110

Nuclear Instruments and Methods in Physics Research A296 (1990) 110-114North-Holland

A REVIEW OF THE STANFORD MARK III INFRARED FEL PROGRAM

Stephen V. BENSON, Wun Shain FANN, Brett A. HOOPER, John M.J. MADEY and Eric B. SZARMESDepartment of Physics, Duke University, Durham, NC 27706, USA

Bruce RICHMANDepartment of Applied Physics, Stanford University, Stanford, CA 94305, USA

Louis VINTROWharton School of Business, Philadelphia, PA 19104, USA

The performance of the Mark III infrared FEL with a new microwave gun will be reviewed. Operation of the accelerator is nowclose to design values. The Mark III has provided over 2000 hours of laser time to experiments in FEL physics, materials science andmedical physics. Highlights of the experimental program will be presented and the new facility at Duke will be described .

l . Introduction

The Mark III infrared free-electron laser (FEL) wasconceived as a laboratory-scale medium-power laserwhich could be used for studies of medical sciences,material sciences and FEL physics . The basic design ofthe laser/accelerator system has been describedelsewhere [1, 2] . Initial operation of the laser benefitedfrom excellent electron beam brightness and good en-ergy spread . Peak power and gain in agreement withdesign values were achieved . The energy per macro-pulse, the repetition rate and the wavelength tuningrange were all less than design values, however, due tolimitations in two components in the accelerator . Theaverage power was quite sufficient, however, for manyexperiments in medical and materials sciences as well asseveral experiments in FEL physics . Many of the resultsof these studies are detailed in these Proceedings . Beforethe shutdown of the Mark III and subsequent move toDuke University, several improvements were made tothe accelerator system. This paper will discuss the per-formance improvements resulting from c langes in thegun and microwave power distribution system whichhave raised performance almost to design levels . It willthen summarize some of the results of the user programand finally will describe the new installation planned atDuke University .

2. Machine development results

la the first two years of operation at the StanfordPhoton Research Laboratory the Mark ill performance

Elsevier Science Publishers B.V . (North-Rolland)

was limited by two features of the accelerator . The firstlimitation was that the fixed coupler which providedpower to the microwave gun limited the minimum en-ergy of the accelerator to about 35 MeV. This minimumenergy is determined by the power level required toachieve the operating gradient in the microwave gun.One cannot reduce the gradient in the acceleratorwithout reducing the power to the gun below thisminimum level . This problem was easily solved by mak-ing a separate coupler which coupled more power to thegun . This coupler was designed to provide an energyrange of 25 to 35 MeV while the high-energy couplerallowed access to beam energies from 35 to 45 MeV.This capability, combined with the gap tunability of thelaser, allows us to span the wavelength range of 2 to 8Rm with three different electron beam energies, as isshown in fig. 1 . Wavelengths shorter than 2 tLm wereaccessible by means of third-harmonic lasing or externalharmonic generation [3,4] . Changing the coupler andrestarting the laser requires about 2 to 3 h . Since this isan inconvenience in a user facility, we intend to developa variable coupler to provide access to the entire enei byrange within a few minutes.

The second and more serious problem was caused byheating of the cathode in the microwave gun caused byelectron back-bombardment. This limited the macro-pulse length and the repetition rate to 4 l .s and 15 Hz,respectively. The limitation on macropulse length is dueto the heating during the macropulse which causes aramp in the current out of the gun . The current rampmust be less than about 10% or the laser performancewill be degraded . The limit on repetition rate is due tothe average heating of the cathode, which can become

Fig. 1 . Calculated gain vs wavelength for three different elec-tron beam energies. Micropulse lengths were chosen to optimize average power extraction and gain . Parameters are other-

wise as listed in table 1 .

comparable to the power supplied to the cathode by theheater for high repetition rates . The feedback systemwhich maintains a constant gun current cannot operate

stably when this condition arises.The details of the back-bombardment effect are de-

scribed in more detail in another paper in these Pro-

ceedings [5] . Reduction of this effect requires the reduc-

tion of the number of back-bombarding electrons, the

optimization of cathode emissivity and the optimization

of the feedback system controlling the cathode tempera-

ture . Significant reduction in the back-heating power

was achieved by creating a transverse magnetic field in

the cavity, thereby deflecting a fraction of back-

bombarding electrons . However, the magnitude of this

field, and hence the reduction in back-bombardment

power, was limited by the size of the exit aperture of the

gun .The average back-heating problem was alleviated

somewhat by changing the cathode button mount in

order to improve its thermal emissivity . A substantial

further reduction in back-heating was achieved by re-

placing the microwave gun cavity with a new cavity

which has a slotted exit nosepiece. This allows one to

apply a stronger magnetic field at the cathode and

further reduce the self-heating without losing current

from the beam. The magnetic field strength is now

limited by the ability to steer the beam back on axis. In

order to correct the position and angle of the eler.-ironbeam evasmiutted from. the CQun_ it is necessarv to have twobeam.

