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SIRIUS FEL project
Nuclear Instruments and Methods in Physics Research A304 (1991) 255-257 North-Holland
255
SIRIUS FEL project Michel Roche Bourgogne Technologies, Parc Technologique, 9 au de la Découverte, 21000 Dijon, France
Jacques Moret-Bailly Laboratoire de Spectronomie Moléculaire et Instrumentation Laser *, Université de Bourgogne, 6 bd Gabriel, 21000 Digon, France
This article presents a 6 MV electrostatic accelerator-driven free electron laser project, with a wavelength range of 80 to 1000 ~Lm. The study of this facility, called SIRIUS (source intense de rayonnement infra-rouge pour utilisations scientifiques), is based on the facility realized by Elias at the University of California, Santa Barbara. We plan to improve some important technological features .
1. Introduction The electrostatic FELs' characteristics are very dif-
ferent from those of other types of FEL (linacs, storage rings, etc.) . Actually, electrostatic FELs cannot reach short wavelengths, but, as the pulses are long, the frequency resolution may be high. This is the reason why electrostatic FELs are well adapted to high-resolution spectroscopy .
2. General description The characteristics of the FEL we plan to build are, at the beginning, very similar to those of the UCSB facility [11, apart from the linewidth. See table 1 .
Because of their relatively low cost, several types of
cavity may be realized in the vacuum chamber (e.g . Fabry-Perot and annular). The longitudinal modes of the laser can be selected by a Fabry-Peiot interferometer.
The waveguide in the wiggler induces an astigmatism
which will be corrected by cylindrical mirrors.
3. Planned improvements The construction and operation of the Santa Barbara
FEL have shown their failures . We plan to improve some technological features .
* Unité associée au Centre national de la Recherche Scientifique .
3.1 . General design Our facility (see fig. 1) will be as compact as possible, with a very good electrical insulation . The electrical Table 1 Characteristics of the SIRIUS facility Expected performances Wavelength Peak power Pulse duration Repetition rate Linewidth Reproducibility from one pulse to another Mode number Accelerator characteristics Electron energy Current Charging current Accelerating tube gradient Accelerating tube length
between 0.1 and 1 mm between 10 and 40 kW between 5 and 50 lis >10 Hz 0,N/a <10-7
n =I between 3 and 6 MeV 2.6 A 10 mA 15 kV/cm 4m
Wiggler and optical cavity characteristics Period 2.5 cm Gap 1 cm Peak magnetic field 0.1 to 0.5 T Length 3m Amplifying medium parameters Height Width Length Losses
< 0.9 cm <4cm 3m about 1 %
Laser cavity Ring laser Length of the cavity 14 m Focal length of the spherical murors 3.5 m
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
IV. PROPOSALS/STATUS REPORTS
256
M. Roche, J Moret- Badly /SIRIUS FEL project
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M. Roche, J. Moret-Bailly / SIRIUS FEL project
breakdown probability will be reduced by using SF6 under high pressure (10 bar). For the same reason, the accelerating tube must be long enough (> 4 m). But length increase (to 6 m, for example) leads to mechanical problems, and introduces larger electron losses. It seems possible to use a single large tube for electron acceleration and deceleration. This configuration makes mechanical design and residual gases pumping easier. 3.2. High-voltage source The FEL average power is limited by the electron beam power and wiggler heating by lost electrons. To reduce the electron losses, the vacuum will be improved and a more suitable wiggler will be built. The low average power of the Santa Barbara facility is because of a Van de Graaff high-voltage source. We plan to replace it by a static generator working at high frequency (1 MHz), so that heating is the only limitation. The 400 V output from a transistor square-wave modulator will be increased to 175 kV ac average voltage by a high-Q resonator. Capacitors and diodes in a 20-stage multiplier will then increase the output voltage to 6 MV with a current of up to 10 mAo 3.3. High-voltage control The pulse usable length of the Santa Barbara device is limited by the wavelength shift, induced by the electron energy decrease in the wiggler. This is due to the power extracted by the electromagnetic wave, and to the high-voltage