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The microtron in X-ray lithography
820
Nuclear Instruments and Methods in Physics Research B24/25 (1987) 820-825 North-tlolland, Amsterdam
T H E M I C R O T R O N I N X-RAY L I T H O G R A P H Y * C. M I L E I K O W S K Y Scanditronix AB, Uppsala, Sweden
The race-track microtron cart play two important roles in X-ray lithography: It can serve as a low energy injector (at 50-150 MeV) to compact synchrotron/storage rings which are either normal conducting or superconducting; or it can, in a superconducting version, serve as full energy injector (at e.g. 680 MeV) integrated with a surrounding superconducting storage ring, forming a compact synchrotron light source.
1. The need for synchrotron radiation in semiconductor lithograph)' In lithographic methods for production of semiconductor LSI chips and very dense memories by exposure with electromagnetic radiation through a mask, the wavelength, quality and intensity are of paramount importance. In today's mass production the mercury lamp is an excellent source. However, its wavelengths are too long to produce chips with detail dimensions as fine as 0.1-0.2 /.tm which will be required in the near future. For this purpose radiation in the X-ray region is needed. X-rays produced in the "ordinary" way - i.e. by an electron beam hitting a piece of matter - are emitted in all directions. At the distances where the radiation could be used for lithography, the intensities are insufficient for a competitive production rate for chips. Therefore manufacturers have to turn to some other X-ray source. One kind of radiation that can certainly do the job is synchrotron radiation of suitable wavelengths. Here all the required properties can be achieved including high intensity. It is well-known that an electron at high energy whose orbit is bent (in e.g. a magnetic field) emits photons in the tangential direction: synchrotron radiation. Energy and radius of curvature determine the wavelengths and number of photons according to a well-known formula. The number of photons is, of course, also proportional to the number of electrons. Thus, when electrons are circulating in the vacuum of a storage ring they emit synchrotron radiation from * The design of the systems is mainly by B. Anderberg, Scanditronlx, and his team, in cooperation with M. Eriksson, University of Lund. The superconducting magnet system is designed by C. Dustmann and his group at BBC, Mannheim to specifications supplied by Scanditronix.
all the parts of the orbit where it is curved. In storage rings dedicated to X-ray lithography the required synchrotron radiation is created where the orbiting electrons are passing through the fields of the bending magnets, in contrast to storage rings for research where synchrotron radiation also originates in special devices, such as wigglers, undulators etc. inserted in long straight sections of the ring. 2. Some possible 13"pes of compact synchrotron radiation sources dedicated to X-ray lithograph)' and the roles of the microtron The various possible schemes differ in many ways, for instance: the injection into the ring can be either at low energy or at full energy; the low energy injector can be a normal conducting race-track microtron or a linac; the full energy injector can be either a normal conducting synchrotron or a superconducting race-track microtron; and the synchrotron a n d / o r storage ring can be either normal conducting or superconducting. (Here normal conducting or superconducting refers to the main bending magnet coils.) If we assume that all the technically reasonable schemes can be designed to have similar reliability in operation, it turns out that some of them are clearly more costly than the others as to capital cost or operation cost or both. Those will (in the long run) not be competitive in production. Among the technically feasible schemes there are three having an intrinsic potential for lower cost, relatively speaking. Two of them have low energy injection which contributes to their low cost. The third scheme uses full energy injection but this is compensated for costwise as a consequence of other properties of the concept. The three schemes are" (i) low energy injection with a race-track microtron (normal conducting, 50-150 MeV) into a normal conducting compact synchrotron/
C. 3fileikowsky / The microtron in X-ray fithography
storage ring; (it) low energy injection with a race-track microtron (normal conducting, 50-150 MeV) into a superconducting synchrotron/storage ring; and (iii) full energy injection with a superconducting race-track microtron (650-7B24/25 MeV) into a surrounding superconducting storage ring. The same bending magnets are used for both injector and ring integrated. None of these schemes has yet been built in a complete way and tested at commercially interesting wafer production rates, but there are very good reasons to believe that all three are feasible for the goal. Each one of them has its advantages and disadvantages. In this article it is not the intention to try to make an evaluation of the relative merits of the three. Two possible roles for the race-track microtron are obvious from the above schemes: as normal conducting low energy injector and as superconducting full energy injector.
