber 108. A cesium supply source 118 is connected to the cesium introduction unit 114, and in this embodiment, cesium is supplied to the chamber 108 in a gas (vapor) state.

  The plasma electrode 116 is disposed between the insulating flange 120 provided on the flange portion 108d located on the other end side of the main body portion 108a and the insulating flange 106 on the vacuum box 104 side. The plasma electrode 116 is connected to a power source (not shown) having a variable voltage. By controlling the power supply to control the magnitude of the voltage applied to the plasma electrode 116, the plasma distribution in the chamber 108 is controlled, and the amount of negative ions extracted from the chamber 108 is controlled. The plasma electrode 116 has a through hole 116a through which negative ions generated in the chamber 108 can be drawn out of the chamber 108 (in this embodiment, the vacuum box 104 side).

  A pipe 116 b connected to the gas supply source 122 is provided in the vicinity of the plasma electrode 116. That is, the pipe 116 b is located on the other end side of the chamber 108. The gas supply source 122 includes a source gas source (hydrogen gas source) and an inert gas source (argon gas source). That is, the source gas and the inert gas from the gas supply source 122 are supplied into the chamber 108 from the other end side of the main body 108a through the pipe 116b.

  The vacuum box 104 is located on the downstream side of the chamber 108 from which the negative ion beam is extracted (the other end side of the chamber 108). The vacuum box 104 can hold the inside in a vacuum state, like the chamber 108. In the vacuum box 104, an electrode 124 such as an extraction electrode, a Faraday cup (not shown) for measuring the beam amount of the negative ion beam, a steering coil (not shown) for changing the trajectory of the negative ion beam, and the like are arranged. ing. A safety valve 126 is provided on the side wall of the vacuum box 104. The safety valve 126 is closed when the pressure in the chamber 108 (in the vacuum box 104) is lower than a predetermined threshold value, and the pressure in the chamber 108 (in the vacuum box 104) is higher than the predetermined threshold value. To open. The predetermined threshold is set higher than the pressure outside the chamber 108.

  In the negative ion source device 100 described above, when generating negative ions, the chamber 108 and the vacuum box 104 are first evacuated by a vacuum pump. Next, a source gas (hydrogen gas) is supplied into the chamber 108 from the gas supply source 122, and cesium gas is supplied into the chamber 108 from the cesium supply source 118. The amount of cesium supplied by the cesium supply source 118 may be adjusted according to the amount of the negative ion beam to be extracted. Since the work function is reduced on the surface of the substance to which cesium has adhered, cesium has a function of promoting the generation of negative ions. FIG. 2 shows a cesium deposition layer C adhering to the inner wall surface of the chamber 108 and the surface of the plasma electrode 116.

  Next, a current is passed through the plasma generation unit 112, and a voltage is applied between the plasma generation unit 112 and the chamber 108. When a voltage is applied between the filament 112b heated by the current flow and the chamber 108, thermoelectrons are emitted from the filament 112b to the chamber 108, and arc discharge occurs. The thermoelectrons collide with the hydrogen gas filled in the chamber 108 and eject electrons to make the hydrogen gas into plasma.

  Among electrons existing in the plasma, fast electrons and slow electrons are discriminated by a magnet. Negative ions are generated by the reaction of slow electrons or electrons on the surface of the plasma electrode 116 with hydrogen molecules, hydrogen atoms, or hydrogen ions in the plasma. The negative ions thus generated are drawn out of the chamber 108 through the opening of the plasma electrode 116 and introduced into the cyclotron 2 via the vacuum box 104.

  By the way, since the filament 112b is consumed (deteriorated) with the operation of the negative ion source device 100, it is necessary to replace the plasma generation unit 112 with the passage of a predetermined period. Therefore, the plasma generation unit 112 in the negative ion source device 100 will be described.

  First, the supply of hydrogen gas from the gas supply source 122 and the supply of cesium gas from the cesium supply source 118 are stopped. Next, an inert gas is supplied into the chamber 108 by the gas supply source 122. At this time, due to the presence of the safety valve 126, the pressure of the inert gas in the chamber 108 becomes higher than the pressure outside the chamber 108.

  Next, a current is passed through the filament 112b to cause the filament 112b to generate heat. Thereby, cesium adhering to the surface of the filament 112b evaporates, and cesium is removed from the surface of the filament 112b. Next, the plasma generator 112 is removed from the lid 108b while continuing the supply of the inert gas into the chamber 108 by the gas supply source 122. At this time, since the pressure inside the chamber 108 is higher than the pressure outside the chamber 108, the atmosphere outside the chamber 108 is prevented from flowing into the chamber 108 through the through hole 108e of the lid portion 108b. Next, a new plasma generator 112 prepared in advance is attached to the lid 108b. Thus, the exchange of the plasma generation unit 112 is completed.