decrease during the pulses. Therefore, we need both to maintain the high voltage constant during the pulse and to increase it to compensate for the energy transfer from the electron beam to the electromagnetic wave. The problem is composed of two parts. (1) The electrons lose energy, when leaving the cathode, in the wiggler and in the collector. To reach the cathode again, they need an energy increase, which is given by a generator placed between the collector and the cathode. The collector efficiency should be high. This can be achieved by using a sufficient number of plates, and dc generators maintaining constant potentials between them. If the electron trajectories are well
257
computed, the electrons "choose" the correct collector plate, and therefore the correct dc generator number. (2) The electron energy in the wiggler is defined by the high-voltage potential. This depends mostly on the electron losses. Because the high-voltage source is neither powerful nor fast enough, we will use, after Elias [1], a high-voltage terminal capacitance to compensate for the electron losses. A pulse transformer will then be connected between the cathode and the high-voltage terminal to obtain the correct cathode potential variation. Its primary will be connected to a generator built to provide a convenient pulse shape, the shape being dependent on the instantaneous high voltage, results of previous pulses, etc. 3.4. The wiggler The electron beam bends through 180 0 in a magnet before passing through the wiggler. The bend is made under favourable conditions, because the beam homogeneity has not yet been destroyed by the wiggler. The wiggler type has not yet been chosen. Weare studying iron and ironless pulsed wigglers. The general design allows an easy wiggler change (with the 180 0 magnet). It will also be possible to change or modify the wiggler without breaking the vacuum because a thin-wall vacuum tube has been studied. This may be important for modifying the tapering versus the demanded operation mode. The cooling of this thin wall will be realized bya liquid which is vaporized at its contact. Therefore it is possible to get a very strong cooling.
4. Conclusion We will take into account the experience acquired on the existing FELs for the construction of SIRIUS. Every researcher will have the possibility of using the facility, placed under an International Scientific Commission control.
Reference [I] L.R. Elias and G.J. Ramian, in: Free Electron Generators of Coherent Radiation, eds. c.A. Brau, S.F. Jacobs and M.O. Scully, Int. Soc. Opt. Eng. 453 (1983) 137.
IV. PROPOSALS/STATUS REPORTS
255
SIRIUS FEL project Michel Roche Bourgogne Technologies, Parc Technologique, 9 au de la Découverte, 21000 Dijon, France
Jacques Moret-Bailly Laboratoire de Spectronomie Moléculaire et Instrumentation Laser *, Université de Bourgogne, 6 bd Gabriel, 21000 Digon, France
This article presents a 6 MV electrostatic accelerator-driven free electron laser project, with a wavelength range of 80 to 1000 ~Lm. The study of this facility, called SIRIUS (source intense de rayonnement infra-rouge pour utilisations scientifiques), is based on the facility realized by Elias at the University of California, Santa Barbara. We plan to improve some important technological features .
1. Introduction The electrostatic FELs' characteristics are very dif-
ferent from those of other types of FEL (linacs, storage rings, etc.) . Actually, electrostatic FELs cannot reach short wavelengths, but, as the pulses are long, the frequency resolution may be high. This is the reason why electrostatic FELs are well adapted to high-resolution spectroscopy .
2. General description The characteristics of the FEL we plan to build are, at the beginning, very similar to those of the UCSB facility [11, apart from the linewidth. See table 1 .
Because of their relatively low cost, several types of
cavity may be realized in the vacuum chamber (e.g . Fabry-Perot and annular). The longitudinal modes of the laser can be selected by a Fabry-Peiot interferometer.
The waveguide in the wiggler induces an astigmatism
which will be corrected by cylindrical mirrors.
3. Planned improvements The construction and operation of the Santa Barbara
FEL have shown their failures . We plan to improve some technological features .
* Unité associée au Centre national de la Recherche Scientifique .
