Visible and Ultraviolet Radiation Generation Using

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JOURNAL

IEEE

OF QUANTUM ELECTRONICS, VOL. QE-20, NO. 12, DECEMBER 1984

variousoptical echoes. He demonstratedthetwo-photonechoand, in 1977, was a codiscoverer of the tri-level echo. In 1978 he joined the staff of Avco Everett Research Laboratory, Everett, MA. Since then he has been involved in a variety of projects related t o his background in nonlinear optics and atomicspectroscopy. Hemade major contributions to the Avco-Exxon uranium isotope separation program and developed a practical technique of photographing far-IR laser intensity distributions. He developed the theory of a thermoacoustic instability which occurs in the gas of a pulsed COz laser. He is currently working on various aspects of stimulated Raman scattering, especially wavefront preservation. He has published over 30 journal papers in nonlinear optics and atomic spectroscopy. Dr. Flusberg is a member of the American Physical Society and the Optical Society of America.

Visible and Ultraviolet Radiation Generation Using a Gas-Loaded Free-Electron Laser ANNE-MARIE FAUCHET, JOSEPH FEINSTEIN, FELLOW, IEEE, AVRAHAM COVER, AND RICHARD H. PANTELL, FELLOW, IEEE

Abstract-Gas loadingofafree-electron laser modifies the phasematching condition while the small-signal gain expression remains the same when written in appropriate form. This permits a wider parameter space than the vacuum FEL, which is particularlyadvantageous at shorter wavelength operation.Scattering of theelectrons by the gas limits the interaction length, but available gains are still high enough to allow oscillation build-up. For example, a 0.5-pn wavelength helical wiggler FEL utilizing a 41 MeV electron beam is restricted to a length of 14 cm, has a small signal gain of 21 percent, and builds up to saturation in less than 1 p?.. Tunability of this device over several microns is easily obtained by changing the gas pressure.

INTRODUCTION HE free-electron laser (FEL) [ I ] is a coherent radiation source in which the photons are generated by an electron beam wiggled by a periodic, transverse, static magnetic field called a wiggler, oranundulator insideanopticalcavityas shown in Fig. 1. Since the active medium in this device is a beam of free particles, and since the electron beam and wiggler characteristics are left t o be chosen by the designer, it holds the promiseof a veryeasily and widely tunableradiation sourcecompared tothemostcommon lasers in which the

T

SENIOR MEMBER, IEEE,

A X

0

c

2

Y

Fig. 1. Schematic of afree-electron laser. The electron beam ( e 3 is wiggled inside the optical cavityconsisting of the two mirrors M I and M z . The dashed array of magnet pairs forming the wiggler should be imagined rotated by 90" out of the plane of the paper to produce a motion of the electrons in the plane of the paper. The optical wave is linearly polarized in the plane of the paper as indicated by the electric field vector E if the wiggler is linear.

activemedium is anatomic ormolecularmediumwhose energy level system cannot betailored to the user's needs. Furthermore, electron beamsused in a variety of devices, some ofwhichareanalogous tothe free-electron laser [2] are known to beverypowerful andefficientsourcesofmicrowaves. It was thereforehoped,whenthefree-electron laser principle was conceived [ l ] , that it would be applicable to Manuscript received May 4, 1984. This work was supported by the designing powerfulradiationsourcestunable over wide porU.S. Office of Naval Research under Contract N00014-78-C-0403. tions of a spectrum spanning from millimeterto X-rays. A.-M. Pauchet was with theDepartment of ElectricalEngineering, The wavelength of oscillation of a free-electron laser i s given Stanford University, Stanford, CA 94305. She is now with National Synchrotron Light Source, BrookhavenNational Laboratory, Brook- by avelocitysynchronism conditionbetweentheelectrohaven, NY 11793. magnetic wave and the electrons, which can be approximately J. Feinstein and R. H. Pantell are with the Department of Electrical written Engineering, Stanford University, Stanford, CA 94305. A. Gover is with the Department of Electrical Engineering, Tel-Aviv University, Tel-Aviv, Israel.

