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The UCLA IR FEL project
Nuclear Instruments and Methods in Physics Research A 331 (1993) 228-231 North-Holland
NUCLEAR iNSTRUMENTS & METHODS tN PHYSICS RESEARCH Section A
T h e U C L A I R F E L project * G. S. N. P.
B a r a n o v a, N. B a r o v b p. Davis c, M. F a u v e r b B. Gitter b, G. H a i r a p e t i a n ~, H a r t m a n b , M. H o g a n b , S. I v a n c h e n k o v d , C. Joshi c , A. K h l e b n i k o v d , P. K w o k b,b L u h m a n n Jr. c, S. P a r k b, C. Pellegrini b, j. R o s e n z w e i g b, K. S c h e n k b, j. Smolin , T r a n b G. Travish b and A. V a r f o l o m e e v d
a D.U. Efremov Scientific Research Institute of Electrophysical Apparatus, St. Petersburg, Russian Federation b Particle Beam Physics Laboratory, Department of Physics, University of California, Los Angeles, CA 90024, USA c Department of Electrical Engineering, University of California, Los Angeles, CA 90024, USA Laboratory of Coherent Radiation, Russian Science Center, L V.. Kurchatov Institute, Moscow, Russian Federation
A 10.6 ~m free electron laser (FEL) operating in the high gain regime is under construction at UCLA. FEL physics significant to future short wavelength operation will be emphasized including optical guiding, superradiance, saturation and self-amplified spontaneous emission (SASE). A 5 MeV rf photocathode gun illuminated by a UV laser will supply a high brightness electron beam which will be injected into a plane wave transformer (PWT) linac for acceleration to 20 MeV. Recent measurements of the gun emittance as well as quantum efficiency are presented. The undulator is of a modified hybrid design producing ~ 7.5 kG peak field on axis with 5 mm gap spacing and 1.5 cm pole period. Simulation results which include three-dimensional effects are furnished. The present status and future plans of the project are summarized.
1. Overview The Particle B e a m Physics Laboratory at U C L A is constructing a short period F E L which will operate in the high gain regime. The focus of the work thus far has been the production of a high brightness electron beam suitable for this and future FELs. The next stage of the project will concern itself with F E L physics; of primary interest are issues of significance to future short wavelength devices. P h e n o m e n a to be studied include self-amplified spontaneous emission (SASE), superradiance, optical (refractive) guiding and saturation. A compact high brightness electron source is currently being tested at the U C L A D e p a r t m e n t of Physics [1,2]. A n S-band rf gun employing a copper photocathode produces a 4.5 M e V high brightness beam [3]. The copper photocathode, while easy to handle and robust, requires illumination by a U V laser. Short (2-50 ps) pulses of U V (266 nm) are produced using a frequency quadrupled N d : Y A G laser which is pulse compressed [4]. A 5 M e V test stand complete with b e a m diagnostics is currently operating. Emittance and quantum efficiency measurements of the gun are presented in the next section. A drawing of the test beamline is given in fig. 1. * Work supported by US DOE Grant No. DOE-DE-FG0392ER40693.
The complete beamline including the plane wave transformer linac [5], associated optics and experiment insertion region is shown in fig. 2. The undulator is complete and has been tuned and tested. Further details and simulation results are presented in subsequent sections. Conclusions and future plans are discussed in the final section.
