JP2015018609A - Microwave ion source and ion extraction part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance plasma density in the plasma chamber of a microwave ion source.SOLUTION: A microwave ion source 10 includes a plasma chamber 12 including a microwave introduction part 16 and an ion extraction part 18, and a magnetic field generator 40 for generating a magnetic field M in the plasma chamber 12 toward the direction of travel P of microwaves. The ion extraction part 18 includes an ion beam extraction surface 34 crossing the magnetic field M, a coating part 52 forming at least a part of the ion beam extraction surface 34, and a magnetic material 50 provided on the back of the coating part 52. The coating part 52 is formed of a secondary electron discharge material.

Description

  The present invention relates to a microwave ion source and an ion extraction unit suitable for the microwave ion source.

  Ion sources that use microwaves for plasma generation are known. Microwave is introduced into the vacuum plasma chamber. The source gas supplied to the plasma chamber is excited by microwaves to generate plasma. Ions are extracted from the plasma. The ions thus extracted from the ion source are used, for example, for an ion implantation process. Also known is a method for efficiently generating plasma by applying a magnetic field for causing electron cyclotron resonance to the plasma chamber.

JP-A-6-36696 JP 2005-251743 A

  When a magnetic field is applied, charged particles in the generated plasma have a helical motion along the magnetic field. Some particles can reach the wall of the plasma chamber and be lost from the plasma. In order to draw more ions out and use them, it is desirable to suppress such plasma loss.

  One exemplary objective of certain aspects of the present invention is to improve the plasma density in the plasma chamber of a microwave ion source.

  A microwave ion source according to an aspect of the present invention includes a plasma chamber including a microwave introduction unit and an ion extraction unit, and a magnetic field generator that generates a magnetic field directed in the traveling direction of the microwave in the plasma chamber. Prepare. The ion extraction portion includes an inner wall surface that intersects the magnetic field, a secondary electron emission material layer that forms at least a part of the inner wall surface, and a magnetic body provided behind the secondary electron emission material layer, .

  According to this aspect, since the magnetic field is collected in the magnetic material, charged particles in the plasma moving along the magnetic field can be guided to the secondary electron emission material layer in front of the magnetic material. When charged particles hit the secondary electron emission material layer, secondary electrons are emitted into the plasma chamber. Since the secondary electron emission material layer is provided in the ion extraction portion, the electron density in the ion extraction portion is increased, and thereby positive ions flow toward the ion extraction portion so as to approach an electrical equilibrium state. Therefore, it is possible to improve the plasma density of the plasma chamber, particularly the ion extraction portion. Therefore, the amount of ions extracted from the ion source can be increased.

  The inner wall surface may be provided with an ion extraction opening, and the secondary electron emission material layer may be formed adjacent to the ion extraction opening.

  In this way, secondary electrons can be supplied in the vicinity of the ion extraction opening, which is particularly effective in improving the plasma density and increasing the amount of ions extracted.

  The magnetic body may be covered with the secondary electron emission material layer and stored in the ion extraction portion.

  In this way, the magnetic material can be protected by the secondary electron emission material layer. It is possible to prevent the magnetic material from being damaged by the plasma and to prevent the magnetic material components from being mixed into the plasma.

  Another aspect of the present invention is an ion extraction unit for extracting ions from a plasma chamber. A magnetic field is applied to the plasma chamber. The ion extraction portion includes an inner wall surface that intersects the magnetic field, a secondary electron emission material layer that forms at least a part of the inner wall surface, and a magnetic body provided behind the secondary electron emission material layer, .

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to the present invention, the plasma density in the plasma chamber of the microwave ion source can be improved.

1 is a cross-sectional view schematically illustrating a microwave ion source according to an embodiment of the present invention. It is an exploded view of the ion extraction part shown in FIG. It is a figure which illustrates the magnetic field which arises in a plasma chamber by the magnetic field generator in the microwave ion source which concerns on one embodiment of this invention. It is a figure which illustrates the magnetic field which arises in a plasma chamber by the magnetic field generator in the microwave ion source which concerns on a comparative example.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description, the same elements are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate.