0018-9197/84/1200-1332$01.00 0 1984 IEEE

FAUCHET e t al.: VISIBLE AND ULTRAVIOLET RADIATION GENERATION

where X = wavelength of operation, X, = wiggler period, and = z-component of the relative velocity betweenthe wave and the electron divided by c, the velocity of light in vacuum. As will be shown later (1) can be more exactly rewritten in terms of the electron beam energy and the wiggler characteristics according to

Bz,

with y = total electron energy divided by m e c 2 , where me = electron rest mass:

eBoX,

a, = -.

271~1,~ '

e = electron charge, and Bo = wiggler magnetic field. Equation (2) is valid for a helical wiggler generating a circularly polarized static magnetic field. It is valid for a linear wiggler, generating a linearly polarized fieldif Bo is taken to be the rms value of the magnetic field. Such a laser has been operated at StanfordUniversity [3]with43.5 MeV electrons,a wiggler period of 3.2 cm, and a magnetic field of 2.4 kG, generating 3.4 pm radiation. Equation (1) shows that to generate wavelengths shorter than the 3.5 pm value of the Stanford experiment, either the wiggler period has to be decreased, or the electron beamenergyincreasedverydrastically if one wants to design an ultraviolet or X-ray radiation source. Present magnetic technology does not provide us with wigglers of a period significantlyshorter than 2 cm;ontheotherhand increasing the electron beamenergyrapidlybecomesverycostly. Furthermore, the gain scales as X1I2 for a fixed length and optimalGaussianbeam [4], thus decreasingwith shorter wavelength.Since it is alsoproportional to the normalizedcurrentdensity, storagerings,whichprovideelectronbeams with a largenormalizedcurrentdensity, have to be used to overcome the gain decrease in the ultravioletpart of the spectrum. Tofurther enhance the gain, theFELmay be built in an optical klystron configuration which has a higher gain but smallerenergyacceptance [5]. If onewants to limittheelectron energyrequirement toabout 40 MeV, which at the present time seems to be the upper limit of the energy that could be produced in the near future by a cornpactcommercialaccelerator,one cannothopeto generate radiationshorterthanabout2pm[6] using afree-electron laser similar to the Stanford one. On the other hand, velocitysynchronism betweenanoptical wave and a relativisticelectronbeam, the directions of propagationofwhichmakeanangle Bc, is readilyachieved byintroducinga gas ofindex n suchthatthe Cherenkov condition is met as follows: n/3 cos Bc = 1.

(4)

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It has been known for a number of years that if the electromagnetic wave is slowed down (by adding a dielectric liner) inaubitron [8], [9], the microwaveanalogueof thefreeelectronlaser,theelectronbeamenergyrequired to obtain a given output microwave frequency is lowered. In the opticalpart of the spectrum it hasbeenproposedseveraltimes [lo]-[12] to add a gaseousmedium to a standard FEL to obtainthe sameeffect. It canbeseen from (1) thatthe relative z-velocity of the wave and the electrons can then be madearbitrarily small by slowing downtheoptical wave insteadofaccelerating the electrons. Anotherpictureof such a device would be that the pitch angle of the helix described by the electrons in a helical wiggler is the Cherenkov angle, so that the stimulated Cherenkov interaction is in fact occurring, but without any walk-off. It will be shown later underwhichcircumstances thelatterinterpretationofthe interaction holds. The purpose of this paper is to present the design of two gas-loaded free-electron lasers which would generate pressuretunableradiation of approximately 0.2 pm and 0.5 pm, respectively, using a 40 MeV accelerator presently under construction at Stanford. This paper will take the vacuum FEL viewpoint and show how it is modified when a gaseous medium, which slows down the optical wave, is added. Section I will discuss the changesin the expressionsfor thequantities ofinterestdue to the presence of the gas. Section I1 will illustrate how the added degree of freedom given by the presence of the gas allows it to reach a more attractive parameterspace than in the vacuumcase.Section I11 will discuss the problems introduced by the gas and how they can be overcome. Section IV will present the parameters of the two gasloaded FEL's designed to operate on a 40 MeV accelerator at 0.5 pm and 0.2 pm. Ail quantities are in MKS units except when otherwise specified. SECTIONI: ANALYSIS As already mentioned, the optical wave is slowed down by a gas of index n inside the FEL wiggler. The analysis of the gas-loaded FELtherefore proceeds as forthe vacuum FEL except that the k-vector of the optical wave now has amplitude kopt = on/c. Any of the methods used for the vacuum FEL areapplicable. The gas-loaded FELhasbeenpartially analyzedin [lo] and [ I l l . The small signal gain can be rederived using Madey's theorem [ 131 , which is applicable here, as can be seen from [14]. This yields for the small signal gain in the case of a helical wiggler