3. The normal conducting race-track microtron as low energy injector to compact synchrotron-storage rings, which are either normal conducting or superconducting The race-track microtron is excellent for this task because of its high electron beam quality, its ease of operation and its compactness. The beam quality is determined by the well defined energy, the low energy spread and the small emittance [1]. As a reminder, the race-track microtron operates according to the following principle. Electrons from the electron gun are injected into the linac, accelerated by it, and returned to the linac by one of the two main bending magnets and two small displacing magnets. After the second linac transit the beam passes into the second main bending magnet where it makes a 180 ° turn, at a larger radius than before, and goes back to the first bending magnet, where it is again bent 180 ° and sent into the linac. There it is accelerated
821
a third time etc. The distance between the linac and the bending magnets is chosen such that the beam always comes back to the linac at the right phase for repeated acceleration. So, if the energy gain in the linac is, for example, 5 MeV per transit and there are 20 passages, i.e. 20 orbits, the final energy is 100 MeV. The beam can be deflected and extracted after the last linac transit, but, if so desired, also from orbits with lower energy. The beam is focused by proper end magnet field forms and a number of quadrupole magnets. The main parameters for three race-track microtrons for different maximum injection energies are given in table 1.
4. The superconducting microtron as full energy injector integrated with the storage ring for X-ray lithography Full energy injection is commonly regarded as a technically more straightforward, safer and superior method of injection compared to low energy injection. Experience available in the world seems to support this view. In many schemes, however, the higher cost of full energy injection into a ring makes such injection uncompetitive" for production purposes. A compact synchrotron light source which has the potential for the desired quality and intensity of the synchrotron light, reliability and reasonable cost is the integrated race-track microtron storage ring (figs. 2 and 3). This system is a race-track microtron using superconducting bending magnets and a race-track shaped storage orbit outside the race-track microtron. The bending magnets are extending slightly outside the radius needs for the microtron and are used also for the storage orbit. Thus the full energyinjector occupies the area inside the storage orbit. A characteristic property of microtrons is that the magnetic fields in the bending magnets - as well as in the contramagnets and adjusting magnets - are kept
Table 1 Main parameters for three race-track microtrons a) Maximum electron energy (MeV) Beam pulse current (rnA) Pulse length (,us) Pulse repetition frequency (Hz) Ernittance at max. energy ( > 85% of current) Vertical (mm mrad) Horizontal (mm mrad) Energy spread at max. energy Weight of accelerator (t) Size of accelerator (height x width × depth (m)) Size of cabinet for klystron and klystron modulator a)
50 25 0.05-5 1-250
100 15 0.05-5 1-250
150 10 0.05-5 1-250
0.2rr 0.2~4-2 × 10 - 3 3 1.6 x 2.5 × 0.8 2.0 × 2.4 X 1.3
0.1~r 0.1rr 4-1 x 10"3 6 1.7 x 2.9 x 1.1 2.0 X 3.0 X 1.3
0.07~" 0.07~r + 0.7 x 10 - 3 16 1.8 × 3.3 x 1.4 2.0 X 3.6 × 1.3
~) Remaining auxiliary equipment needs the following space (height × width × depth): gun modulator: 2.0 × 1-2× 1.3 m3; control system interface: 2.0×0.8x 1.3 m3; control console: 1.2xl.6x 1.I m3; and 4 electronic cabinets each 2.0x0.6x0.8 m3. VII. ACCELERATOR TECHNOLOGY
822
C. Mileikowskl' / The microtron in X-ray lithography
I •
.?-