  FIG. 1 is a cross-sectional view schematically illustrating a microwave ion source 10 according to an embodiment of the present invention. A microwave ion source 10 generates high-density plasma by inputting microwave power in the direction of magnetic field into a plasma chamber 12 to which a magnetic field satisfying electron cyclotron resonance (ECR) or a magnetic field higher than the magnetic field is applied to generate ions with high density. Ion source to be extracted. The microwave ion source 10 is configured to generate a plasma of a source gas by the interaction between a magnetic field and a microwave, and to extract ions from the plasma to the outside of the plasma chamber 12.

  As is well known, the strength of the magnetic field that satisfies the ECR condition is uniquely determined with respect to the microwave frequency used, and a magnetic field of 87.5 mT (875 gauss) is required when the microwave frequency is 2.45 GHz. It is. Hereinafter, for convenience of explanation, a magnetic field that satisfies the ECR condition may be referred to as a resonance magnetic field.

  The microwave ion source 10 is used, for example, as an ion source for an ion implantation apparatus or a particle beam therapy apparatus. The microwave ion source 10 is used as a monovalent ion source, for example. The microwave ion source 10 can also be used as an ion source for a proton accelerator or an X-ray source.

  The plasma chamber 12 is a vacuum chamber configured to generate and maintain plasma in its internal space. Hereinafter, the internal space of the plasma chamber 12 may be referred to as a plasma generation space 14.

  The plasma chamber 12 has a cylindrical shape having both ends. Hereinafter, the direction from one end to the other end of the plasma chamber 12 may be referred to as an axial direction for convenience. In addition, a direction orthogonal to the axial direction may be referred to as a radial direction, and a direction surrounding the axial direction may be referred to as a circumferential direction. However, these do not necessarily mean that the plasma chamber 12 has a rotationally symmetric shape. The axial length of the plasma chamber 12 may be longer or shorter than the radial length of the end portion of the plasma chamber 12.

  The plasma chamber 12 includes a microwave introduction unit 16 and an ion extraction unit 18. The microwave introduction part 16 and the ion extraction part 18 are opposed to each other with the plasma generation space 14 in between.

  The plasma chamber 12 includes a side wall 20 that connects the microwave introduction unit 16 and the ion extraction unit 18 and surrounds the plasma generation space 14. Side wall portions 20 are fixed to the outer peripheral portions of the microwave introduction portion 16 and the ion extraction portion 18. The side wall 20 is made of a nonmagnetic metal material such as stainless steel or aluminum.

  Note that the side wall 20 may include a cooling unit for cooling the side wall 20. This cooling part may be built in the side wall part 20 or may be attached outside the side wall part 20. Moreover, in order to protect the side wall part 20 of the plasma chamber 12 from a plasma, the liner (for example, boron nitride) which coat | covers the inner surface of the side wall part 20 may be provided.

  A plasma generation space 14 is defined in the plasma chamber 12 by the microwave introduction part 16, the ion extraction part 18, and the side wall part 20. Note that the plasma chamber 12 is configured such that the microwave introduction part 16, the ion extraction part 18, and the side wall part 20 integrally surround the plasma generation space 14, and thus the side wall part 20 connected to the ion extraction part 18. The end can also be regarded as part of the ion extraction portion 18. Similarly, the end of the side wall portion 20 connected to the microwave introduction portion 16 can be regarded as a part of the microwave introduction portion 16.

  The plasma chamber 12 has, for example, a cylindrical shape. In this case, the microwave introduction part 16 and the ion extraction part 18 are substantially disk-shaped, and the side wall part 20 is substantially cylindrical. The plasma chamber 12 may have any shape as long as plasma can be appropriately accommodated.

  The microwave introduction unit 16 includes a vacuum window 22 and a vacuum window support unit 24 that supports the vacuum window 22. The vacuum window 22 seals the inside of the plasma chamber 12 to a vacuum. One side of the vacuum window 22 faces the plasma generation space 14, and the other side of the vacuum window 22 is directed to the microwave supply system or the waveguide 26. The propagation direction P of the microwave is perpendicular to the vacuum window 22. In the present embodiment, the vacuum window 22 occupies the entire microwave introducing portion 16, but the vacuum window 22 may be formed in a part (for example, the central portion) of the microwave introducing portion 16.

  The vacuum window 22 is made of a dielectric such as alumina or boron nitride. In this embodiment, the vacuum window 22 has a two-layer structure, for example, a boron nitride window inner layer 28 faces the plasma generation space 14, and an alumina window outer layer 30 is adjacent to the window inner layer 28. The window inner layer 28 covers the window outer layer 30 in order to protect the window outer layer 30 from electrons flowing back to the plasma chamber 12 from the outside of the plasma chamber 12 through the ion extraction opening 32. The vacuum window support 24 is made of a nonmagnetic metal material such as stainless steel or aluminum.

  On the other hand, at least one ion extraction opening 32 is formed in the ion extraction portion 18. The ion extraction opening 32 is, for example, a slit elongated in a direction perpendicular to the paper surface. The ion extraction opening 32 is formed in the central portion of the ion extraction portion 18. The ion extraction opening 32 is formed at a position facing the vacuum window 22 across the plasma generation space 14. The vacuum window 22, the plasma generation space 14, and the ion extraction opening 32 are arranged along the central axis of the plasma chamber 12.

  As will be described in detail later, the ion extraction portion 18 includes an outer portion 48, a magnetic body 50, and a covering portion 52 that is a secondary electron emission material layer. In this document, a portion of the inner wall surface of the ion extraction portion 18 that faces the microwave introduction portion 16 may be referred to as an ion beam extraction surface 34.

  An extraction electrode system including at least one extraction electrode 36 for extracting ions to the outside of the plasma chamber 12 is provided outside the ion extraction opening 32 in the axial direction. The extraction electrode 36 is opposed to the ion extraction portion 18 with an extraction gap 38 in the axial direction. The extraction electrodes 36 are each formed, for example, in an annular shape, and have an opening at the center for allowing ions extracted from the plasma chamber 12 to pass through.

  As shown in FIG. 1, the microwave ion source 10 includes a magnetic field generator 40 for generating a magnetic field in the plasma chamber 12. The magnetic field generator 40 is disposed so as to surround the side wall portion 20 of the plasma chamber 12 in order to generate a magnetic field directed in the axial direction on the central axis of the plasma chamber 12. The direction of the line of magnetic force is indicated by an arrow M in FIG. The magnetic force line direction M by the magnetic field generator 40 is the same direction as the microwave propagation direction P. In addition, the magnetic field M has a resonance magnetic field or higher intensity at least at a part on the central axis of the plasma chamber 12. However, the magnetic field generator 40 can generate a magnetic field lower than the resonance magnetic field in at least a part on the axis of the plasma chamber 12.

  The magnetic field generator 40 includes a donut-shaped coil 42 surrounding the plasma chamber 12 and a yoke 44 attached to the coil 42. The coil 42 is formed in an annular shape, and a conducting wire is wound in the circumferential direction of the plasma chamber 12. The yoke 44 is provided adjacent to the outer periphery of the coil 42 and both ends in the axial direction. Further, the magnetic field generator 40 includes a coil power supply (not shown) for causing a current to flow through the coil 42.

  The magnetic field generator 40 may include a plurality of coils arranged along the axial direction of the plasma chamber 12 instead of including one coil 42 as illustrated. Further, the magnetic field generator 40 may include a permanent magnet together with the one or more coils or instead of the one or more coils.

  The magnetic field generator 40 may include a magnet support (not shown) for supporting the magnetic field generator 40 with respect to the plasma chamber 12. In this case, the magnetic field generator 40 is disposed so as to surround the plasma chamber 12 by the magnet support portion. In the present embodiment, the magnetic field generator 40 is supported so that a radial gap 46 is formed between the plasma chamber 12 and the magnetic field generator 40. When the magnetic field generator 40 and the plasma chamber 12 are thus separated, different potentials may be applied to the magnetic field generator 40 and the plasma chamber 12. For this purpose, the microwave ion source 10 may include a high voltage power source (not shown) for applying a potential to each of the plasma chamber 12 and the magnetic field generator 40. The magnetic field generator 40 may be directly attached to the outer surface of the plasma chamber 12.

  FIG. 2 is an exploded view of the ion extraction unit 18 shown in FIG. As shown in FIGS. 1 and 2, the ion extraction portion 18 includes an outer portion 48, a magnetic body 50, and a covering portion 52. FIG. 3 is a diagram illustrating a magnetic field generated in the plasma chamber 12 by the magnetic field generator 40 in the microwave ion source 10 according to this embodiment.

  The outer portion 48 defines the outer shape of the ion extraction portion 18. The outer portion 48 is formed with an ion extraction opening 32 at the center thereof, and the outer portion 48 defines the shape of the ion extraction opening 32. The outer portion 48 is made of a nonmagnetic metal material such as stainless steel or aluminum. The outer portion 48 may be a refractory metal such as tungsten or a refractory metalloid conductor such as graphite. When a high voltage is applied to the plasma chamber 12, the same high voltage is also applied to the outer portion 48, which is a part of the plasma chamber 12. Therefore, the outer portion 48 may be called a plasma electrode.

  The magnetic body 50 is between the outer portion 48 and the covering portion 52 in the axial direction, and is sandwiched between the outer portion 48 and the covering portion 52. The magnetic body 50 is arranged in an axial symmetry so as to surround the ion extraction opening 32. The magnetic body 50 is a soft magnetic body such as an iron material. The magnetic body 50 is located between the ion extraction opening 32 and the magnetic field generator 40 in the radial direction. In this way, a magnetic circuit is configured to feed back the axial magnetic field generated in the plasma chamber 12 from the ion extraction opening 32 to the magnetic field generator 40 via the magnetic body 50. Therefore, in this magnetic circuit, the magnetic body 50 is provided behind the covering portion 52.

  A magnetic field passing through the magnetic body 50 is illustrated in FIG. For comparison, FIG. 4 illustrates a magnetic field when the magnetic body 50 is not provided and the ion extraction unit 18 is a nonmagnetic material. The comparison of these magnetic fields will be described later.

  The covering portion 52 forms the above-described ion beam extraction surface 34. The covering portion 52 is also a protective layer that covers the surface of the magnetic body 50 in order to protect the magnetic body 50 from plasma.

  In the present embodiment, the covering portion 52 is made of a secondary electron emission material, for example, a material having a high secondary electron emission coefficient such as alumina, magnesium oxide, or boron nitride. Here, a material having a high secondary electron emission coefficient means at least one electron when ions are incident on the material at an energy level (for example, several eV to several tens eV, or about 10 eV) required for ions in this application. A material that releases Thus, the covering portion 52 is formed of a material having a higher secondary electron emission capability than the material forming the inner surface of the side wall portion 20 of the plasma chamber 12 or the inner surface of the microwave introduction portion 16.

  The secondary electron emission material may be an alkaline earth metal oxide containing at least two alkaline earth metal elements. Such alkaline earth metal oxides can have a high secondary electron emission capability. Therefore, it is desirable that the alkaline earth metal element is an element having a large number of electrons. Therefore, the first element of at least two alkaline earth metal elements may be strontium or barium. In addition, the second element of the at least two alkaline earth metal elements may be selected from the group consisting of magnesium, calcium, strontium, and barium, and may be an element different from the first element. Alternatively, the at least two alkaline earth metal elements may include strontium and an element selected from the group consisting of magnesium, calcium, and barium. The alkaline earth metal oxide may be strontium calcium oxide or strontium magnesium oxide.

  As shown in FIG. 2, the outer portion 48 is formed with a concave portion 54 for accommodating the magnetic body 50 and the covering portion 52. The concave portion 54 is dug down from the plasma generation space 14 side so as to leave the outer peripheral portion of the outer portion 48 and the periphery of the ion extraction opening 32.

  The magnetic body 50 and the covering portion 52 are sized so as to be fitted into the recess 54 of the outer portion 48. Therefore, each of the magnetic body 50 and the covering portion 52 is a plate having an outer shape corresponding to the outer peripheral portion of the outer portion 48 and having an opening corresponding to the ion extraction opening 32 in the central portion. One of the surfaces of the magnetic body 50 is in contact with the bottom surface 56 of the recess 54, and the surface of the magnetic body 50 on the opposite side is in contact with the covering portion 52. The covering portion 52 covers the entire magnetic body 50, and the magnetic body 50 is not exposed to the plasma generation space 14. Thus, the covering portion 52 shields the magnetic body 50 from the plasma.

  The covering portion 52 forms the same plane as the outer peripheral portion of the outer portion 48 and the periphery of the ion extraction opening 32, and when the outer portion 48 is attached to the side wall portion 20, the outer peripheral portion of the covering portion 52 and the magnetic body 50 is 20 and the outer portion 48. The outer portion 48 is attached to the side wall portion 20 by appropriate attachment means (not shown) such as a tie rod.

  The operation of the microwave ion source 10 according to this embodiment will be described. Microwaves are introduced into the plasma chamber 12 through the vacuum window 22. The source gas is excited by the microwave, and plasma is generated in the plasma chamber 12. Ions are extracted out of the plasma chamber 12 through the ion extraction opening 32.

  By the way, as illustrated in FIG. 4, the microwave ion source generally generates and confines plasma by a magnetic field 62 directed in a generally axial direction generated by a solenoid coil installed outside the plasma chamber 60. This magnetic field 62 crosses the ion beam extraction surface 64 made of a nonmagnetic metal material. The charged particles in the plasma move along the magnetic field 62 (for example, so as to wrap around the magnetic field 62). Some charged particles hit the extraction surface 64 and heat the extraction surface 64. Since such charged particles are trapped on the extraction surface 64 and lost from the plasma, the plasma density in the plasma chamber 60 is unlikely to increase.

  In contrast, in the present embodiment, as described above, the ion beam extraction surface 34 is formed by the covering portion 52 that is a member having a high secondary electron emission coefficient. Therefore, when charged particles collide with the covering portion 52, secondary electrons are emitted into the plasma generation space 14. The electron density in a region of the plasma generation space 14 adjacent to the ion extraction unit 18 is increased, so that ions flow into the region so as to approach an electrical equilibrium state. Thus, the plasma density of the ion extraction unit 18 can be improved, and the amount of ions extracted from the microwave ion source 10 (that is, the amount of ion beam current that can be extracted) can be increased.

  In the present embodiment, the magnetic body 50 is provided behind the covering portion 52 that is a secondary electron emission source. Therefore, as illustrated in FIG. 3, the magnetic field M can be attracted to the magnetic body 50. For example, when the ion extraction part 18 is made of a nonmagnetic material, the magnetic field lines that have passed through the side wall part 20 to the outside of the plasma chamber 12 and the magnetic field lines that have passed through the outside of the plasma chamber 12 are extracted. Can be collected into the surface 34. Alternatively, the magnetic field lines that have passed through the ion extraction opening 32 can be directed to the ion beam extraction surface 34. Charged particles are guided to the covering portion 52 along the magnetic field lines, and a large amount of secondary electrons are emitted from the covering portion 52. A large amount of ions are attracted toward the ion extraction unit 18 so as to realize an electrical equilibrium state. Therefore, by providing the magnetic body 50, the plasma density enhancement effect at the ion extraction portion 18 can be further enhanced.

  According to the present embodiment, the covering portion 52 is formed so as to be adjacent to the ion extraction opening 32. Specifically, the covering portion 52 is formed in an axial symmetry so as to surround the ion extraction opening 32. Secondary electrons are directly supplied in the vicinity of the ion extraction opening 32, and the plasma density in the ion extraction opening 32 can be improved. Therefore, the arrangement of the covering portion 52 is particularly effective in increasing the amount of ion beam current that can be extracted from the microwave ion source 10.

  According to the present embodiment, the magnetic body 50 is protected from plasma by the covering portion 52. Therefore, the magnetic body 50 is protected from sputtering by the plasma, and the contamination of the plasma by the material of the magnetic body 50 and consequently the contamination of the ion beam can be prevented. It should be noted that the surface of the magnetic body 50 may be exposed to the plasma generation space 14 in applications where such contamination problems are not important.

  According to this embodiment, the magnetic body 50 is housed in the outer portion 48, and the size of the pull-out gap 78 does not change before and after the magnetic body 50 is mounted. Generally, the extraction gap 78 is optimally adjusted for ion extraction. Therefore, in this embodiment, there is also an advantage that it is not necessary to perform such adjustment again after the magnetic body 50 is mounted.