2. The 5 MeV test stand High brightness photocathode guns have great potential as beam sources for F E L s [6]. Characterizing the performance of these devices has been a priority at a number of institutes. Recent measurements of the U C L A gun have yielded fruitfu! results. Dark current, emittance and quantum efficiency have been measured. These investigations were carried out on a 5 M e V beamline consisting of the gun, quadrupole triplet, dipole bending magnet and various b e a m diagnostics. The present operating parameters are given in table 1. The U C L A S-band rf copper photocathode gun has been described in detail eIsewhere [7]. Briefly, the gun is a one and a half cell structure with a high peak accelerating field ( ~ 100 M V / m ) . Solenoidal focusing is used to control the highly divergent b e a m after it exits the gun. Dark current is an issue in this and other photocathode guns due to the high fields used to
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
G. Baranov et aI. / The UCLA IR FEL project Table 1 Measured operating parameters for the UCLA 5 MeV test stand. The system is operated with single bunches at a repetition rate up to 5 Hz Electron beam (measured) Electron beam energy Peak charge Pulse duration (rms) Normalized emittance (rms)
229
Dark Current Spectrum 10
"G ;">-
0
8
0 0
56
_<5 MeV ~ 1 nC ~ 50 ps ~ 12-rr mm mrad
0 ®
®0
C
4 o•
=o
O
o o
2
"o
control space charge derived emittance growth. P r o p e r gun conditioning can help r e d u c e dark current, and m e a s u r e m e n t s of the dark c u r r e n t s p e c t r u m are necessary to quantify the problem. The dark current spect r u m as m e a s u r e d for the U C L A gun after conditioning is given in fig. 3. U s i n g the scanning q u a d r u p o l e t e c h n i q u e the normalized rms e m i t t a n c e of ~ 50 ps (rms) b e a m s was m e a s u r e d to b e ~ 12~ m m mrad. T h e m e a s u r e d emittance is e x p e c t e d to go down substantially w h e n shorter pulses ( ~ 2 ps rms) are used. Simulations using the P A R M E L A code predict an emittance of 5 ~ m m m r a d for the 2 ps case [8]. E m i t t a n c e m e a s u r e m e n t s w e r e
0ooo °o0
~,
,°'~ ....
1.5
1
2
2.5
(MeV/c)
P
3
3,5
Fig. 3. The dark current energy spectrum for the UCLA gun.
m a d e with a 3 M e V b e a m to avoid complications by dark current. The choice of c a t h o d e materials has received m u c h attention in the literature [9]. While materials such as CsSb can offer high q u a n t u m efficiency ( Q E ) and ease of manufacturing, they require high vacuum ( ~ 10-11 Torr) and have a limited lifetime. O n the o t h e r hand,
FARADAY I CUP
STRIPUNE ] 5 MeV RF GUN
DIPOLE BENDING MAGNET
MONITOR I
SOLENO, D I
'
'
~.~
I
| ~,~j~'
ILASER COUPLING
'
t
~
__
_ _
ADDITIONAL DIAGNOSTICS
Fig. 1. The UCLA 5 MeV test stand beamline showing the gun, magnets and diagnostics. The laser mirror box is used to 111 " _ umlnate the cathode on axis ( ~ 3°) while ports on the gun allow for 70° illumination.
5 MeV RF GUN I LASER COUPLING
20 MeV LINAC I
DIPOLE
PHOSPHOR SCREEN
I
ISTRIPUNE / MONITOR
"~_.~
DIAGNOSTICBEAMLINE
-
~
Fig. 2. The full UCLA FEL project beamline showing the gun, linac and experiment inertion area. The second beamline will be used for diagnostics and other experiments. V. PROJECTS/PROPOSALS
230
G. Baranov et a L / The UCLA IR FEL project
simple metals such as copper are inexpensive, easy to work with and rugged, but have poor quantum efficiency ( ~ 10-5). Since copper cathodes require expensive U V laser illumination, enhancement of the quantum efficiency is desirable. Currently, the U C L A gun utilizes a removable unpolished copper cathode. The test stand features two sets of laser ports; one on axis (perpendicular to the cathode) another at 70 ° from the axis. Enhancement of the quantum efficiency by orders of magnitude was predicted for the off axis illumination [10]. Preliminary measurements show a QE of 2.7 × 10 .5 on axis and 10.5 × 10 -5 at 70 ° which implies an enhancement ratio of 3.9. This ratio is substantially smaller than the one predicted by theory; however, these measurements are preliminary. In order to preserve the short pulse length ( ~ few ps), a scheme is now being devised to "tilt" the pulse appropriately. In this way, the impact of the laser pulse will be uniform in time across the face of the cathode. Because there are two laser ports (left and right) at 70 ° it is also possible to illuminate the cathode symmetrically. Finally, it has been reported that cathode surface finish affects photoelectron production [11]. Cathodes with various finishes (i.e. polished, grated, etc.) are currently being produced and will be tested soon.