Fig. 1. Race-track microtron, 50 MeV. constant in time. There is no ramping like in synchrotrons of the field. This makes the machine particularly suitable for superconducting technology. There is a long experience of superconducting magnets with fields constant in time. The design problems of eddy currents in adjacent parts at ramping and the losses due to them are not present. 4.1. Main parameters and design features The maximum bending radius in the race-track microtron is 0.45 m and the dipole field is slightly over 5 T. In the storage part the dipole field is slightly above 4 T with field index n = 0.5 and the radius correspondingly larger than in the microtron: 0.55 m. The linae in the microtron is 1.8 m long and gives a resonant energy gain of 24.3 MeV. Its frequency is 3 GHz. From a cylindrical Pierce-type electron gun the electrons are injected at - 100 kV into the linac via X - Y steering magnets and three iron shielded focussing solenoids to compensate for space charge effects. The number of orbits in the race-track injector is 28. Orbit no. 1 (24.3 MeV) is handled separately due to its relatively low energy. It is brought through a separate
loop in the vertical plane back to the linac. All the following orbits go the normal way in the race-track microtron. Each orbit has a steering magnet in the horizontal plane and one in the vertical plane. The full energy of the electrons becomes 680 MeV. It should be pointed out here that the bending field in this machine is almost 5 times the field in room temperature microtrons but also the linac energy is almost 5 times higher than in the room temperature microtrons. Therefore the frequency is the same, 3 GHz, in both cases and so are the electron optics and the orbit positions. Also the beam loading is similar in the two cases. A deflection magnet extracts the last orbit of the accelerator part. The full energy injection of the electrons from the microtron into the storage part is done in a conventional way by means of a pulsed septum magnet and two kickers. The pulse current of 5 mA and the repetition rate of 10 Hz of the microtron give a very short filling time. In a typical case it is less than 10 s. The stored current will be 200 mA but the machine is designed with margins for a higher current. With the full energy injection there is furthermore the possibility to keep a high circulating current con-
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>
7-
Fig, 2. Schematic illustration of integrated superconducting microtron/storage ring 680 MeV (full energy rejection).
/
!
oo
824
C. Mileikowsl%v/ The microtron hi X-ra~'lithography
LENGTH 7 rn HEIfiTH 1.8 rn DEPTH
2,6m
Fig. 3. Exterior view of integrated microtron/storage ring.
tinuously by frequent refilling of new electrons. Synchrotron radiation loss is on the average 36 keV per turn which is compensated for by means of a 500 MHz cavity. The horizontal and vertical emittances are each 0.3 mm mrad (full coupling). The energy spread is 0.0008 and the beam dimension ox =oy = 0.6 mm. The critical wavelength of the synchrotron radiation is 9.4 and the radiation power is 6.8 kW at 200 mA. The synchrotron light beams are dumped in a water cooled copper-profile in the vacuum chamber except where there are holes for the required beams; 10 equally spaced along each 180°-portion of the stored orbit with corresponding channels in the magnet yokes for the vacuum vessel ports. This machine is thus a compact synchrotron light
FINALOILREHOVAL SYSTEH ~ He COMPRESSOR \ ~ ~
source for lithography with a length of 7 m, a depth of 2.6 m and a height of 1.8 m. The machine is mounted on a rigid common frame. The weight is - 65 t (about the same as a 50 MeV normal conducting medical cyclotron). Most of this weight is due to the iron in the two bending magnets. The machine is to a considerable extent self-shielding. The walls of the room should, however, be about 2 m thick, if made by ordinary concrete. A possible lay-out of the rooms is shown in fig. 4. 4.2. Remarks about some of the subsystems
The superconducting bending magnets use the same superconducting material as large accelerators do, such
POWEI~DISTRIBUTION /UNIT / ~ [UBI[LE / /SO0 HHz SUPPLY
I..~/~tV.~/.~//A
"',..DIPOLEHAONET POWER SUPPLY
Fig. 4. Integrated microtron/storage ring illustrating possible single level layout.