3.1 . General design Our facility (see fig. 1) will be as compact as possible, with a very good electrical insulation . The electrical Table 1 Characteristics of the SIRIUS facility Expected performances Wavelength Peak power Pulse duration Repetition rate Linewidth Reproducibility from one pulse to another Mode number Accelerator characteristics Electron energy Current Charging current Accelerating tube gradient Accelerating tube length
between 0.1 and 1 mm between 10 and 40 kW between 5 and 50 lis >10 Hz 0,N/a <10-7
n =I between 3 and 6 MeV 2.6 A 10 mA 15 kV/cm 4m
Wiggler and optical cavity characteristics Period 2.5 cm Gap 1 cm Peak magnetic field 0.1 to 0.5 T Length 3m Amplifying medium parameters Height Width Length Losses
< 0.9 cm <4cm 3m about 1 %
Laser cavity Ring laser Length of the cavity 14 m Focal length of the spherical murors 3.5 m
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
IV. PROPOSALS/STATUS REPORTS
256
M. Roche, J Moret- Badly /SIRIUS FEL project
c
T cd G
C
4J . 0
_ =
Y ô
a u oW ü E -a
~ 3
O
W
ô_
U u û ou E o û 7
û
en1 " C m m m ~ p, p C `° C = « û û Eye_ p Ea ~u 2 p C â o . ÔY ô - .=~ =4 > > > u Z ~2 2 $ û
Qv33aaa.t Pmm aw ==V, .oc:mao
-N
tn N
NN
C
û
00 c_ - T
e_ c
N
E > U épm v a n u~= o o u
Ô _4
°o E E u T ',u p u û ~
C E
O a C mm~ Q . C C ~ u ô : Z ` Z '> > >$ ~~ u o 01 Z
u
a u û u E. 2~ -= ^ »a _° û C °° U û û u , -, 0 « ~ ra : s Zz =
ct
" rf ~O~aeP O ^--
M. Roche, J. Moret-Bailly / SIRIUS FEL project
breakdown probability will be reduced by using SF6 under high pressure (10 bar). For the same reason, the accelerating tube must be long enough (> 4 m). But length increase (to 6 m, for example) leads to mechanical problems, and introduces larger electron losses. It seems possible to use a single large tube for electron acceleration and deceleration. This configuration makes mechanical design and residual gases pumping easier. 3.2. High-voltage source The FEL average power is limited by the electron beam power and wiggler heating by lost electrons. To reduce the electron losses, the vacuum will be improved and a more suitable wiggler will be built. The low average power of the Santa Barbara facility is because of a Van de Graaff high-voltage source. We plan to replace it by a static generator working at high frequency (1 MHz), so that heating is the only limitation. The 400 V output from a transistor square-wave modulator will be increased to 175 kV ac average voltage by a high-Q resonator. Capacitors and diodes in a 20-stage multiplier will then increase the output voltage to 6 MV with a current of up to 10 mAo 3.3. High-voltage control The pulse usable length of the Santa Barbara device is limited by the wavelength shift, induced by the electron energy decrease in the wiggler. This is due to the power extracted by the electromagnetic wave, and to the high-voltage decrease during the pulses. Therefore, we need both to maintain the high voltage constant during the pulse and to increase it to compensate for the energy transfer from the electron beam to the electromagnetic wave. The problem is composed of two parts. (1) The electrons lose energy, when leaving the cathode, in the wiggler and in the collector. To reach the cathode again, they need an energy increase, which is given by a generator placed between the collector and the cathode. The collector efficiency should be high. This can be achieved by using a sufficient number of plates, and dc generators maintaining constant potentials between them. If the electron trajectories are well
257
computed, the electrons "choose" the correct collector plate, and therefore the correct dc generator number. (2) The electron energy in the wiggler is defined by the high-voltage potential. This depends mostly on the electron losses. Because the high-voltage source is neither powerful nor fast enough, we will use, after Elias [1], a high-voltage terminal capacitance to compensate for the electron losses. A pulse transformer will then be connected between the cathode and the high-voltage terminal to obtain the correct cathode potential variation. Its primary will be connected to a generator built to provide a convenient pulse shape, the shape being dependent on the instantaneous high voltage, results of previous pulses, etc. 3.4. The wiggler The electron beam bends through 180 0 in a magnet before passing through the wiggler. The bend is made under favourable conditions, because the beam homogeneity has not yet been destroyed by the wiggler. The wiggler type has not yet been chosen. Weare studying iron and ironless pulsed wigglers. The general design allows an easy wiggler change (with the 180 0 magnet). It will also be possible to change or modify the wiggler without breaking the vacuum because a thin-wall vacuum tube has been studied. This may be important for modifying the tapering versus the demanded operation mode. The cooling of this thin wall will be realized bya liquid which is vaporized at its contact. Therefore it is possible to get a very strong cooling.
4. Conclusion We will take into account the experience acquired on the existing FELs for the construction of SIRIUS. Every researcher will have the possibility of using the facility, placed under an International Scientific Commission control.
Reference [I] L.R. Elias and G.J. Ramian, in: Free Electron Generators of Coherent Radiation, eds. c.A. Brau, S.F. Jacobs and M.O. Scully, Int. Soc. Opt. Eng. 453 (1983) 137.
IV. PROPOSALS/STATUS REPORTS