3. The 20 MeV beamline and undulator Once characterization of the gun and beam are finished, a 20 MeV beamline including the PWT linac will be assembled (see fig. 2). The main beamline as well as a second beamline will contain various beam diagnostics: strip-line monitors, phosphor screens, Cherenkov and optical transition radiation detectors, Faraday cups, etc. Successful operation of the complete beamline will lead to undulator insertion and FEL operation. A novel high performance undulator has been constructed for U C L A by the collaborators at the Kurchatov Institute. The design utilizes N d - F e - B blocks in a hybrid configuration with vanadium permedure yokes. In addition, SmCo s blocks placed between the yokes enhance the pole tip field. The samarium cobalt blocks counter the flux from the N d - F e - B magnets within the yokes. Thus, saturation of the yoke is reduced. At the same time, the blocks enhance the flux within the pole gaps. In this way a higher field is produced than possible with a conventional hybrid design [12]. The linear undulator is 60 cm long with a 1.5 cm period, 5 mm fixed gap spacing and 7.5 kG peak field. The mechanical assembly allows for precise tuning of the field: a field uniformity better than _+0.25% has been achieved [13]. The second integral of the undulator field (along the axis) shows that the electron beam
Table 2 Design parameters for the UCLA FEL. Electron beam figures are based on simulation results Electron beam (simulation) Electron beam energy Energy spead (rms) Peak current Pulse duration (rms) Normalized emittance (rms)
~ 20 MeV 0.1% 200 A ~ 2 ps ~ 5~ mm mrad
Undulator (measured) Undulator period Total length Pole face gap (fixed) Peak field on axis FEL parameter (p)
1.5 cm 60 cm 5 mm 7.5 kG ~ 1 × 10-2
deflection Ax = 150 ~m satisfies the requirement that it be less that the beam waist ~rx = 200 ~zm. The initial F EL will be configured as a single pass amplifier. Calculations indicate that the SASE signal will be weak and may be difficult to detect. A pulsed CO a laser will be used as a seed in order to study saturation effects and simplify detection. A T E A - C O 2 laser with peak power in the megawatt range and associated optics are being tested for this purpose. In order to preserve emittance, the electron beam will be sent directly through the undulator: no bends are present in the beamline before the undulator. This necessitates the use of a mirror with a hole in it to introduce the seed laser into the beamline. Numerical simulations have been used to determine the FEL's expected performance. Primarily, Tran and Wurtele's T D A code has been used [14]. Utilizing the parameters given in table 2, the code predicts an exponential power growth length of 8.7 cm. This corre-
Gain vs. Distance Along Undulator
1os 107 106 1o5
~
"~ 104 (.5
~
Present
1o3
FEL
102 , ~ 1 o1
10° 10-1 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1,8
2
Distance Along Undulator [m]
Fig. 4. A plot of the power gain as a function of undulator length for the UCLA FEL produced by the TDA code. Injection energy for this simulation was 19 MeV with a beam current of 200 A.
G. Baranov et a L / The UCLA IR FEL project
Table 3 Simulation results for the UCLA FEL from the TDA code. Parameters used are as }hose in table 2 FEL (simulation) Output radiation wavelength Power gain (e-folding) length SASE saturation length Peak power at saturnation SASE peak power output after 60 cm
10.6 I.~m 8.7 cm 160 cm ~ 50 MW ~ 1.7 kW
sponds to a gain of 103 over the undulator length. Saturation occurs at ~ 1.6 m with a peak power of ~ 50 MW. A plot of the gain as a function of distance down the undulator is given in fig. 4. Assuming a peak spontaneous noise level of 1.75 W, which corresponds to the radiation emitted in one gain length of the undulator, then the peak power at output is 1.7 KW. This low average power necessitates the use of cryogenic detectors. For this and other reasons it is likely that initial operation will be with the seed laser. A n input power of a few tens of kW is sufficient to saturate the FEL. Simulation results are summarized in table 3. Saturation cannot be achieved from S A S E with the present undulator. However, after suitable testing of the first undulator a second undulator will be obtained. Running as an optical klystron [15] the U C L A F E L will be able to achieve saturation (or high extraction efficiency) starting from SASE.
4. Conclusions and future work The U C L A F E L project has a viable high brightness source to be used in conjunction with a completed short period undulator. The quantum efficiency enh a n c e m e n t findings are of significance for high brightness sources in general and for F E L s in particular. The e n h a n c e m e n t implies that a lower power, and hence a less complex laser system, can be used. Alternatively, the additional laser power can be used to drive a higher current beam. O n c e operational, the F E L will be used to study p h e n o m e n a of significance to future short wavelength
231
devices. D u e to the lack of suitable mirrors, the oscillator configuration is presently impractical for short wavelengths. Hence, the U C L A device will operate as an amplifier in the high gain regime. The lack of suitable sources at various wavelengths makes the feasibility of startup from spontaneous emission (SASE mode) important [16]. The need to mitigate diffraction also makes the study of optical guiding significant. These phenomena, as well as saturation, superradiance and sidebands, will be studied at U C L A .