(5)

where lo= electron beam peak current, L= interaction length, and A = optical mode area; B is given by Theoptical wave doesexperience gain bystimulatedCherenkov interaction [ 7 ] , however the gainis small in this simple configuration due to the walk-off between the two beams. The advantage of this mechanism is a very broad wavelength tunability range, and a simple tuning by changing the pressure where o = optical frequency and z = direction of propagation. of thegas. The index n differs from unity by only a few parts in lo4 for

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-20, NO. 12, DECEMBER 1984

the gases and pressure range considered, so that the n in (5) tron energy distribution by can be ignored. Setting 8 equal to - 1.3 to maximize the gain andreplacing the fundamental constants by their numerical values, (5) becomes (in MKS units) where AE is the full width at half-maximum of the distribution. Equation (12) is readily obtained by using the energy relationship y = [l - p j - p i ] ‘1’ and by noting that the norThe phase-matching condition (corresponding to 8 = 0) be- malized transverse velocity is pl= &/?. By limiting the phasetween the optical wave and a given electron becomes slippage t o n we finally obtain for theefficiency

where OP is the pitch angle of the helical trajectory described Using the resonance condition and letting N , be the number by the electron in the wiggler field, and is given by of wiggler periods, (13) can be rewritten as

For relativistic electrons O P is a small angle, so that (8) can be rewritten

In summary, the addition of a gas to the FEL does not alter the small signal gain but does change the synchronism condition from (2) to (10). If we put a lower value on X, of about 1 cm for conventional wigglers we see that for X smaller than 1 pm h/h, will be less than A gassuch as Hz at 1 .O atm and a temperature of 273 K has an index n - 1 of 1.368 X at 1.0 pm, increasing with decreasing h. Thus, at wavelengths shorter than 1 pm n - 1 is at least comparable to XJX,, and dominates as X decreases. We see that when progressing towards the UV, the phase-matching condition tends towards the Cherenkov condition given by (4). The wiggler field then serves mainly t o provide a geometry where there is an angle Bc 8, betweentheelectroninstantaneous velocityvector and the optical mode k vector while maintaining a unity filling factor over theinteractiondistance.The degreeof freedom thereby introduced into the choice of wiggler parameters and y allowshigher gain throughtransittimebunching as evidenced in (7). Thus, the “gas-loaded FEL” is really a magnetic bremsstrahlung, free-electron laser in the far IR where h/h, >> n - 1, asynergisticmagnetic bremsstrahlung-Cherenkov freeelectron laser in the near IR to longer wavelength part of the spectrum where n - 1 = X/h,, and a purely Cherenkov freeelectron laser in the visible to UV part of the spectrum. The efficiency of the device can be estimated easily in the small signal limit from the argument showing that efficiency and energy acceptance are almost equal [ 151. This argument applies equally well to the gas-loaded FEL. Let us now rederive the energyacceptance.The phase-slippage due tothe energy spread of the electron beamis