  According to this embodiment, as can be seen from FIG. 3, the magnetic body 50 allows the axial magnetic field M of the plasma chamber 12 to escape in the radial direction to reduce or prevent leakage of the axial magnetic field M to the extraction gap 38. it can. In this way, the magnetic body 50 also functions as a magnetic shield that shields the leakage magnetic field from the plasma chamber 12 to the extraction gap 38. According to the knowledge of the present inventor, when the axial leakage magnetic field in the extraction gap 38 is large, discharge is likely to occur between the outer portion 48 (plasma electrode) and the extraction electrode 36. This is thought to be because electrons that move around the leakage magnetic field in the extraction gap 38 are one of the main factors of the discharge between the two electrodes. Therefore, according to the present embodiment, by providing the magnetic body 50 as a magnetic shield, it is possible to suppress discharge in the extraction gap 38. This is useful for stably operating the microwave ion source 10.

  In the above, this invention was demonstrated based on the Example. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiment, and various design changes are possible, various modifications are possible, and such modifications are within the scope of the present invention. By the way.

  In the above-described embodiment, the outer portion 48, the magnetic body 50, and the covering portion 52 are separate members, and can be individually replaced as necessary. However, two members or all of the outer portion 48, the magnetic body 50, and the covering portion 52 that are adjacent to each other may be integrated by, for example, different material bonding. Since the microscopic gap between adjacent members can be eliminated, such integration is effective in protecting the magnetic body 50 from plasma (or preventing contamination of the plasma).

  In the above-described embodiment, the outer portion 48, the magnetic body 50, and the covering portion 52 are each a single member, but may be configured by a plurality of pieces. For example, the magnetic body 50 and / or the covering portion 52 may be divided into two pieces, that is, a central portion surrounding the ion extraction opening 32 and an outer peripheral portion thereof. If it does in this way, only the central part which is easy to receive wear can be exchanged as needed.

  The magnetic body 50 and / or the covering portion 52 may not be provided on the entire ion beam extraction surface 34 and may be provided partially in the radial direction or the circumferential direction. For example, the magnetic body 50 may be a plurality of annular magnetic bodies arranged concentrically at intervals in the radial direction, or a plurality of magnetic pieces arranged radially at intervals in the circumferential direction. But you can. Similarly, the covering portion 52 may be a plurality of secondary electron emission portions arranged concentrically with a space therebetween in the radial direction, or a plurality of secondary electron emission portions arranged radially with a space in the circumferential direction. A secondary electron emission part may be sufficient.

  Further, the covering portion 52 may form at least a part of the inner wall surface at the end of the side wall portion 20 connected to the ion extracting portion 18. The magnetic body 50 may be provided behind the covering portion 52. The magnetic body 50 may be disposed between the ion extraction unit 18 and the magnetic field generator 40 on the radially outer side of the ion extraction unit 18. Or the magnetic body 50 may be arrange | positioned at the axial direction outer side of the ion extraction part 18, and may be provided adjacent to the outer side of the outer side part 48, for example.

  In the above-described embodiment, plasma is generated in the plasma chamber 12 by microwave discharge. However, the plasma generation means of the ion source to which the present invention can be applied is not limited to the microwave. The present invention can be applied to any ion source that generates a magnetic field that intersects the inner wall surface of the ion extraction unit.

  10 microwave ion source, 12 plasma chamber, 14 plasma generation space, 16 microwave introduction part, 18 ion extraction part, 32 ion extraction opening, 34 ion beam extraction surface, 40 magnetic field generator, 48 outer part, 50 magnetic body, 52 Covering part.

Claims (4)

A plasma chamber having a microwave introduction section and an ion extraction section;
A magnetic field generator for generating a magnetic field directed in the traveling direction of the microwave in the plasma chamber,
The ion extraction portion includes an inner wall surface that intersects the magnetic field, a secondary electron emission material layer that forms at least a part of the inner wall surface, and a magnetic body provided behind the secondary electron emission material layer, A microwave ion source comprising:   2. The microwave ion source according to claim 1, wherein an ion extraction opening is provided in the inner wall surface, and the secondary electron emission material layer is formed adjacent to the ion extraction opening. .   3. The microwave ion source according to claim 1, wherein the magnetic body is covered with the secondary electron emission material layer and accommodated in the ion extraction portion. An ion extraction unit for extracting ions from the plasma chamber, wherein a magnetic field is applied to the plasma chamber;
An inner wall crossing the magnetic field;
A secondary electron emission material layer forming at least a part of the inner wall surface;
And a magnetic body provided behind the secondary electron emission material layer.

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