References [1] F. Aghamir et al., Nucl. Instr. and Meth. A304 (1991) 155. [2] J. Dodd et al., Nucl. Instr. and Meth. A318 (1992) 178: [3] J.S. Fraser and R.L. Sheffield, IEEE J. Quantum Electron QE-23 (1987) 1489. [4] P. Maine et al., IEEE J. Quantum Electron. QE-24 (2) (1988) 403. [5] D.A. Swenson, Europ. Particle Accelerator Conf., Rome, Italy, 1988, vol. 2, ed. S. Tazzari, p. 1418. [6] S. Hartman et al., IEEE 1991 Particle Accelerator Conf. Proc., p. 2967. [7] J.G. Davis, C.J. Josh} and C. Pellegrini, Intense Microwave and Particle Beams III, ed. H.E. Brandt (Elsevier). [8] L. Young and K.T.. McDonald, IEEE Trans. Electron Devices ED-35 (1988) 2052. [9] See, for instance, H. Riege, CERN, CLIC-NOTE 105 (1989). [10] K.L. Kliewer, Phys. Rev. B14 (1975) 1414. [11] J. Endriz, Stanford Electronics Lab. Tech. Rep. 5207-3 (1970). [12] K. Halbach, J. de Phys. Coll. (1983) CI-211. [13] Here undulator field uniformity is reported as the maximum deviation of the pole tip fields (on axis) from the mean pole tip field. [14] T.M. Tran and J.S. Wurtele, Computer Phys. Commun. 54 (1989) 263; and T.M. Trans and J.S. Wurtele, Phys. Rep. 195 (1990) 1. [15] J.C. Gallardo and C. Pellegrini, Opt. Commun. 77 (1990) 45. [16] C. Pellegrini et al., these Proceedings (14th Int. Free Electron Laser Conf., Kobe, Japan, 1992) Nucl. Instr. and Meth. A331 (1993) 223.
V. PROJECTS/PROPOSALS
NUCLEAR iNSTRUMENTS & METHODS tN PHYSICS RESEARCH Section A
T h e U C L A I R F E L project * G. S. N. P.
B a r a n o v a, N. B a r o v b p. Davis c, M. F a u v e r b B. Gitter b, G. H a i r a p e t i a n ~, H a r t m a n b , M. H o g a n b , S. I v a n c h e n k o v d , C. Joshi c , A. K h l e b n i k o v d , P. K w o k b,b L u h m a n n Jr. c, S. P a r k b, C. Pellegrini b, j. R o s e n z w e i g b, K. S c h e n k b, j. Smolin , T r a n b G. Travish b and A. V a r f o l o m e e v d
a D.U. Efremov Scientific Research Institute of Electrophysical Apparatus, St. Petersburg, Russian Federation b Particle Beam Physics Laboratory, Department of Physics, University of California, Los Angeles, CA 90024, USA c Department of Electrical Engineering, University of California, Los Angeles, CA 90024, USA Laboratory of Coherent Radiation, Russian Science Center, L V.. Kurchatov Institute, Moscow, Russian Federation
A 10.6 ~m free electron laser (FEL) operating in the high gain regime is under construction at UCLA. FEL physics significant to future short wavelength operation will be emphasized including optical guiding, superradiance, saturation and self-amplified spontaneous emission (SASE). A 5 MeV rf photocathode gun illuminated by a UV laser will supply a high brightness electron beam which will be injected into a plane wave transformer (PWT) linac for acceleration to 20 MeV. Recent measurements of the gun emittance as well as quantum efficiency are presented. The undulator is of a modified hybrid design producing ~ 7.5 kG peak field on axis with 5 mm gap spacing and 1.5 cm pole period. Simulation results which include three-dimensional effects are furnished. The present status and future plans of the project are summarized.
