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Status report on the low-frequency photo-injector and on the infrared FEL experiment (ELSA)
Ni-.clear Instruments and Methods in Physics Research A296 (1990) 209-216 North-Holland
209
Section II. Proposed experiments STATUS REPORT ON THE LOW-FREQUENCY PHOTO-INJECTOR AND ON FEL EXPERIMENT (ELSA)
EI
R. DEI-CAS, P. BALLEYGUIER, A. BERTIN, M.A . BEUVE, A. BINET, A. BLOQUET, R. BOIS, C. BONETTI, J.P. DE BRION, F. COÇU, J. DI CRESCENZO, P. FOURDIN, J. FREHAUT, M. GUILLOUD, G. HAOUAT, A. HERSCOVICI, D. IRACANE, R. JOLY, S. .OLY, J.C . JOUYS, J.P. LAGET, C. LASPALLES, H. LEBOUTET, J.G. MARMOUGET, D. MASSERON, Y. DE PENQUER, Y. PRANAL, J. SIGAUD, S. STRIBY, D. VÉRON, J . VOUILLARMET, J. AUCOUTURIER *, A. BENSUSSAN *, M. SIMON *, A. DUBROVIN +, G. LE MEUR +, J.C. ADAM § and A. HÉRON §
Commissariat à l'Energie Atomique, Service PTN, Centre d'Etudes de Bruyères-le-Châtel, B.P. no 12, 91680 Bruyères-le-Châtel, France
The photo-injector presented at the FEL-88 Conference is now completely assembled. Performances of the main components will be outlined. The photo-injector beam dynamics has been simulated by using different codes and the results will be compared . By integrating the main components under development (photo-injector, 433 MHz rf cavity, rf generator, wiggler and optical cavity) a FEL experiment (called ELSA), presently under construction, will be performed in the 20 Wm range. The expected performances will be presented.
l. Introduction To reach a high-efficiency regime in a high-reakpower free electron laser, a specific injector delivering high-quality electron beams, a low-frequency rf linac and a flexible tapered wiggler have to be developed. A research and development program has been initiated in 1987 and was described in refs . [1,2]. Compared to a conventional thermionic gun, a pho-
tocathode illuminated by an adequate pulsed laser is able to produce high-brightness bunched beams [3]. For high-intensity bunches, space-charge forces will result in emittance growth. This effect, together with rf dynamics forces due to E-field time variation, can be minimized by using long bunches and, consequently, a lowfrequency injector . For technical reasons, a frequency of 144 MHz was chosen for the rf gun cavity with the drawback of a maximum azcelerated E-field limited to 15-20 MV /m. For bunches containing 5-10 nC, the optimized cell geometry, with respect to beam qualities, seems to be still an open question . To get the highest efficiency in converting the electron-beam energy into photon energy in the wiggler, the peak current should be as large as possible and the * Present address: Thomson-CSF, Bagneux, France . + Present address : LAL, Orsay, France . § Present address : Ecole Polytechnique, