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Design and construction of an induction linac for a mm wave free electron laser for fusion research
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A 341 (1994) 412-416 North-Holland
Section A
Design and construction of an induction linac for a mm wave free electron laser for fusion research M. Shiho a, *, S. Kawasaki b , K. Sakamoto a, H. Maeda A. Tokuchi e, Y. Yamashita f , S . Nakajima g
a,
H. Ishizuka ", Y. Watanabe
d,
° Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, Naka-machi, Ibaraki-ken 311-01, Japan n Saitama Univ ., Faculty of Set ., 225 Hismo-Okubo, Urawa, 388 Japan Fukuoka Inst. of Tech., Wapro, Higashi-ku, Fukuoka, 811-02 Japan d Nissei Sangyo Co. Ltd, 1-24-14 Nishishinnbashi, Minato-ku, Tokyo, 105 Japan e Ntehicon Co. Ltd., 2-3-1 Yagura, Kusatsu, Shiga-ken, 525 Japan f Hitachi Metals Ltd., 5200 Mikaltn, Kumagaya, Saitama-ken, 360 Japan a Hitachi Ltd, Kokubucho, Hitachi-shi, Ibaraki-ken, 316 Japan
A new induction accelerator is designed and being constructed for application to fusion research . The main characteristics of the new induction linac are: (1) Foils made of a new Fe-based nanocryostalline soft magnetic material "Finemet", which has a higher saturation flux density and lower power loss than other commercially available Fe based amorphous alloys, are used . (2) The maximum beam energy and current will be 2.5 MeV, and 3 kA respectively; the accelerating voltage can be kept constant within ±1% during the pulse; the gradient of the voltage in time dV, ldt can be controlled in the range of ±20% . (3) The acceleration units are divided into two parts; one is for the beam generation and the other is for post acceleration of the beam . The latter is constructed so that the acceleration column may be separated from the driving circuit We expect FEL radiation with a power of several hundred megawatts in the frequency range of 30-120 GHz, and tokamak heating research is planned with the JAERI medium size tokamak JFT-2M . 1. Introduction With various successful results [1], there is a continuous demands for a high power and tunable radiation source for electron cyclotron resonance heating (ECRH) in fusion research . From resent experimental results of MTX [21, it is verified that very intense millimeter wave radiation of multi-hundreds megawatts is well absorbed by the tokamak plasma . This fact means that the millimeter FEL has a great potential for electron cyclotron resonance heating (ECRH) for fusion plasma . Although we have been doing several research work on FEL with an induction linac of 1 MeV energy, the amplification ability is limited by the beam energy [3]. From calculations, especially in relation to the wiggler tapering, it follows that a beam energy of 2-3 MeV is at least required to get multi-megawatts millimeter wave radiation [4]. For the ECRH research in the JFT-2M tokamak of JAERI, we started the construction of a new induction accelerator with the following requirements ; (1) The pulse length is as long as possible ; (2) The beam energy * Corresponding author
is 2.3-6 MeV; (3) High repetition rate is possible ; (4) The acceleration wave form can be varied ; (5) The shape of the accelerating electric field is controlled to obtain a good quality electron beam . As a first step to these requirements, an induction accelerator of 2.5 MeV is now being constructed [4], which consists of a beam generation unit of 1.25 MeV, and a post-acceleration unit of 1.25 MeV . This paper describes the magnetic material used, and the main characteristics of the accelerator .
2. Magnetic materials Fig. 1 shows a schematic view of the new induction linac. For the magnetic switches and the cores of the acceleration modules, a Fe based amorphous alloy, "Finemet", which is available from Hitachi Metals Ltd., is used [6]. This materials has a nanocrystalline fine grain structure, and has almost the same saturation induction as commercially available Fe-based amorphous such as AC-10 of TDK Co . Ltd. or 2605S-2 of Allied Signal Inc., and has a core loss, as low as Co-based amorphous materials. In addition, the magnetostriction of Finemet is less than 1/10 of that of
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)EII10-J
M Shcho et
al. /Nucl. Instr. and
Meth. in Phys. Res. A 341 (1994) 412-416
Fe-based amorphous materials. Fig. 2 shows the halfcycle core loss of the Fe-based amorphous core, Cobased amorphous core, and of the Finemet core for various time constants. For each test sample, the core diameter is 120 mm, the thickness is 25 mm and the packing factor K is about 70%. We have also measured similar core loss data with a larger diameter (- 600 mm) test sample ; for the Finemet and Co based cores, we obtain similar values as shown in Fig . 2, but for the conventional Fe-based core we always observe an increase of the core loss by factor of 2-2.5 . High magnetostriction of the conventional Fe-based amorphous material might be the reason that the larger test sample reveals a high core loss . We have fabricated the pulse transformers No . 1 and No . 2 and magnetic switches No . 1 to No . 3 in
(A) PULSE
413
twofold, one with Finemet and the other with the Fe-based core, and now testing is under way to compare the performance of the pulse forming network. From the preliminary result it can be said that in the pulse line where the pulse width of the passing pulse is long (0 .5-2 Its), e.g ., pulse transformer No. 1 and magnetic switch No . 1 in Fig. la, the change in performance is small; but for magnetic switch No . 3, and pulse transformer No . 2 where the pulse width of the passing pulse is short (0 .1-0 .2 ws), the Finemet core reveals much better performance. In this context we have chosen Finemet as the core material for the accelerating module, where the pulse width of the passing pulse is short . For each core, the ribbon thickness is 20 wm, and the ribbon is coated with 2 wm Si02.
Tunable Pulse Forming Network
TRANS
N0 .1
MAGNETIC
SWITCH
No .1
MAGNETIC
SWITCH
No .2
MAGNETIC
SWITCH
No .3
OUTPUT 86'G,ä :% . FI.
300H160ns
ä 604 l IN 7 -5 -5-5 ,
(25Q ,9P) INPUT CABLE
_ISC
30kV,2us
300kv,2us
(B)
HIGH
CABLE
(50Q ,4 P)
MOVABLE
INSULATOR/
PFN
I'rl
~GGGi G.ili ..
30DkV,500ns
(1 " 2) TRANSMISSION
LINE
PULSE
TRANS.
No .2
150kV,160ns
Accelerator VOLTAGE
PULSE
INPUT
(1 .25MV)
HIGH
VOLTAGE
PULSE
INPUT
(1 .25MV)
-
BEAM
ACCELERATING COLUMN
BEAM
GENERATION
UNIT
POST
ACCELERATION
UND
Fig. 1. Schematic view of the induction accelerator.
VII. ACCELERATORS
M. Sheho et al. /Nucl. Instr. and Meth . i n Phys. Res. A 341 (1994) 412-416
414
3. Tunable pulse forming network
Fig. 2. Half cycle core loss of the Finemet, Fe-based amorphous and Co-based amorphous core.
The main characteristics of the pulse line is that the impedance of the pulse line can be tuned by changing the capacitance of the pulse transmission line . We call this a tunable pulse forming network (TPFN) . The TPFN is shown in Fig. Ia . Fig. 3 shows the detailed structure of the tunable capacitor; there are (9 x 2) movable insulators inside the tunable capacitor; each movable insulator is inserted or extracted from outside the capacitor. The capacitance of each part can be changed from 0.28 nF to 4 nF . By changing each capacitance value, we can change the output wave form . The gradient of the voltage in time, dV,/dt, can be controlled in the range of ± 20% . Fig. 4 shows a computer simulation result of the output voltage wave form after the pulse transformer No . 2 in Fig. I a. For each case of ramp-up and ramp-down, the capacitance value for each of the nine capacitors is indicated . In real operation, the choice of the capacitance is determined by a computerized procedure when the required ramp-up/ ramp-down rate is fixed.
MOVABLE
INSULATOR
HIGH VOLTAGE ELECTRODE
HIGH
VOLTAGE INPUT ,
300kv,5001)s
HIGH VOLTAGE OUTPUT l50kV,160ns LOW VOLTAGE ,ELECTRODE INSULATOR LIQUID (PURE WATER)
Fig. 3. Detailed structure of the tunable capacitor
M. Shrho et al. /Nucl. Instr. and Meth . in Phys. Res. A 341 (1994) 412-416
400 .00, 350 .00
CASE 2 CONSTAN
IMPEDANCE
CASE 3 DECREASE CASE 4 INCREAS
IMP6DA1` IMPEDA
415
300 .00 250 .00 200 .00
MM 0
150 .00 100 .00 50 .00
I
0 .00 -50 .00 -100 .00
2 .40
2 .46
2 .52
2 .58
2 .70 -\Iq
2 .64
EQUIVALENT CIRCUIT OF TUNABLE PEN 8 2nH
15 4nH 30 9nH
inductance L1
L2
L3
14 6nH
14 6nH 14 6nH
L4
L5
L6
30 9nH 30 .9nH L7
tunable range 2 5-6 1 1 .9-4 6 1.9-4 6 0 2-2 9 0.2-2 9 0 2--2 9 nF nF nF nF nF nF C1
C2
C3
L8
30 9nH
8 6nH
L9
L10
1 .9-4 6 1 9--4 .6 2 5--6 1 rTF nF nF
C4
C5
C6
C7
C8
1 8nF
3 8nF
4 2nF
6 2nF-f- J
1 1nF
2 5nF
2 2nF
2.5nF +--
ramp down
2 5nF
2 2nF
2 5nF
1 .1 nF
1 4nF
ramp up
6 1 nF
4 2nF
3 8nF
1 8nF
1 .4nF
C9
^
Fig. 4. Simulated waveform of normal, ramp-up, and ramp-down case .
Fig. 5. Experimental wave form of normal and ramp-up operation. VII. ACCELERATORS
416
M. Shiho et al. / Nucl . Instr. and Meth in Phys. Res. A .341 (1994) 412-416
Fig. 5 shows the experimental result of the output voltage form . In normal operation, the rise time is about 20 ns, and the half width of the pulse is 160 ns . In the flat top, the fluctuation of the wave form is within 1 .5%. In ramp-up operation, the amplitude was increased with about 17% at the end of the pulse. With this ramp-up/ramp-down capability of the pulse forming network, we can compensate the impedance change of the beam diode, which has often been observed in the operation of an induction linac. 4. Accelerator The accelerator consists of two different accelerating parts ; one is the beam generating part, and the other is a post-acceleration part . At present the accelerator is under construction, and it will be completed in May 1994 . The beam generating unit consists of four accelerating cavities of conventional style described elsewhere [7]. In each cavity, 8 pieces of Finemet core, with a 600 mm diameter and a 2 .5 cm thickness, are used . A 250-300 kV pulse voltage is supplied from the pulse line, and each cavity generates a 250-300 kV well tailored pulse voltage. With this unit, an electron beam of 1-1.25 MeV, and 3 kA will be generated. The post-acceleration part also has four core blocks consisting of eight cores, but they are all contained in the same box. Inside the box, an accelerating column is placed . The accelerating column is a ceramic tube with many metallic electrodes having an aperture structure, which is very similar to that of an accelerating tube used in a high voltage electron microscope . The accelerating voltage generated by the block of cores are distributed to the electrode, and the electron beam passing through the tube is continuously accelerated by the voltage between each of the electrodes . A conventional accelerating cavity has only one accelerating gap [7], and one cannot change the accelerating electric field. However, with this type of accelerating column, by changing the number and the shape of the electrode, we can change the shape of the accelerating electric field and chose the optimum beam optics . In addition to this, we can avoid the acceleration cavity to become a resonance cavity for the intense electromagnetic waves generated by the electron beam as observed in a conventional induction cavity, which play a harmful role on the stability of the beam transport [7]. 5. Discussion The induction linac we describe here has two main characteristics which a conventional induction linac
does not have : (a) the impedance of the pulse line is tunable, and the output wave form can be changed, i .e ., ramped up or ramped down; (b) the accelerating column has an independent structure to the voltage driving structure unlike a conventional inducation linac, which gives us more freedom in designing the beam optics . With characteristic (a), the impedance change of the beam during the pulse, will be compensated, as often observed in the electron accelerator . It is of interest that, if we use this type of induction accelerator for ion acceleration, characteristic (a) will act as a tool for beam compression/ beam expansion. With characteristic (b), we get much more freedom to designing the beam optics in the accelerator . With a suitable choice of the shape and the spacing of the electrodes, we expect that the accelerating structure would not be a resonator for (harmful) em-waves which will cause instable beam transport . As for the fusion heating experiment, the schedule is roughly described; at present the construction of the pulse line is completed, and the accelerating units are now under construction ; they will be completed in May 1994 . As a first step the beam energy is presently set at 2.5 MeV. For the second state, the construction of a 2.5-3 .5 MeV post-acceleration unit is planned in 1995 . The pulse repetition rate will be gradually increased to 1 kHz by 1995 . With the linac, a several hundred megawatt of output power millimeter wave FEL is expected in the frequency range of 30-120 GHz [3], which corresponds to the first and second electron cyclotron frequency of the JAERI medium size tokamak JFT-2M in its usual operation range . Fusion heating experiment are planned to start in late 1995 .
References [1] K. Hoshmo et al ., Phys . Rev. Lett 69 (1992) 2208 . [2] S.L . Allenn et al ., Proc. 14th Int. Nat. Conf. on Plasma Physics and Cont . Nuclear Fusion Research, Wurzburg, Germany, 30 Sept -7 Oct. 1992, IAEA-CN-56/E-1-4 [3] K. Sakamoto et a] ., J. Appl . Phys ., to be published . [4] S. Kawasaki et al ., these Proceedings (15th Int. Free Electron Laser Conf., The Hague, The Netherlands, 1993) Nucl . Instr. and Meth . A 341 (1994) 316. [5] S. Kawasaki et al ., Proc IEEE Particle Accelerator Conf . '93, Washington, DC, 17-20 May 1993 . [6] S. Nakajima, S. Arakawa, Y Yamashita and M. Shiho, Nucl . Instr and Meth Res. A 331 (1993) 318 [7] D.S . Prone and the Beam Research Group, IEEE Trans. Nucl . Sci. NS-32 (5) (1985) 3144 .
Nuclear Instruments and Methods in Physics Research A 341 (1994) 412-416 North-Holland
Section A
Design and construction of an induction linac for a mm wave free electron laser for fusion research M. Shiho a, *, S. Kawasaki b , K. Sakamoto a, H. Maeda A. Tokuchi e, Y. Yamashita f , S . Nakajima g
a,
H. Ishizuka ", Y. Watanabe
d,
° Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, Naka-machi, Ibaraki-ken 311-01, Japan n Saitama Univ ., Faculty of Set ., 225 Hismo-Okubo, Urawa, 388 Japan Fukuoka Inst. of Tech., Wapro, Higashi-ku, Fukuoka, 811-02 Japan d Nissei Sangyo Co. Ltd, 1-24-14 Nishishinnbashi, Minato-ku, Tokyo, 105 Japan e Ntehicon Co. Ltd., 2-3-1 Yagura, Kusatsu, Shiga-ken, 525 Japan f Hitachi Metals Ltd., 5200 Mikaltn, Kumagaya, Saitama-ken, 360 Japan a Hitachi Ltd, Kokubucho, Hitachi-shi, Ibaraki-ken, 316 Japan
A new induction accelerator is designed and being constructed for application to fusion research . The main characteristics of the new induction linac are: (1) Foils made of a new Fe-based nanocryostalline soft magnetic material "Finemet", which has a higher saturation flux density and lower power loss than other commercially available Fe based amorphous alloys, are used . (2) The maximum beam energy and current will be 2.5 MeV, and 3 kA respectively; the accelerating voltage can be kept constant within ±1% during the pulse; the gradient of the voltage in time dV, ldt can be controlled in the range of ±20% . (3) The acceleration units are divided into two parts; one is for the beam generation and the other is for post acceleration of the beam . The latter is constructed so that the acceleration column may be separated from the driving circuit We expect FEL radiation with a power of several hundred megawatts in the frequency range of 30-120 GHz, and tokamak heating research is planned with the JAERI medium size tokamak JFT-2M . 1. Introduction With various successful results [1], there is a continuous demands for a high power and tunable radiation source for electron cyclotron resonance heating (ECRH) in fusion research . From resent experimental results of MTX [21, it is verified that very intense millimeter wave radiation of multi-hundreds megawatts is well absorbed by the tokamak plasma . This fact means that the millimeter FEL has a great potential for electron cyclotron resonance heating (ECRH) for fusion plasma . Although we have been doing several research work on FEL with an induction linac of 1 MeV energy, the amplification ability is limited by the beam energy [3]. From calculations, especially in relation to the wiggler tapering, it follows that a beam energy of 2-3 MeV is at least required to get multi-megawatts millimeter wave radiation [4]. For the ECRH research in the JFT-2M tokamak of JAERI, we started the construction of a new induction accelerator with the following requirements ; (1) The pulse length is as long as possible ; (2) The beam energy * Corresponding author
is 2.3-6 MeV; (3) High repetition rate is possible ; (4) The acceleration wave form can be varied ; (5) The shape of the accelerating electric field is controlled to obtain a good quality electron beam . As a first step to these requirements, an induction accelerator of 2.5 MeV is now being constructed [4], which consists of a beam generation unit of 1.25 MeV, and a post-acceleration unit of 1.25 MeV . This paper describes the magnetic material used, and the main characteristics of the accelerator .
2. Magnetic materials Fig. 1 shows a schematic view of the new induction linac. For the magnetic switches and the cores of the acceleration modules, a Fe based amorphous alloy, "Finemet", which is available from Hitachi Metals Ltd., is used [6]. This materials has a nanocrystalline fine grain structure, and has almost the same saturation induction as commercially available Fe-based amorphous such as AC-10 of TDK Co . Ltd. or 2605S-2 of Allied Signal Inc., and has a core loss, as low as Co-based amorphous materials. In addition, the magnetostriction of Finemet is less than 1/10 of that of
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)EII10-J
M Shcho et
al. /Nucl. Instr. and
Meth. in Phys. Res. A 341 (1994) 412-416
Fe-based amorphous materials. Fig. 2 shows the halfcycle core loss of the Fe-based amorphous core, Cobased amorphous core, and of the Finemet core for various time constants. For each test sample, the core diameter is 120 mm, the thickness is 25 mm and the packing factor K is about 70%. We have also measured similar core loss data with a larger diameter (- 600 mm) test sample ; for the Finemet and Co based cores, we obtain similar values as shown in Fig . 2, but for the conventional Fe-based core we always observe an increase of the core loss by factor of 2-2.5 . High magnetostriction of the conventional Fe-based amorphous material might be the reason that the larger test sample reveals a high core loss . We have fabricated the pulse transformers No . 1 and No . 2 and magnetic switches No . 1 to No . 3 in
(A) PULSE
413
twofold, one with Finemet and the other with the Fe-based core, and now testing is under way to compare the performance of the pulse forming network. From the preliminary result it can be said that in the pulse line where the pulse width of the passing pulse is long (0 .5-2 Its), e.g ., pulse transformer No. 1 and magnetic switch No . 1 in Fig. la, the change in performance is small; but for magnetic switch No . 3, and pulse transformer No . 2 where the pulse width of the passing pulse is short (0 .1-0 .2 ws), the Finemet core reveals much better performance. In this context we have chosen Finemet as the core material for the accelerating module, where the pulse width of the passing pulse is short . For each core, the ribbon thickness is 20 wm, and the ribbon is coated with 2 wm Si02.
Tunable Pulse Forming Network
TRANS
N0 .1
MAGNETIC
SWITCH
No .1
MAGNETIC
SWITCH
No .2
MAGNETIC
SWITCH
No .3
OUTPUT 86'G,ä :% . FI.
300H160ns
ä 604 l IN 7 -5 -5-5 ,
(25Q ,9P) INPUT CABLE
_ISC
30kV,2us
300kv,2us
(B)
HIGH
CABLE
(50Q ,4 P)
MOVABLE
INSULATOR/
PFN
I'rl
~GGGi G.ili ..
30DkV,500ns
(1 " 2) TRANSMISSION
LINE
PULSE
TRANS.
No .2
150kV,160ns
Accelerator VOLTAGE
PULSE
INPUT
(1 .25MV)
HIGH
VOLTAGE
PULSE
INPUT
(1 .25MV)
-
BEAM
ACCELERATING COLUMN
BEAM
GENERATION
UNIT
POST
ACCELERATION
UND
Fig. 1. Schematic view of the induction accelerator.
VII. ACCELERATORS
M. Sheho et al. /Nucl. Instr. and Meth . i n Phys. Res. A 341 (1994) 412-416
414
3. Tunable pulse forming network
Fig. 2. Half cycle core loss of the Finemet, Fe-based amorphous and Co-based amorphous core.
The main characteristics of the pulse line is that the impedance of the pulse line can be tuned by changing the capacitance of the pulse transmission line . We call this a tunable pulse forming network (TPFN) . The TPFN is shown in Fig. Ia . Fig. 3 shows the detailed structure of the tunable capacitor; there are (9 x 2) movable insulators inside the tunable capacitor; each movable insulator is inserted or extracted from outside the capacitor. The capacitance of each part can be changed from 0.28 nF to 4 nF . By changing each capacitance value, we can change the output wave form . The gradient of the voltage in time, dV,/dt, can be controlled in the range of ± 20% . Fig. 4 shows a computer simulation result of the output voltage wave form after the pulse transformer No . 2 in Fig. I a. For each case of ramp-up and ramp-down, the capacitance value for each of the nine capacitors is indicated . In real operation, the choice of the capacitance is determined by a computerized procedure when the required ramp-up/ ramp-down rate is fixed.
MOVABLE
INSULATOR
HIGH VOLTAGE ELECTRODE
HIGH
VOLTAGE INPUT ,
300kv,5001)s
HIGH VOLTAGE OUTPUT l50kV,160ns LOW VOLTAGE ,ELECTRODE INSULATOR LIQUID (PURE WATER)
Fig. 3. Detailed structure of the tunable capacitor
M. Shrho et al. /Nucl. Instr. and Meth . in Phys. Res. A 341 (1994) 412-416
400 .00, 350 .00
CASE 2 CONSTAN
IMPEDANCE
CASE 3 DECREASE CASE 4 INCREAS
IMP6DA1` IMPEDA
415
300 .00 250 .00 200 .00
MM 0
150 .00 100 .00 50 .00
I
0 .00 -50 .00 -100 .00
2 .40
2 .46
2 .52
2 .58
2 .70 -\Iq
2 .64
EQUIVALENT CIRCUIT OF TUNABLE PEN 8 2nH
15 4nH 30 9nH
inductance L1
L2
L3
14 6nH
14 6nH 14 6nH
L4
L5
L6
30 9nH 30 .9nH L7
tunable range 2 5-6 1 1 .9-4 6 1.9-4 6 0 2-2 9 0.2-2 9 0 2--2 9 nF nF nF nF nF nF C1
C2
C3
L8
30 9nH
8 6nH
L9
L10
1 .9-4 6 1 9--4 .6 2 5--6 1 rTF nF nF
C4
C5
C6
C7
C8
1 8nF
3 8nF
4 2nF
6 2nF-f- J
1 1nF
2 5nF
2 2nF
2.5nF +--
ramp down
2 5nF
2 2nF
2 5nF
1 .1 nF
1 4nF
ramp up
6 1 nF
4 2nF
3 8nF
1 8nF
1 .4nF
C9
^
Fig. 4. Simulated waveform of normal, ramp-up, and ramp-down case .