1. Overview The Particle B e a m Physics Laboratory at U C L A is constructing a short period F E L which will operate in the high gain regime. The focus of the work thus far has been the production of a high brightness electron beam suitable for this and future FELs. The next stage of the project will concern itself with F E L physics; of primary interest are issues of significance to future short wavelength devices. P h e n o m e n a to be studied include self-amplified spontaneous emission (SASE), superradiance, optical (refractive) guiding and saturation. A compact high brightness electron source is currently being tested at the U C L A D e p a r t m e n t of Physics [1,2]. A n S-band rf gun employing a copper photocathode produces a 4.5 M e V high brightness beam [3]. The copper photocathode, while easy to handle and robust, requires illumination by a U V laser. Short (2-50 ps) pulses of U V (266 nm) are produced using a frequency quadrupled N d : Y A G laser which is pulse compressed [4]. A 5 M e V test stand complete with b e a m diagnostics is currently operating. Emittance and quantum efficiency measurements of the gun are presented in the next section. A drawing of the test beamline is given in fig. 1. * Work supported by US DOE Grant No. DOE-DE-FG0392ER40693.
The complete beamline including the plane wave transformer linac [5], associated optics and experiment insertion region is shown in fig. 2. The undulator is complete and has been tuned and tested. Further details and simulation results are presented in subsequent sections. Conclusions and future plans are discussed in the final section.
2. The 5 MeV test stand High brightness photocathode guns have great potential as beam sources for F E L s [6]. Characterizing the performance of these devices has been a priority at a number of institutes. Recent measurements of the U C L A gun have yielded fruitfu! results. Dark current, emittance and quantum efficiency have been measured. These investigations were carried out on a 5 M e V beamline consisting of the gun, quadrupole triplet, dipole bending magnet and various b e a m diagnostics. The present operating parameters are given in table 1. The U C L A S-band rf copper photocathode gun has been described in detail eIsewhere [7]. Briefly, the gun is a one and a half cell structure with a high peak accelerating field ( ~ 100 M V / m ) . Solenoidal focusing is used to control the highly divergent b e a m after it exits the gun. Dark current is an issue in this and other photocathode guns due to the high fields used to
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
G. Baranov et aI. / The UCLA IR FEL project Table 1 Measured operating parameters for the UCLA 5 MeV test stand. The system is operated with single bunches at a repetition rate up to 5 Hz Electron beam (measured) Electron beam energy Peak charge Pulse duration (rms) Normalized emittance (rms)
229
Dark Current Spectrum 10
"G ;">-
0
8
0 0
56
_<5 MeV ~ 1 nC ~ 50 ps ~ 12-rr mm mrad
0 ®
®0
C
4 o•
=o
O
o o
2
"o
control space charge derived emittance growth. P r o p e r gun conditioning can help r e d u c e dark current, and m e a s u r e m e n t s of the dark c u r r e n t s p e c t r u m are necessary to quantify the problem. The dark current spect r u m as m e a s u r e d for the U C L A gun after conditioning is given in fig. 3. U s i n g the scanning q u a d r u p o l e t e c h n i q u e the normalized rms e m i t t a n c e of ~ 50 ps (rms) b e a m s was m e a s u r e d to b e ~ 12~ m m mrad. T h e m e a s u r e d emittance is e x p e c t e d to go down substantially w h e n shorter pulses ( ~ 2 ps rms) are used. Simulations using the P A R M E L A code predict an emittance of 5 ~ m m m r a d for the 2 ps case [8]. E m i t t a n c e m e a s u r e m e n t s w e r e
0ooo °o0
~,
,°'~ ....
1.5
1
2
2.5
(MeV/c)
P
3
3,5
Fig. 3. The dark current energy spectrum for the UCLA gun.
m a d e with a 3 M e V b e a m to avoid complications by dark current. The choice of c a t h o d e materials has received m u c h attention in the literature [9]. While materials such as CsSb can offer high q u a n t u m efficiency ( Q E ) and ease of manufacturing, they require high vacuum ( ~ 10-11 Torr) and have a limited lifetime. O n the o t h e r hand,
FARADAY I CUP
STRIPUNE ] 5 MeV RF GUN
DIPOLE BENDING MAGNET
MONITOR I
SOLENO, D I
'
'
~.~
I
| ~,~j~'
ILASER COUPLING
'
t
~
__
_ _
ADDITIONAL DIAGNOSTICS
Fig. 1. The UCLA 5 MeV test stand beamline showing the gun, magnets and diagnostics. The laser mirror box is used to 111 " _ umlnate the cathode on axis ( ~ 3°) while ports on the gun allow for 70° illumination.