C. Mileikowsl~r / The microtron in X-rat lithograplLr
as HERA for example, but the coil configuration is developed by BBC, Mannheim, specially to meet the microtron requirement. The specifications for the superconducting magnets are determined by the requirements of the accelerator and are of the following type: - The field slope along the straight edge of the D-shaped magnet must exceed a certain steepness. - The field in the accelerator region shall be homogeneous with a given precision at an exact value just above 5 T. - The field in the storage orbit shall have an exact value, just above 4 T with a field index of 0.5. The integrated field in the storage orbit must have a specified value in Tin. - The fringe field outside a reverse field area between the superconducting D-magnets must not exceed a quite small, given value. Apertures, vertical and horizontal, shall have given values. - There shall be 10 channels of given diameter in the return yoke etc. BBC has designed a configuration which obeys these criteria. The magnets are also quench safe designed with regard to the "hot spot" criterium. The total magnetically stored energy may be deposited in the coils without damage even under the worst conditions. The insulation -
-
825
system is based on kapton, both conductor insulation and pancake insulation. The coils are flat wound and cured. They rest in stainless steel cases which give stability and serve as liquid helium vessels. The helium refrigeration system was worked out by BBC and Sulzer for Scanditronix. The vacuum system has all surfaces in metal or ceramics and is bakeable in the sections outside the bending magnets (except for some O-rings in pump valves). The operating pressure shall be less than 10 -9 Torr. The diagnostic system for the beam from electron gun to linac, for the accelerator and for the storage ring consists of a variety of diagnostic elements; such as current transformers, cross wires, Q-electrodes, beam position monitors, frequency analyzers, optical chips, TV-cameras etc. Clearing electrodes clear the beam from trapped ions. The whole system is controlled by a computerized control system.
Reference [1] M. Eriksson, IEEE Trans. Nucl. Sci. NS-30 (1983) 2070.
VII. ACCELERATOR TECHNOLOGY
Nuclear Instruments and Methods in Physics Research B24/25 (1987) 820-825 North-tlolland, Amsterdam
T H E M I C R O T R O N I N X-RAY L I T H O G R A P H Y * C. M I L E I K O W S K Y Scanditronix AB, Uppsala, Sweden
The race-track microtron cart play two important roles in X-ray lithography: It can serve as a low energy injector (at 50-150 MeV) to compact synchrotron/storage rings which are either normal conducting or superconducting; or it can, in a superconducting version, serve as full energy injector (at e.g. 680 MeV) integrated with a surrounding superconducting storage ring, forming a compact synchrotron light source.
1. The need for synchrotron radiation in semiconductor lithograph)' In lithographic methods for production of semiconductor LSI chips and very dense memories by exposure with electromagnetic radiation through a mask, the wavelength, quality and intensity are of paramount importance. In today's mass production the mercury lamp is an excellent source. However, its wavelengths are too long to produce chips with detail dimensions as fine as 0.1-0.2 /.tm which will be required in the near future. For this purpose radiation in the X-ray region is needed. X-rays produced in the "ordinary" way - i.e. by an electron beam hitting a piece of matter - are emitted in all directions. At the distances where the radiation could be used for lithography, the intensities are insufficient for a competitive production rate for chips. Therefore manufacturers have to turn to some other X-ray source. One kind of radiation that can certainly do the job is synchrotron radiation of suitable wavelengths. Here all the required properties can be achieved including high intensity. It is well-known that an electron at high energy whose orbit is bent (in e.g. a magnetic field) emits photons in the tangential direction: synchrotron radiation. Energy and radius of curvature determine the wavelengths and number of photons according to a well-known formula. The number of photons is, of course, also proportional to the number of electrons. Thus, when electrons are circulating in the vacuum of a storage ring they emit synchrotron radiation from * The design of the systems is mainly by B. Anderberg, Scanditronlx, and his team, in cooperation with M. Eriksson, University of Lund. The superconducting magnet system is designed by C. Dustmann and his group at BBC, Mannheim to specifications supplied by Scanditronix.
all the parts of the orbit where it is curved. In storage rings dedicated to X-ray lithography the required synchrotron radiation is created where the orbiting electrons are passing through the fields of the bending magnets, in contrast to storage rings for research where synchrotron radiation also originates in special devices, such as wigglers, undulators etc. inserted in long straight sections of the ring. 2. Some possible 13"pes of compact synchrotron radiation sources dedicated to X-ray lithograph)' and the roles of the microtron The various possible schemes differ in many ways, for instance: the injection into the ring can be either at low energy or at full energy; the low energy injector can be a normal conducting race-track microtron or a linac; the full energy injector can be either a normal conducting synchrotron or a superconducting race-track microtron; and the synchrotron a n d / o r storage ring can be either normal conducting or superconducting. (Here normal conducting or superconducting refers to the main bending magnet coils.) If we assume that all the technically reasonable schemes can be designed to have similar reliability in operation, it turns out that some of them are clearly more costly than the others as to capital cost or operation cost or both. Those will (in the long run) not be competitive in production. Among the technically feasible schemes there are three having an intrinsic potential for lower cost, relatively speaking. Two of them have low energy injection which contributes to their low cost. The third scheme uses full energy injection but this is compensated for costwise as a consequence of other properties of the concept. The three schemes are" (i) low energy injection with a race-track microtron (normal conducting, 50-150 MeV) into a normal conducting compact synchrotron/
C. 3fileikowsky / The microtron in X-ray fithography
storage ring; (it) low energy injection with a race-track microtron (normal conducting, 50-150 MeV) into a superconducting synchrotron/storage ring; and (iii) full energy injection with a superconducting race-track microtron (650-7B24/25 MeV) into a surrounding superconducting storage ring. The same bending magnets are used for both injector and ring integrated. None of these schemes has yet been built in a complete way and tested at commercially interesting wafer production rates, but there are very good reasons to believe that all three are feasible for the goal. Each one of them has its advantages and disadvantages. In this article it is not the intention to try to make an evaluation of the relative merits of the three. Two possible roles for the race-track microtron are obvious from the above schemes: as normal conducting low energy injector and as superconducting full energy injector.
3. The normal conducting race-track microtron as low energy injector to compact synchrotron-storage rings, which are either normal conducting or superconducting The race-track microtron is excellent for this task because of its high electron beam quality, its ease of operation and its compactness. The beam quality is determined by the well defined energy, the low energy spread and the small emittance [1]. As a reminder, the race-track microtron operates according to the following principle. Electrons from the electron gun are injected into the linac, accelerated by it, and returned to the linac by one of the two main bending magnets and two small displacing magnets. After the second linac transit the beam passes into the second main bending magnet where it makes a 180 ° turn, at a larger radius than before, and goes back to the first bending magnet, where it is again bent 180 ° and sent into the linac. There it is accelerated
821
a third time etc. The distance between the linac and the bending magnets is chosen such that the beam always comes back to the linac at the right phase for repeated acceleration. So, if the energy gain in the linac is, for example, 5 MeV per transit and there are 20 passages, i.e. 20 orbits, the final energy is 100 MeV. The beam can be deflected and extracted after the last linac transit, but, if so desired, also from orbits with lower energy. The beam is focused by proper end magnet field forms and a number of quadrupole magnets. The main parameters for three race-track microtrons for different maximum injection energies are given in table 1.