wv. ...+.~~ the

dipoles after the gun . We replaced the first quadrupole

after the gun with a dipole and used a corrector on the

second quadrupole in place of a second dipole;. This

arrangement does not allow as strong a field at the

cathode as we would like. If the injector is rearranged to

allow two dipoles and two quadrupoles before the alpha

magnet and two quadrupoles after the alpha magnet,

the magnetic field could be increased to a level which

S. V. Benson et al. / The Stanford Mark 111 infrared FEL program

would eliminate most of the back-bombardment. Thisdesign will be used in the FEL now being built atVanderbilt University and will eventually be used onthe Mark III . With the present arrangement, it is stillpossible to increase the magnetic field over that possiblewith the old gun and raise the total duty cycle of thegun by a factor of 2.5 . This means that one can nowoperate with a 10 [ts macropulse at 15 Hz or a 5 psmacropulse at 30 Hz. The new parameters for theelectron accelerator and laser are listed in table 1 .

The new gun cavity also incorporates two otherchanges which have enhanced its performance. Thecathode nosepiece is now machined out of a single pieceof copper. This design has eliminated a problem in theold gun of poor thermal contact between the nosepeiceand the rest of the gun cavity . The poor thermal contactresulted in a strong dependence in the cavity frequencywith input power. The improved thermal design hasreduced this dependence so that the system can now berestarted from a brief shutdown in a couple of minutes .The time from the interlock enable to lasing is typicallyless than a minute . The time before the gun is stabilizedat running temperature is usually about 5 min. Wefound this to be extremely valuable in machine physicsstudies where frequent changes to the machine config-uration are made.

The interior surface of the gun is now diamondturned . This has raised the threshold far rf breakdownby approximately 50`x . As part of the accelerator start-

1 . EXISTING EXPERIMENTS

Table 1Mark III operating parameters

Electron beamMacropulse length 9 [LsMicropulse length 2-3 psMicropulse repetition rate 2857 MHzBeam energy 26-45 MeVMacropulse energy spread 0.7%Average current 100-200 mAPeak current 20-40 AHorizontal emittance (BYE) 10îr mmmradVertical emittance (BYE) 4ir mm mradMaximum repetition rate 30 Hz

LaserDemonstrated tuning range 1.4-8 .1 pmGain 20-100RßF_nergy per pulse 1--200 m1Repetition rate single shot - 30 HzPeak power 0.5-2 MWMacropulse length 0.5-8 [Ls

Micropulse length 0.5-3 .0 psSpectral bandwidth 0.5-1.0% FWHMStrehl ratio > 0.8Optical cavity length 184 cmRayleigh range 73 cm

3

300.

â

S. V.

Fig . 2 . Calculated limits on macropulse energy and peak powerfor the Mark III with the new microwave gun . Sharp dropoffin energy at wavelengths shorter than 4 tLm is due to mirror

damage limitations.

up procedure, the gun is processed up to 1509 ofoperating gradient at full pulse length . This reduces thelikelihood of arcing in subsequent operations . The timefor this procedure has been reduced from over 1 hour toless than 30 min .

With the longer pulses available from the accelera-tor, one might expect much higher pulse energies fromthe Mark III laser. We have in fact operated the laserwith 250 mJ of outcoupled energy per pulse at 3.8 Rm.The laser macropulse was 8 ps long . At shorter wave-lengths, however, we were incapable of operating atenergies this high due to mirror damage limitations .This is shown in fig . 2 . In this figure, we have plottedthe theoretical maximum for the energy and peak poweroutput vs wavelength from the Mark III operated at 15Hz. To produce this curare, the efii,iency was assumedto be equal to the 112N limit and the undulator param-eter and beam energy were chosen to maximize theoutput power. The length of the electron beam macro-pulse was assumed to be 9 l.s which is the flat top of themodulator pulse forming network . The absorption and

2-'

Wavelength(gm)