5 MeV RF GUN I LASER COUPLING
20 MeV LINAC I
DIPOLE
PHOSPHOR SCREEN
I
ISTRIPUNE / MONITOR
"~_.~
DIAGNOSTICBEAMLINE
-
~
Fig. 2. The full UCLA FEL project beamline showing the gun, linac and experiment inertion area. The second beamline will be used for diagnostics and other experiments. V. PROJECTS/PROPOSALS
230
G. Baranov et a L / The UCLA IR FEL project
simple metals such as copper are inexpensive, easy to work with and rugged, but have poor quantum efficiency ( ~ 10-5). Since copper cathodes require expensive U V laser illumination, enhancement of the quantum efficiency is desirable. Currently, the U C L A gun utilizes a removable unpolished copper cathode. The test stand features two sets of laser ports; one on axis (perpendicular to the cathode) another at 70 ° from the axis. Enhancement of the quantum efficiency by orders of magnitude was predicted for the off axis illumination [10]. Preliminary measurements show a QE of 2.7 × 10 .5 on axis and 10.5 × 10 -5 at 70 ° which implies an enhancement ratio of 3.9. This ratio is substantially smaller than the one predicted by theory; however, these measurements are preliminary. In order to preserve the short pulse length ( ~ few ps), a scheme is now being devised to "tilt" the pulse appropriately. In this way, the impact of the laser pulse will be uniform in time across the face of the cathode. Because there are two laser ports (left and right) at 70 ° it is also possible to illuminate the cathode symmetrically. Finally, it has been reported that cathode surface finish affects photoelectron production [11]. Cathodes with various finishes (i.e. polished, grated, etc.) are currently being produced and will be tested soon.
3. The 20 MeV beamline and undulator Once characterization of the gun and beam are finished, a 20 MeV beamline including the PWT linac will be assembled (see fig. 2). The main beamline as well as a second beamline will contain various beam diagnostics: strip-line monitors, phosphor screens, Cherenkov and optical transition radiation detectors, Faraday cups, etc. Successful operation of the complete beamline will lead to undulator insertion and FEL operation. A novel high performance undulator has been constructed for U C L A by the collaborators at the Kurchatov Institute. The design utilizes N d - F e - B blocks in a hybrid configuration with vanadium permedure yokes. In addition, SmCo s blocks placed between the yokes enhance the pole tip field. The samarium cobalt blocks counter the flux from the N d - F e - B magnets within the yokes. Thus, saturation of the yoke is reduced. At the same time, the blocks enhance the flux within the pole gaps. In this way a higher field is produced than possible with a conventional hybrid design [12]. The linear undulator is 60 cm long with a 1.5 cm period, 5 mm fixed gap spacing and 7.5 kG peak field. The mechanical assembly allows for precise tuning of the field: a field uniformity better than _+0.25% has been achieved [13]. The second integral of the undulator field (along the axis) shows that the electron beam
Table 2 Design parameters for the UCLA FEL. Electron beam figures are based on simulation results Electron beam (simulation) Electron beam energy Energy spead (rms) Peak current Pulse duration (rms) Normalized emittance (rms)
~ 20 MeV 0.1% 200 A ~ 2 ps ~ 5~ mm mrad
Undulator (measured) Undulator period Total length Pole face gap (fixed) Peak field on axis FEL parameter (p)
1.5 cm 60 cm 5 mm 7.5 kG ~ 1 × 10-2
deflection Ax = 150 ~m satisfies the requirement that it be less that the beam waist ~rx = 200 ~zm. The initial F EL will be configured as a single pass amplifier. Calculations indicate that the SASE signal will be weak and may be difficult to detect. A pulsed CO a laser will be used as a seed in order to study saturation effects and simplify detection. A T E A - C O 2 laser with peak power in the megawatt range and associated optics are being tested for this purpose. In order to preserve emittance, the electron beam will be sent directly through the undulator: no bends are present in the beamline before the undulator. This necessitates the use of a mirror with a hole in it to introduce the seed laser into the beamline. Numerical simulations have been used to determine the FEL's expected performance. Primarily, Tran and Wurtele's T D A code has been used [14]. Utilizing the parameters given in table 2, the code predicts an exponential power growth length of 8.7 cm. This corre-
Gain vs. Distance Along Undulator
1os 107 106 1o5
~
"~ 104 (.5
~
Present
1o3
FEL
102 , ~ 1 o1
10° 10-1 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1,8
2
Distance Along Undulator [m]
Fig. 4. A plot of the power gain as a function of undulator length for the UCLA FEL produced by the TDA code. Injection energy for this simulation was 19 MeV with a beam current of 200 A.