4. The superconducting microtron as full energy injector integrated with the storage ring for X-ray lithography Full energy injection is commonly regarded as a technically more straightforward, safer and superior method of injection compared to low energy injection. Experience available in the world seems to support this view. In many schemes, however, the higher cost of full energy injection into a ring makes such injection uncompetitive" for production purposes. A compact synchrotron light source which has the potential for the desired quality and intensity of the synchrotron light, reliability and reasonable cost is the integrated race-track microtron storage ring (figs. 2 and 3). This system is a race-track microtron using superconducting bending magnets and a race-track shaped storage orbit outside the race-track microtron. The bending magnets are extending slightly outside the radius needs for the microtron and are used also for the storage orbit. Thus the full energyinjector occupies the area inside the storage orbit. A characteristic property of microtrons is that the magnetic fields in the bending magnets - as well as in the contramagnets and adjusting magnets - are kept
Table 1 Main parameters for three race-track microtrons a) Maximum electron energy (MeV) Beam pulse current (rnA) Pulse length (,us) Pulse repetition frequency (Hz) Ernittance at max. energy ( > 85% of current) Vertical (mm mrad) Horizontal (mm mrad) Energy spread at max. energy Weight of accelerator (t) Size of accelerator (height x width × depth (m)) Size of cabinet for klystron and klystron modulator a)
50 25 0.05-5 1-250
100 15 0.05-5 1-250
150 10 0.05-5 1-250
0.2rr 0.2~4-2 × 10 - 3 3 1.6 x 2.5 × 0.8 2.0 × 2.4 X 1.3
0.1~r 0.1rr 4-1 x 10"3 6 1.7 x 2.9 x 1.1 2.0 X 3.0 X 1.3
0.07~" 0.07~r + 0.7 x 10 - 3 16 1.8 × 3.3 x 1.4 2.0 X 3.6 × 1.3
~) Remaining auxiliary equipment needs the following space (height × width × depth): gun modulator: 2.0 × 1-2× 1.3 m3; control system interface: 2.0×0.8x 1.3 m3; control console: 1.2xl.6x 1.I m3; and 4 electronic cabinets each 2.0x0.6x0.8 m3. VII. ACCELERATOR TECHNOLOGY
822
C. Mileikowskl' / The microtron in X-ray lithography
I •
.?-
Fig. 1. Race-track microtron, 50 MeV. constant in time. There is no ramping like in synchrotrons of the field. This makes the machine particularly suitable for superconducting technology. There is a long experience of superconducting magnets with fields constant in time. The design problems of eddy currents in adjacent parts at ramping and the losses due to them are not present. 4.1. Main parameters and design features The maximum bending radius in the race-track microtron is 0.45 m and the dipole field is slightly over 5 T. In the storage part the dipole field is slightly above 4 T with field index n = 0.5 and the radius correspondingly larger than in the microtron: 0.55 m. The linae in the microtron is 1.8 m long and gives a resonant energy gain of 24.3 MeV. Its frequency is 3 GHz. From a cylindrical Pierce-type electron gun the electrons are injected at - 100 kV into the linac via X - Y steering magnets and three iron shielded focussing solenoids to compensate for space charge effects. The number of orbits in the race-track injector is 28. Orbit no. 1 (24.3 MeV) is handled separately due to its relatively low energy. It is brought through a separate
loop in the vertical plane back to the linac. All the following orbits go the normal way in the race-track microtron. Each orbit has a steering magnet in the horizontal plane and one in the vertical plane. The full energy of the electrons becomes 680 MeV. It should be pointed out here that the bending field in this machine is almost 5 times the field in room temperature microtrons but also the linac energy is almost 5 times higher than in the room temperature microtrons. Therefore the frequency is the same, 3 GHz, in both cases and so are the electron optics and the orbit positions. Also the beam loading is similar in the two cases. A deflection magnet extracts the last orbit of the accelerator part. The full energy injection of the electrons from the microtron into the storage part is done in a conventional way by means of a pulsed septum magnet and two kickers. The pulse current of 5 mA and the repetition rate of 10 Hz of the microtron give a very short filling time. In a typical case it is less than 10 s. The stored current will be 200 mA but the machine is designed with margins for a higher current. With the full energy injection there is furthermore the possibility to keep a high circulating current con-
Z 0 ©
M ¢'3
©
e-" r'rl
>
7-
Fig, 2. Schematic illustration of integrated superconducting microtron/storage ring 680 MeV (full energy rejection).