-ß- Peak Power-"- Pulse Energy

Benson et al. / The Stanford Mark 111 infrared FELprogram

0.6

WU

0.2

l

3

5

7

ßWavelength(Etm)

Fig. 3 . Calculated limits on the macropulse energy and peakpower for an optical cavity 2.05 m in length with a Raleigh

range of 53 em .

scatter of the mirrors was assumed to be 2% and thetotal cavity losses were assumed to be one fourth of thesmall-signal gain . If the fluence on the mirrors wasgreater than the damage threshold for our mirrors (50Jcm-2 ~ts-1/2), the macropulse length was reduced un-til the fluence was less than the damage threshold . Theenergy vs wavelength at wavelengths longer than about4 lim was limited by the energy of the electron beam. Atwavelengths shorter than 4 lim, the energy was limitedby optical damage of the mirrors . As can be seen fromthe figure, the peak power increases with energy butdecreases as the gain decreases at short wavelength . It ispossible to operate briefly above the curve in fig. 2, butthe mirrors are eventually damaged . The actual. maxi-mum achieved from the laser depends on many factors .It is typically one half to two thirds of the 1/2N limit .With a strong sideband, it can equal or exceed thislimit.

It is possible to get more energy at shorter wave-lengths by either increasing the optical damage thresholdor by increasing the optical-mode size at the mirrors .We intend to study both alternatives at Duke. Onepossibility for a higher damage threshold material isdiamond-turned gold . We have fabricated copper mir-rors with an imbedded gold slug 1 mm thick on whichthe optical surface is diamond-turned. Damagethresholds for these mirrors and other vendor-suppliedmirrors will be determined at Duke . The cavity lengthand Rayleigh range will also be changed. The cavitylength will be lengthened by two rf periods to 205 cmand the Rayleigh range will be reduced to 53 cm. Theexpected energy and power for this cavity configurationis shown in fig . 3 for a damage threshold equal to themirrors used at Stanford . We should be able to operatedown to 3 ~Lm with little possibility of damaging themirrors .

Several projects were undertaken to extend the lasercapabilities and to study the physics of FELs. Some ofthese are reported in these Proceedings. The Mark IIIand the Rockwell system were operated in a MOPAconfiguration in order to study the physics issues insuch a device and to benchmark simulation codes [6] .The Mark III accelerator was operated in a photo-cathode mode and used to drive the Rockwell laser [7] .The use of intracavity elements to modulate the loss ofthe optical cavity and to dump the stored energy wasstudied [8] . The characteristics of the coherent harmon-ics emitted from the laser were studied [9] . Finally, themacropulse structure was studied using autocorrelationtechniques [10] .

3 . User program

Though flexible, continuously tunable sources existin the visible from 1 [tm to about 250 nm, the availabil-

3 0 .5

0 .4

2'.. 0.3v30a. 0.2

1~a.

-"-Pulse Energy -0 .1---*--Peak Power

0 ~ 0.00 2 4 b 8 10

ity of tunable sources in the infrared is quite limited .Picosecond dye lasers can be shifted down to the in-frared with quite high peak powers but quite pooraverage power and stability . Further, the 30 to 100 pspulses are too long for probing most chemical transi-tions . Nanosecond pulses can also be shifted withslightly more average power but the pulse structure andenergy per pulse is not adequate for medical applica-tions . The Mark III has therefore been used extensivelyas a research facility for applications of free-electron-laser light in the infrared. Approximately 600 h of beamtime were devoted to applications in materials science,chemistry and medicine in 1988 . Two applications ofthe Mark III to non-linear spectroscopy of poly-acetylene and acetanilide are reported in these Proceed-ings by Fann et al . [111 . Efforts to extend the useablewavelength range via harmonic generation in crystalsand applications of the shorter-wavelength light aredescribed by Hooper et al. [4] . In addition, a broadrange of experiments were performed in the field oflaser-tissue interactions by the University of Utah. Agood summary of the biomedical research is given inref. [121 .

Both hard and soft tissue were cut both in vitro andin vivo . Quantitative studies were carried out to char-acterize the cutting efficiency as a function of fluenteand wavelength. It was found that there was an opti-mum fluente for cutting which was just above thethreshold for plasma formation . Too high or low afluente reduced the cutting efficiency. The optimumcutting wavelength was not at the peak in the waterabsorption curve as one might expect but at a slightlylonger wavelength .

Spectrographic studies were made of the plasmaformation in tissue and other materials in order tocharacterize the nature of the laser-tissue interaction .