Fig. 5. Experimental wave form of normal and ramp-up operation. VII. ACCELERATORS
416
M. Shiho et al. / Nucl . Instr. and Meth in Phys. Res. A .341 (1994) 412-416
Fig. 5 shows the experimental result of the output voltage form . In normal operation, the rise time is about 20 ns, and the half width of the pulse is 160 ns . In the flat top, the fluctuation of the wave form is within 1 .5%. In ramp-up operation, the amplitude was increased with about 17% at the end of the pulse. With this ramp-up/ramp-down capability of the pulse forming network, we can compensate the impedance change of the beam diode, which has often been observed in the operation of an induction linac. 4. Accelerator The accelerator consists of two different accelerating parts ; one is the beam generating part, and the other is a post-acceleration part . At present the accelerator is under construction, and it will be completed in May 1994 . The beam generating unit consists of four accelerating cavities of conventional style described elsewhere [7]. In each cavity, 8 pieces of Finemet core, with a 600 mm diameter and a 2 .5 cm thickness, are used . A 250-300 kV pulse voltage is supplied from the pulse line, and each cavity generates a 250-300 kV well tailored pulse voltage. With this unit, an electron beam of 1-1.25 MeV, and 3 kA will be generated. The post-acceleration part also has four core blocks consisting of eight cores, but they are all contained in the same box. Inside the box, an accelerating column is placed . The accelerating column is a ceramic tube with many metallic electrodes having an aperture structure, which is very similar to that of an accelerating tube used in a high voltage electron microscope . The accelerating voltage generated by the block of cores are distributed to the electrode, and the electron beam passing through the tube is continuously accelerated by the voltage between each of the electrodes . A conventional accelerating cavity has only one accelerating gap [7], and one cannot change the accelerating electric field. However, with this type of accelerating column, by changing the number and the shape of the electrode, we can change the shape of the accelerating electric field and chose the optimum beam optics . In addition to this, we can avoid the acceleration cavity to become a resonance cavity for the intense electromagnetic waves generated by the electron beam as observed in a conventional induction cavity, which play a harmful role on the stability of the beam transport [7]. 5. Discussion The induction linac we describe here has two main characteristics which a conventional induction linac
does not have : (a) the impedance of the pulse line is tunable, and the output wave form can be changed, i .e ., ramped up or ramped down; (b) the accelerating column has an independent structure to the voltage driving structure unlike a conventional inducation linac, which gives us more freedom in designing the beam optics . With characteristic (a), the impedance change of the beam during the pulse, will be compensated, as often observed in the electron accelerator . It is of interest that, if we use this type of induction accelerator for ion acceleration, characteristic (a) will act as a tool for beam compression/ beam expansion. With characteristic (b), we get much more freedom to designing the beam optics in the accelerator . With a suitable choice of the shape and the spacing of the electrodes, we expect that the accelerating structure would not be a resonator for (harmful) em-waves which will cause instable beam transport . As for the fusion heating experiment, the schedule is roughly described; at present the construction of the pulse line is completed, and the accelerating units are now under construction ; they will be completed in May 1994 . As a first step the beam energy is presently set at 2.5 MeV. For the second state, the construction of a 2.5-3 .5 MeV post-acceleration unit is planned in 1995 . The pulse repetition rate will be gradually increased to 1 kHz by 1995 . With the linac, a several hundred megawatt of output power millimeter wave FEL is expected in the frequency range of 30-120 GHz [3], which corresponds to the first and second electron cyclotron frequency of the JAERI medium size tokamak JFT-2M in its usual operation range . Fusion heating experiment are planned to start in late 1995 .
References [1] K. Hoshmo et al ., Phys . Rev. Lett 69 (1992) 2208 . [2] S.L . Allenn et al ., Proc. 14th Int. Nat. Conf. on Plasma Physics and Cont . Nuclear Fusion Research, Wurzburg, Germany, 30 Sept -7 Oct. 1992, IAEA-CN-56/E-1-4 [3] K. Sakamoto et a] ., J. Appl . Phys ., to be published . [4] S. Kawasaki et al ., these Proceedings (15th Int. Free Electron Laser Conf., The Hague, The Netherlands, 1993) Nucl . Instr. and Meth . A 341 (1994) 316. [5] S. Kawasaki et al ., Proc IEEE Particle Accelerator Conf . '93, Washington, DC, 17-20 May 1993 . [6] S. Nakajima, S. Arakawa, Y Yamashita and M. Shiho, Nucl . Instr and Meth Res. A 331 (1993) 318 [7] D.S . Prone and the Beam Research Group, IEEE Trans. Nucl . Sci. NS-32 (5) (1985) 3144 .