It is t o be noted that (13) is valid for both the vaccum freeelectron laser andthe gas-loaded FEL. However, theoften quotedformula, q AE/E = 1/2N, isvalid onlyin vacuum. One can also see from (14) that for a given number of wiggler periods the efficiency of the gas-loaded free-electron laser is smaller than in vacuum. For example, for Hz gas at 1.0 atm, 293 K, for a wavelength of 0.5 pm and for A, = 3 cm,n - 1 is 1.5 X the energyacceptance is thusreducedby a factor of 10 compared to a vacuum FEL that would have the same numberof wiggler periods.However,(13)does not impose astringentmonochromaticityrequirementontheelectron beam because the gas-loaded FEL can have a very small numberof wiggler periods, as will be seen later.Inboth cases L/X is thequantity of physical significance indetermining the allowed phase slip and therefore theenergy acceptance. The output power AP,, of the laser when it reaches saturation is obtained from the definitionof efficiency

AP,, = q X power in the electron beam

since the energylost by the electronsinsteady-stateorat saturation should be equal to the output energy of the laser. Another quantity of interest for a free-electron laser amplifier is itsangularacceptance.Angularspread is presentin the electron beam at entrance as a result of transverse thermalvelocities,spacechargeeffects, and lens aberrations. As will be seen later, in the gas-loaded FEL there is also a component due to the scattering of the electrons by the gas molecules.Here we onlycomputethe phase-slippage due to the intrinsic angular spread of the electron beam: electrons enteringthe wiggler at an angle to its axis will slip out of phase with respect to the optical wave. Equation (1 1) applies here, but A(l/@,)is now

where @e is the half-single angular divergence. The resulting phase-slippage is thus The second equality - holds for a helical wiggler for which 0, is constant. The spread in Pz is related to the width of the elec-

FAUCHET et al.: VISIBLE AND ULTRAVIOLET GENERATION RADIATION

Angularacceptance will be computedwhentheothercontributions to the angular spread have been discussed. Amongthecharacteristicsof an FEL,oscillator an importantoneisthe oscillationbuild-uptime. Theoutputof a conventional linac typically consists of electron microbunches of a few picoseconds occurring at the microwave frequency for afewmicroseconds. The microsecond-long macrobunch is repeated at tens or hundreds ofhertz. The oscillation build-up time should therefore be short compared to the macrobunch length to permitsteady-statesaturatedpower level to be reached. We follow the method used in [16] to estimate this quantity. If N is the number of round-trips necessary to reach saturation,thebandwidthofthespontaneous emission is narrowed down t o

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Case A :

2

a,

(1 t

Case B: G

g = 6.24 ~ X~

~

Case C:

For Case A it will be assumed that X,, B o , and X are given, i.e., the wiggler characteristics and the wavelength of operation arespecified, Then, according t o (23), the gain for afixed length is a constant times the factor [n - 1 t(X/hw)] '1'. As already pointed out for short X,n - 1 can be at least one order 1 X2 An=ofmagnitude greater than X/h,. Table I showsanexample 2n w1 using H2at STPwhich gives n - 1 = 1.400 X for X = where w1 is the spot size of Gaussian optical mode. If d2P/ 0.5 pm.Withoutthe gas, therequired y isapproximately d u d a is the spontaneous emissionperunit solid angle and three times as large and the gain for a fixed length is reduced unit bandwidth, the amount of spontaneous power that starts by over two orders of magnitude. up oscillation is For Case B, X,, B o , and y are given, i.e., the wiggler characteristics andtheelectron beamenergy,arefixed.Equation (10) shows that A/A, will be smaller than in the n = 1 case, thus allowing for operation at a shorter wavelength. In addition, (24) also indicates that the gain per unit length at the shorterwavelength is higher due to the factor (1 - ( n - 1) (2y2)/(1 t a;)] - 1inthe gain expression.Table I shows (20) an example for this case and it is seen that for the specified parameter values theadditionofthe gas reduces the waveN is calculated by noting that Mat is the output power at length and increases the gain per unit length by a factor of saturation and therefore should satisfy 12. However, there will be a reduction inusablelengthin the gFELas a result of scattering induced emittance. Finally for Case C, a,, X, and y are given, and (25) shows that thegain per unit length is the same forboth devices. HowThe oscillation build-up time is defined here as the time necesever, (10) yieldsa value for A, which is larger inthe gassary t o reach saturation. Thus it is given by loaded case, which,in turn, impliesasmaller wiggler field rb = NL, (22) as seen from

after N round-trips, where GiFEL is the excess gain per pass (gain minus output and losses). Equation (18) therefore gives the laser bandwidth. The solid angle of emission of a laser is

where L , is the totalcavity length. As one can see from the above discussion all the quantities of interest in designing a free-electron laser can be written in a form where the index of refraction does not appear, leading to the same expressions for both the gas-loaded FEL and the vacuum FEL. However, the numerical values for these quantities change vastly due to the different range allowed for a,, y,A, and A, by the new phase-matching condition. This will be illustrated in the next paragraph.

a,,, = 0.0933 Bo (kc) X, (cm).

(26)

An example of this case is alsogiven in Table I, where it is shown that to achieve the same gain without the gas there is approximatelyafactorof 10 decreasein X,, and acorrespondingincreasein B. Inaddition,the B-fieldtransverse variation for aperiodicarray of magnetsisexp(-2nx/h,,,) where x is the transverse dimension, so that for magnets positioned outside a 5 mm vacuum chamber the fields needed to produce 33 kG and 3.5 kG on axisascalled for inTableI SECTION11: ADVANTAGES OF ADDINGA GAS are 4500 kG and 5.8 kG, respectively.Clearly the first of There are a number of ways to see the effect of the new these values is unrealizable. phase-matching condition on the performance of the FEL by Thus, in Cases A and B the gains per unit length are higher writing thesmall signal gainin three different forms,follows. as in the gas-loaded case and appreciably so when h/A, l

5 kG; X

n-1

= 0.5 pm; I o / A ]

86

243

7

0.24

G(%)/L3(m)

Case 0 : X,

=

1 . 2 5 cm; B o

1 0 0 A/cmZ

= 1.400~10'~

43

=

1 0 kc; 7

= SO;

= 100

I,,/A,

A

/

~

n-1 = 1 . 6 9 1 ~ 1 0 ' ~

X(pm1

GC%1/L3(m)

Case C :

2.3

0.2

13

153

a,, = 1; Y = 80; X = 0.5 p m ; r a / A i G/L3

= 38.1

= 100

A/cmz;

%/m3

n-1 = 1 . 4 0 0 ~ 1 0 ' ~

X,(cml B,(kCl

0.32 33

thus

~

Z

3.1

3.5

where E is the particle energy in megaelectronvolts and X . is the radiation length in the medium. Hydrogen provides the in Case C; for a givengain a morepractical wiggler canbe least scattering for a specified index y1, and thus we consider used. hydrogen for the gas. For E = 50 MeV electrons, for example, Another way to see the advantage of the gas-loaded FEL is Os after traversing 30 cm of Hz gas at STP is 1.4 mrad. The to note from the synchronism condition that if we choose a corresponding phase-slippage is wiggler of characteristics X, = 2.5 cm, Bo = 5 kG, and y = 86, the vacuum FEL wouldgenerate 4 ,um radiationwhereas if loaded with 0.9 atm of hydrogen at 293 K it would generate 0.25 pm radiation. To generate 0.25 pm radiation in vacuum r 2 with the same wiggler parameters one would have to raise the electron energy to 175 MeV. In this section the advantages of a gas have been considered, and in Section I11 the disadvantages Allowing for a phase-slippage due to scatteringduringthe are presented. interactionof n/2 leads toanapproximateformulaforthe SECTION111: DRAWBACKS allowed length In the same region of space simultaneously present are an electronbeam,astrongstaticmagneticfield,a gas, and a potentiallyintenseelectromagnetic wave. The gaseous mediumcannot be modeledby a single quantity,itsindexof where T, the gas temperature, is taken as 293 K. Examples refraction.Variousotherphysicalphenomenamightoccur ofvaluesof L calculated from(29)for 1.0 atm ofgas are which must be taken into account. The electron beam suffers given in Table 11. Thus,thetotallength L is limited.However it hasbeen collisions with the molecules of the medium and this results in angular scattering of the electron beam, energy spread, and shown [6] , [I 81, in the case of the vacuum FEL, that by energy loss. Ionization of the gas, both by the electron beam using smaller beam areas and by the increase in current made andtheopticalbeam,createsfree chargesin the gas that possible byan increase inemittance,the small signal gain couldbemadeindependent of the laser lengthwhilethe might modify the focusing properties of the electron beam. efficiency and power output scaled,respectively,as theinThe index of refractionandthereforethesynchronismcondition might bemodifiedduringoscillationbuild-updue to verse and inverse cube of thelength withinlimits.Similar considerations will beused here t o obtain high gains and gas ionization,densityfluctuations,and nonlinear optics