  After replacement of the plasma generation unit 112, the chamber 108 and the vacuum box 104 are evacuated again, and hydrogen gas and cesium gas are supplied into the chamber 108 by the gas supply source 122 and the cesium supply source 118, respectively. Thus, the negative ion beam can be generated again. In this case, the cesium deposition layer C adhering to the inner wall surface of the chamber 108 is evaporated again to gas by the heat of plasma generated in the chamber 108, and the cesium can be reused. Therefore, the supply amount of cesium gas supplied from the cesium supply source 118 can be small.

  In the present embodiment as described above, the pressure of the inert gas supplied from the gas supply source 122 into the chamber 108 is adjusted by the safety valve 126 so as to be higher than the pressure outside the chamber 108. Therefore, even when the plasma generation unit 112 is removed from the through hole 108e for replacement of the plasma generation unit 112, the inert gas is discharged from the inside of the chamber 108 to the outside through the through hole 108e. The Therefore, there is almost no possibility that the atmosphere flows into the chamber 108 through the through hole 108e, and the reaction between the cesium existing in the chamber 108 and the atmosphere is suppressed. Accordingly, since work and time for removing cesium that has reacted with the atmosphere from the inside of the chamber 108 are not required, the plasma generation unit 112 can be replaced easily and quickly. In addition, since the reaction between the atmosphere and cesium is suppressed, the consumption of cesium can be greatly reduced.

  In this embodiment, the safety valve 126 closes when the pressure in the chamber 108 is less than a predetermined magnitude, and opens when the pressure in the chamber 108 is equal to or greater than a predetermined magnitude. In this case, when the inert gas is supplied from the gas supply source 122 into the chamber 108, the pressure in the chamber 108 can be easily adjusted.

  In the present embodiment, the safety valve 126 is provided in the vacuum box 104. Since the vacuum box 104 generally has a larger installation space than the chamber 108, it is possible to increase the degree of design freedom when installing the safety valve 126.

  In the present embodiment, the plasma generator 112 is detachably attached to the through hole 108e of the lid 108b. For this reason, when the plasma generating unit 112 is removed from the lid part 108b, the area in which the chamber 108 is opened is reduced, so that the possibility that the atmosphere flows into the chamber 108 through the through hole 108e is further reduced.

  In the present embodiment, the lid portion 108b in which the through hole 108e is formed is located on one end side of the chamber 108, while the pipe 116b connected to the gas supply source 122 is located on the other end side of the chamber 108. Yes. Therefore, an inert gas is supplied into the chamber 108 from the opposite side of the chamber 108 from the through hole 108e. Therefore, in the chamber 108, the inert gas flows in almost one direction toward the through hole 108e, so that the possibility that the atmosphere flows into the chamber 108 through the through hole 108e is further reduced.

  As mentioned above, although embodiment of this invention was described in detail, this invention is not limited to above-described embodiment. For example, a substance other than cesium may be used as long as it promotes the production of negative ions and has a reactivity with the atmosphere. The state of the promoting substance supplied into the chamber 108 may be any of solid, liquid, and gas.

  The inert gas supplied into the chamber 108 by the gas supply source 122 may be other than argon.

  The safety valve 126 may be provided in the chamber 108 (the main body portion 108a and the lid portion 108b). In this case, since the safety valve 126 operates according to the pressure in the chamber 108, the pressure in the chamber 108 can be adjusted more accurately by the safety valve 126.

  In addition to the safety valve 126, means capable of adjusting the pressure inside the chamber 108 to be higher than the pressure outside the chamber 108 may be used. The negative ion source device 100 may not include the safety valve 126 as long as the pressure in the chamber 108 is higher than the pressure outside the chamber 108 by adjusting the flow rate of the inert gas by the gas supply source 122.

  The plasma generation unit 112 may be an internal antenna type instead of the filament 112b. In this case, plasma is generated in the chamber 108 by applying a high frequency to the antenna.

  The pipe 116b connected to the gas supply source 122 is not located on the other end side of the chamber 108, but may be provided at another location. For example, the pipe 116b may be provided on one end side of the chamber 108, or may be provided between one end and the other end of the chamber 108.