/
!
oo
824
C. Mileikowsl%v/ The microtron hi X-ra~'lithography
LENGTH 7 rn HEIfiTH 1.8 rn DEPTH
2,6m
Fig. 3. Exterior view of integrated microtron/storage ring.
tinuously by frequent refilling of new electrons. Synchrotron radiation loss is on the average 36 keV per turn which is compensated for by means of a 500 MHz cavity. The horizontal and vertical emittances are each 0.3 mm mrad (full coupling). The energy spread is 0.0008 and the beam dimension ox =oy = 0.6 mm. The critical wavelength of the synchrotron radiation is 9.4 and the radiation power is 6.8 kW at 200 mA. The synchrotron light beams are dumped in a water cooled copper-profile in the vacuum chamber except where there are holes for the required beams; 10 equally spaced along each 180°-portion of the stored orbit with corresponding channels in the magnet yokes for the vacuum vessel ports. This machine is thus a compact synchrotron light
FINALOILREHOVAL SYSTEH ~ He COMPRESSOR \ ~ ~
source for lithography with a length of 7 m, a depth of 2.6 m and a height of 1.8 m. The machine is mounted on a rigid common frame. The weight is - 65 t (about the same as a 50 MeV normal conducting medical cyclotron). Most of this weight is due to the iron in the two bending magnets. The machine is to a considerable extent self-shielding. The walls of the room should, however, be about 2 m thick, if made by ordinary concrete. A possible lay-out of the rooms is shown in fig. 4. 4.2. Remarks about some of the subsystems
The superconducting bending magnets use the same superconducting material as large accelerators do, such
POWEI~DISTRIBUTION /UNIT / ~ [UBI[LE / /SO0 HHz SUPPLY
I..~/~tV.~/.~//A
"',..DIPOLEHAONET POWER SUPPLY
Fig. 4. Integrated microtron/storage ring illustrating possible single level layout.
C. Mileikowsl~r / The microtron in X-rat lithograplLr
as HERA for example, but the coil configuration is developed by BBC, Mannheim, specially to meet the microtron requirement. The specifications for the superconducting magnets are determined by the requirements of the accelerator and are of the following type: - The field slope along the straight edge of the D-shaped magnet must exceed a certain steepness. - The field in the accelerator region shall be homogeneous with a given precision at an exact value just above 5 T. - The field in the storage orbit shall have an exact value, just above 4 T with a field index of 0.5. The integrated field in the storage orbit must have a specified value in Tin. - The fringe field outside a reverse field area between the superconducting D-magnets must not exceed a quite small, given value. Apertures, vertical and horizontal, shall have given values. - There shall be 10 channels of given diameter in the return yoke etc. BBC has designed a configuration which obeys these criteria. The magnets are also quench safe designed with regard to the "hot spot" criterium. The total magnetically stored energy may be deposited in the coils without damage even under the worst conditions. The insulation -
-
825
system is based on kapton, both conductor insulation and pancake insulation. The coils are flat wound and cured. They rest in stainless steel cases which give stability and serve as liquid helium vessels. The helium refrigeration system was worked out by BBC and Sulzer for Scanditronix. The vacuum system has all surfaces in metal or ceramics and is bakeable in the sections outside the bending magnets (except for some O-rings in pump valves). The operating pressure shall be less than 10 -9 Torr. The diagnostic system for the beam from electron gun to linac, for the accelerator and for the storage ring consists of a variety of diagnostic elements; such as current transformers, cross wires, Q-electrodes, beam position monitors, frequency analyzers, optical chips, TV-cameras etc. Clearing electrodes clear the beam from trapped ions. The whole system is controlled by a computerized control system.
Reference [1] M. Eriksson, IEEE Trans. Nucl. Sci. NS-30 (1983) 2070.
VII. ACCELERATOR TECHNOLOGY