Nuclear Instruments and Methods in Physics Research A304 (1991) 255-257

255North-Holland

SIRIUS FEL projectMichel RocheBourgogne Technologies, Parc Technologique, 9 au de la Découverte, 21000 Dijon, France

Jacques Moret-BaillyLaboratoire 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 thefacility 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, storagerings, etc.) . Actually, electrostatic FELs cannot reachshort wavelengths, but, as the pulses are long, thefrequency resolution may be high. This is the reasonwhy electrostatic FELs are well adapted to high-resolu-tion spectroscopy .

2. General description

The characteristics of the FEL we plan to build are,at the beginning, very similar to those of the UCSBfacility [11, apart from the linewidth. See table 1 .

Because of their relatively low cost, several types ofcavity may be realized in the vacuum chamber (e.g .Fabry-Perot and annular). The longitudinal modes ofthe laser can be selected by a Fabry-Peiot interferome-ter.

The waveguide in the wiggler induces an astigmatismwhich will be corrected by cylindrical mirrors.

3. Planned improvements

The construction and operation of the Santa BarbaraFEL have shown their failures . We plan to improvesome technological features .

* Unité associée au Centre national de la Recherche Scien-tifique .

3.1 . General design

0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)

Our facility (see fig. 1) will be as compact as possi-

ble, with a very good electrical insulation . The electrical

Table 1Characteristics of the SIRIUS facility

Expected performancesWavelengthPeak powerPulse durationRepetition rateLinewidthReproducibility from one

pulse to anotherMode number

n =I

Accelerator characteristicsElectron energyCurrentCharging currentAccelerating tube gradientAccelerating tube length

Wiggler and optical cavity characteristicsPeriodGapPeak magnetic fieldLength

Amplifying medium parametersHeightWidthLengthLosses

between 0.1 and 1 mmbetween 10 and 40 kWbetween 5 and 50 lis>10 Hz0,N/a <10-7

between 3 and 6 MeV2.6 A10 mA15 kV/cm4m

2.5 cm1 cm0.1 to 0.5 T3m

< 0.9 cm<4cm3mabout 1 %

Laser cavityRing laserLength of the cavity

14 mFocal length of the spherical murors 3.5 m

IV. PROPOSALS/STATUS REPORTS

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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, forexample) leads to mechanical problems, and introduceslarger electron losses.

It seems possible to use a single large tube forelectron 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 electronbeam power and wiggler heating by lost electrons. Toreduce the electron losses, the vacuum will be improvedand a more suitable wiggler will be built. The lowaverage power of the Santa Barbara facility is becauseof a Van de Graaff high-voltage source. We plan toreplace it by a static generator working at high frequency(1 MHz), so that heating is the only limitation. The 400V output from a transistor square-wave modulator willbe increased to 175 kV ac average voltage by a high-Qresonator. Capacitors and diodes in a 20-stage multi­plier will then increase the output voltage to 6 MV witha current of up to 10 mAo

3.3. High-voltage control

The pulse usable length of the Santa Barbara deviceis limited by the wavelength shift, induced by the elec­tron energy decrease in the wiggler. This is due to thepower extracted by the electromagnetic wave, and to thehigh-voltage decrease during the pulses.

Therefore, we need both to maintain the high voltageconstant during the pulse and to increase it to com­pensate for the energy transfer from the electron beamto 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 reachthe cathode again, they need an energy increase, whichis given by a generator placed between the collector andthe cathode. The collector efficiency should be high.This can be achieved by using a sufficient number ofplates, and dc generators maintaining constant poten­tials between them. If the electron trajectories are well

computed, the electrons "choose" the correct collectorplate, and therefore the correct dc generator number.

(2) The electron energy in the wiggler is defined bythe high-voltage potential. This depends mostly on theelectron losses. Because the high-voltage source is neitherpowerful nor fast enough, we will use, after Elias [1], ahigh-voltage terminal capacitance to compensate for theelectron losses. A pulse transformer will then be con­nected between the cathode and the high-voltage termi­nal to obtain the correct cathode potential variation. Itsprimary 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 magnetbefore passing through the wiggler. The bend is madeunder favourable conditions, because the beam homo­geneity has not yet been destroyed by the wiggler. Thewiggler type has not yet been chosen. Weare studyingiron and ironless pulsed wigglers. The general designallows an easy wiggler change (with the 180 0 magnet).It will also be possible to change or modify the wigglerwithout breaking the vacuum because a thin-wallvacuum tube has been studied. This may be importantfor modifying the tapering versus the demanded oper­ation mode. The cooling of this thin wall will berealized 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 onthe existing FELs for the construction of SIRIUS. Ev­ery researcher will have the possibility of using thefacility, placed under an International Scientific Com­mission control.

Reference

[I] L.R. Elias and G.J. Ramian, in: Free Electron Generatorsof Coherent Radiation, eds. c.A. Brau, S.F. Jacobs andM.O. Scully, Int. Soc. Opt. Eng. 453 (1983) 137.

IV. PROPOSALS/STATUS REPORTS