Gallstone ablation was demonstrated and was foundto be acoustic in nature . The plasma formed at thesurface of the gallstone by the laser pulse was found to

S. V. Benson et aL / The Stanford Mark III infrared FELprogram

create a shock wave which caused the stone to fragment .The fragmentation occurred only when the stone wassubmerged in water or some other solvent .

Finally, optical-fiber propagation was studied usingboth sapphire and zirconium fluoride fibers. Sapphire,which has excellent physical pr .°perties for medical ap-plications, performed poorly due to fiber imperfections.We were able to pass over 70 mJ per pulse through azirconium fluoride fiber with nearly optimum transmis-sion, but the fiber became extremely brittle and doesnot seem attractive for medical applications .

4 . Duke university FEL facility

The Mark III components have now been moved toDuke University in North Carolina. They are beingassembled in a former Van de Graaff vault in thephysics building . The layout of the new lab is shown infig . 4 . The system will be more compact than at Stan-ford and will have additional shielding to allow oper-ation at high average power with less risk to personnel .The most important difference, however, will be thenew control system . Diagnostics will be installed whichwill allow the control computer to monitor and correctthe state of the machine . All machine functions will becomputer-controlled . With comprehensive diagnosticscoupled to machine control, closed-loop operation willbe possible . The control system will also be able to takedata on the individual elements in order to develop anappropriate model of the accelerator . The eventual goalis to have a control system incorporating artificial intel-ligence in order to allow flexible setup and control of asystem with changing characteristics or to allow easysetup to machine configurations which have never beenused before .

Oncc the Mark III is operating again at Duke Uni-versity, we intend to continue studies in materials scien-ces, medical sciences and machine physics . Work in the

Control RoomFig . 4 . Layout of the Mark III planned for Lhe Duke University Physics Department building .

I . EXISTING EXPERIMENTS

last field will concentrate on use of the optical cavity tocontrol the nature of the lasing action in the FEL.

Acknowledgernents

We would like to acknowledge the staff of the Stan-ford Photon Research Lab which helped to make thisproject a success . Brian Burdick and James Haydonwere responsible for machine operations, maintenanceand upgrades . Mark Emamian was responsible for all ofthe design work for machine upgrades, with the excep-tion of the microwave gun, which was built and char-acterized by Marcel Marc. This work was funded byLos Alamos National Labs contract 9XFH1726G1,Army Research Office contract DAAG29-84K-0144 andby Navy contract N00014-86K-0823.

References

[1] S.V. Benson, J.M.J . Madey, J . Schultz, M . Marc, W.Wadensweiler and G.A. Westenskow, Nucl . Instr. andMeth. A250 (1986) 39.

S. V. Benson et al. / The Stanford Mark III infrared FEL program

[2] S.V. Benson, J . Schultz, B.A. Hooper, R. Crane and J.M.J .Madey, Nucl. Instr . and Meth . A272 (1988) 22 .S.V . Benson and J.M.J . Madey, Phys. Rev. A39 (1989)1579 .B.A. Hooper, S.V . Benson, R.C . Straight and J.M.J .Madey, these Proceedings (11th Int . Free Electron LaserConf ., Naples, FL, USA, 1989) Nucl . Instr . and Meth .A296 (1990) 797.

[5] C.B. McKee and J.M.J . Madey, ibid., p . 716.[6] A. Bhowmik, M.S. Curtin, W.A. McMullin, S.V. Benson,

J.M.J . Madey, B.A . Richman and L . Vintro, ibid ., p . 20 .[7] M . Curtin, G . Bennett, R. Burke, S . Benson and J.M.J .

Madey, ibid ., p . 69 .[8] S.V . Benson, J.M.J . Madey, E.B. Szarmes, A . Bhowmik, J .

Brown, P . Metty and M.S. Curtin, ibid., p. 762.D.J. Bamford and D.A.G . Deacon, Phys. Rev . Lett . 62(1989) 1106 .

[10] B. Richman, J.M.J . Madey and E.B . Szarmes, First ob-servation of spiking behavior in the time domain in afree-electron laser, Phys . Rev. Lett . 63 (1989) 1682.

[11] W.S. Fann et al ., Proceedings (11th Int. Free ElectronLaser Conf., Naples, FL, USA, 1989) Nucl. Instr. andMeth . A296 (1990) 804.

[12] S. Benson, J.M.J . Madey, R. Straight and B.A . Hooper, J.Laser Appl. 1 (1989) 49.

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