G. Baranov et a L / The UCLA IR FEL project
Table 3 Simulation results for the UCLA FEL from the TDA code. Parameters used are as }hose in table 2 FEL (simulation) Output radiation wavelength Power gain (e-folding) length SASE saturation length Peak power at saturnation SASE peak power output after 60 cm
10.6 I.~m 8.7 cm 160 cm ~ 50 MW ~ 1.7 kW
sponds to a gain of 103 over the undulator length. Saturation occurs at ~ 1.6 m with a peak power of ~ 50 MW. A plot of the gain as a function of distance down the undulator is given in fig. 4. Assuming a peak spontaneous noise level of 1.75 W, which corresponds to the radiation emitted in one gain length of the undulator, then the peak power at output is 1.7 KW. This low average power necessitates the use of cryogenic detectors. For this and other reasons it is likely that initial operation will be with the seed laser. A n input power of a few tens of kW is sufficient to saturate the FEL. Simulation results are summarized in table 3. Saturation cannot be achieved from S A S E with the present undulator. However, after suitable testing of the first undulator a second undulator will be obtained. Running as an optical klystron [15] the U C L A F E L will be able to achieve saturation (or high extraction efficiency) starting from SASE.
4. Conclusions and future work The U C L A F E L project has a viable high brightness source to be used in conjunction with a completed short period undulator. The quantum efficiency enh a n c e m e n t findings are of significance for high brightness sources in general and for F E L s in particular. The e n h a n c e m e n t implies that a lower power, and hence a less complex laser system, can be used. Alternatively, the additional laser power can be used to drive a higher current beam. O n c e operational, the F E L will be used to study p h e n o m e n a of significance to future short wavelength
231
devices. D u e to the lack of suitable mirrors, the oscillator configuration is presently impractical for short wavelengths. Hence, the U C L A device will operate as an amplifier in the high gain regime. The lack of suitable sources at various wavelengths makes the feasibility of startup from spontaneous emission (SASE mode) important [16]. The need to mitigate diffraction also makes the study of optical guiding significant. These phenomena, as well as saturation, superradiance and sidebands, will be studied at U C L A .
References [1] F. Aghamir et al., Nucl. Instr. and Meth. A304 (1991) 155. [2] J. Dodd et al., Nucl. Instr. and Meth. A318 (1992) 178: [3] J.S. Fraser and R.L. Sheffield, IEEE J. Quantum Electron QE-23 (1987) 1489. [4] P. Maine et al., IEEE J. Quantum Electron. QE-24 (2) (1988) 403. [5] D.A. Swenson, Europ. Particle Accelerator Conf., Rome, Italy, 1988, vol. 2, ed. S. Tazzari, p. 1418. [6] S. Hartman et al., IEEE 1991 Particle Accelerator Conf. Proc., p. 2967. [7] J.G. Davis, C.J. Josh} and C. Pellegrini, Intense Microwave and Particle Beams III, ed. H.E. Brandt (Elsevier). [8] L. Young and K.T.. McDonald, IEEE Trans. Electron Devices ED-35 (1988) 2052. [9] See, for instance, H. Riege, CERN, CLIC-NOTE 105 (1989). [10] K.L. Kliewer, Phys. Rev. B14 (1975) 1414. [11] J. Endriz, Stanford Electronics Lab. Tech. Rep. 5207-3 (1970). [12] K. Halbach, J. de Phys. Coll. (1983) CI-211. [13] Here undulator field uniformity is reported as the maximum deviation of the pole tip fields (on axis) from the mean pole tip field. [14] T.M. Tran and J.S. Wurtele, Computer Phys. Commun. 54 (1989) 263; and T.M. Trans and J.S. Wurtele, Phys. Rep. 195 (1990) 1. [15] J.C. Gallardo and C. Pellegrini, Opt. Commun. 77 (1990) 45. [16] C. Pellegrini et al., these Proceedings (14th Int. Free Electron Laser Conf., Kobe, Japan, 1992) Nucl. Instr. and Meth. A331 (1993) 223.
V. PROJECTS/PROPOSALS