FAUCHET e t al.: VISIBLE ULTRAVIOLET AND

GENERATION RADIATION

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imately given bythe energy loss divided bytheionization energy, i.e., 50 keV/m/lS ev/ionization event 'y 3000 ionizationevents/m/electron.Forapeakcurrent of 2500A/cm2 I n t e r a c t i o n Length Interaction L e n g t h in a 5 ps pulseat 350 psintervals(S-bandaccelerator)this 4 f o r X-lwm f o r X=O.Swm yields a rate dneldt = 7 X 1021/cm3/s. The equilibrium plasma 20 c m 28 cm 100 density will be determined by three body recombination pro8 m 11 m 4000 cesses. Taking the recombination coefficient to be a = 1.6 X lo-? cm3/s for H2 at STP and setting dneldt = an: yields an equilibrium density ne = 2 X 1014/cm3. This is almost three orders of magnitude greater than the electron density in the output power fromthe gas-loaded FEL. In addition,the gas-loaded FEL benefitsfromtheotheradvantages of the beam. However, its effect upon refractive index at the optical short FEL's: absence of pulse slippage effects, relaxedmechan- frequencies of interest is only of order one part in tenmillion. Potential instability mechanisms associated with a relativistic icalalignment andstabilityconstraints,shortbuild-uptime, been considered butarenot and lower wiggler cost. Thelatter is reduced both by the beamtraversingaplasmahave significantbecauseof theshort pulselength(1.5mmfor shortness of the device and less demanding values forthe magnetic field strength andthe wiggler period. 5 PSI. We next consider stability requirements on the gas density. If weconsiderelectronenergiesbelow the criticalenergy Index of refraction deviations from the value given by (10) [Ec = (400/2) MeV] ionization is themaincauseofmeanwill cause phase-slippage of the electrons with respect to the energy loss. From Evans [19]the mean-energylosscanbe optical wave. Allowing for a phase-slippage of 71 gives a limitaestimated to be tionof 2 X on An/n orabout 1 percentonthefractional change in index of refraction A(n - l)/n - 1 for 0.5 pm radiadE (MeVlcm) 0 . 3 ~ k n Y3/2] f (30) tion and 14 cm ofinteractionlengthinH2at 1.0 atm and dL A 293 K. Thisimpliesapressurestability andatemperature where p is the density of the gas in g/cm3, 2 is its atomic stability of 1 percent over the about 10 ps of the macrobunch, number, A is its atomic weight, and I is the geometric-mean both easily achieved. At higher duty cycles the thermal effects ionization and excitation potential in megaelectronvolts. For of the small energy loss can causetemperature gradients across 1.O atm of hydrogen at273 K, 50 MeV electrons, andI 2 15 eV the gas. The effect of the refractive index profile in the gas on we find dE/dL N 48 keV/m. For 1 m of gas the mean-energy the optical mode will be equivalent to a lens and may be comwould therefore amount to a reduction of 0.1 percent of the pensated by a change in mirror curvature. The effect upon the initial energy. This number is t o be compared to the intrinsic electronbeam will be averaged out by the wiggler motion. energy spread on the electron beam and the energy acceptance Finally the nonlinear index of refraction, due to the presence of the FEL, those two quantitiesusually being equalby design. of the electromagnetic wave in the gas, is much less than 6 X For the examples given in Table 111, AE/E is around 0.5 perfor hydrogen atSTP. cent; so that the mean-energylossdue to scattering is thus Among the phenomena that could deplete the optical wave, gas breakdown and self-focusing have a higher threshold than negligible. For any design where 0.1 percentwouldbenonnegligible compared to the energy acceptance of the laser, the stimulated scattering phenomena. In stimulated Brillouin scatmean-energy loss couldbe compensated by tapering the wiggler tering the incident optical wave scatters on the density flucor the index of refraction of the gas. The latter technique will tuations it createsin the medium. A fraction of the optical be discussed at the end of this paper. beam is thus reflected by the medium.However, this phenomEnergy spread (or straggling) is also introduced on the beam enon is usually not observed with picosecond optical pulses, both by ionization and radiative losses, the former being once since the incident and scattered pulses are counterpropagating, again dominant below the critical energy. From Landau [213 whichreduces theinteractionlength to afewmillimeters. it can be estimated to be Stimulated Raman scattering remains, which has a particularly large cross-section in hydrogen. However,the single-pass Stokes dAE 22 power produced at saturation is at most 6.5 mW for the 0.5 - (MeV/m) = 15.4 p dL EA pmlaser, the parameters of whichare given inTable 111, whereas the saturation power is about 5 MW leading to a where 22 is the sum of the atomic numbers in a molecule of percentdepletion of theoptical waveif nofeedbackexists the medium and Z A is the sum of the atomic weights. For an at the Stokes wavelength, equal to 0.63 ym for a pump waveatmosphere of hydrogen at 273 K,A E is 1.4 keV/m. This is length of 6.5 pm. Due t o the large shift there will be no diffinegligible compared to the intrinsic energy spread on the en- culty in preventing feedback at the Stokes line. tering beam, so that the energy spread introduced by the gas We thus have shown that angular scattering of the electron can be ignored. beamby the gas molecules is theonly significantdrawback To estimate the effect of the gas ionization on the electron of the gas-loaded free-electron laser. It has to be taken into beam [22] oneneeds to consider itstimestructure.The account in the design of a device, as will be shown in the next microbunches ionize the gas along their path, at a rate approx- section. TABLE I1 INTERACTION LENGTHS FOR THE GAS-LOADED FEL

[-?

$1

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-20, NO. 12, DECEMBER 1984

SECTIONIV: EXPERIMENTAL PARAMETERS Our interest lies in the generation of visible and UV radiation using a medium energy (up to 50 MeV) accelerator. This upper limit on the electron beam energy is chosen so that the whole device (acceleratorand FEL) can be packaged into a compact, commercial unit of the size and cost of a medical linac. The accelerator characteristics are assumed given: the peak current I o , the energy y, the emittance of the electron beam E defined as E

In the gain expression the remaining unknown is L3/A1. Several constraints bear on L and A1. The electron beam will be chosen narrower than the optical beam here. The overlap condition we have chosen is that the electron beam area be equal to the optical mode effective area at both ends of the interaction region (neglecting the emittance growth due to the gas)

Al(O) i.e.,

= nwe@e

where w e is the radius of the electron beam at its focus, @e is its intrinsicangular divergence (half-angle); the intrinsicenergy spread AEIE; the time structure of the electronbeam.The desired wavelength of oscillation h is also assumed chosen. It will beshownhereafterthat gain, efficiency,output power, and build-up time can be written in terms ofgas transsure p , cavity length L C , and cavity losses (i.e., mirror transmission) a. Gas temperature T is also aparameter but will be taken as 293 K hereafter. The Sellmeier equation for H2 gives n - 1 as a function of p and h23 13655.3 111 - [ l/h(pm)]

1.

(33)

In the region of interest h/h,