  The filament 112b does not necessarily have to be heated before the plasma generation unit 112 is replaced.

  DESCRIPTION OF SYMBOLS 1 ... Neutron capture therapy apparatus, 100 ... Negative ion source apparatus, 108 ... Chamber, 108a ... Main body part, 108b ... Lid part, 108e ... Through-hole, 110 ... Magnet, 112 ... Plasma generation part, 112b ... Filament, 116 ... Plasma Electrode 118 ... Cesium supply source 122 ... Gas supply source 126 ... Safety valve

Claims (12)

A first chamber provided with a through-hole communicating between the inside and the outside;
A source gas supply unit for supplying source gas into the first chamber;
A plasma generating unit that is attached to the through hole and generates plasma using the source gas supplied by the source gas supplying unit;
An accelerating substance supply unit for accelerating the production of negative ions and supplying an accelerating substance having reactivity with the atmosphere into the first chamber;
An inert gas supply unit configured to supply an inert gas into the first chamber by control from the control unit after the operation of the plasma generation unit is stopped when the plasma generation unit is detached from the first chamber;
A heating unit that heats the plasma generation unit to be removed under the control of the control unit during the supply of the inert gas into the first chamber by the inert gas supply unit;
During the supply of the inert gas into the first chamber by the inert gas supply unit, the pressure of the inert gas supplied from the inert gas supply unit into the first chamber is the first gas. A negative ion source device comprising: a pressure adjusting unit that adjusts the pressure to be higher than the pressure outside the chamber.   The pressure adjusting unit is a safety valve that closes when the pressure in the first chamber is less than a predetermined magnitude and opens when the pressure in the first chamber is greater than or equal to a predetermined magnitude. The negative ion source device according to claim 1. A second chamber connected to the first chamber on the downstream side from which negative ions generated in the first chamber are extracted;
The negative ion source device according to claim 2, wherein the safety valve is provided in the second chamber.   The negative ion source device according to claim 2, wherein the safety valve is provided in the first chamber. The first chamber has a cylindrical main body portion, a lid portion that closes an open end side of the main body portion and is provided with the through hole,
The negative ion source device according to any one of claims 1 to 4, wherein the plasma generation unit is detachably attached to the lid unit.   The negative gas supply according to any one of claims 1 to 5, wherein the inert gas supply unit supplies an inert gas into the first chamber from a side opposite to the through hole in the first chamber. Ion source device. A method of exchanging Help plasma generator put in the negative ion source device,
The negative ion source device includes:
A first chamber provided with a through-hole communicating between the inside and the outside;
A source gas supply unit for supplying source gas into the first chamber;
The plasma generator that is attached to the through hole and generates plasma using the source gas supplied by the source gas supply unit;
An accelerating substance supply unit for accelerating the production of negative ions and supplying an accelerating substance having reactivity with the atmosphere into the first chamber;
An inert gas supply unit configured to supply an inert gas into the first chamber when the plasma generation unit is removed from the first chamber;
A heating unit that heats the plasma generation unit to be removed during the supply of the inert gas into the first chamber by the inert gas supply unit;
A pressure adjusting unit that adjusts the pressure of the inert gas supplied from the inert gas supply unit into the first chamber to be higher than the pressure outside the first chamber during the removal operation; With
A first step of stopping the operation of the source gas supply unit and the plasma generation unit to be removed;
A second step of supplying an inert gas into the first chamber by the inert gas supply section after the first step;
After the second step, a third step of heating the plasma generation unit to be removed;
After the third step, the plasma generation unit to be removed attached to the through-hole is removed while continuing the supply of the inert gas into the first chamber by the inert gas supply unit. And a fourth step of attaching a new plasma generation unit to the through hole.   The pressure adjusting unit is a safety valve that closes when the pressure in the first chamber is less than a predetermined magnitude and opens when the pressure in the first chamber is greater than or equal to a predetermined magnitude. 8. The method of claim 7, wherein:   The negative ion source device further includes a second chamber connected to the first chamber on a downstream side from which negative ions generated in the first chamber are extracted.
  The method of claim 8, wherein the safety valve is provided in the second chamber.   The method of claim 8, wherein the safety valve is provided in the first chamber.   The first chamber has a cylindrical main body portion, a lid portion that closes an open end side of the main body portion and is provided with the through hole,
  The method according to claim 7, wherein the plasma generator is detachably attached to the lid.   The method according to any one of claims 7 to 11, wherein the inert gas supply unit supplies an inert gas into the first chamber from a side opposite to the through hole in the first chamber. .

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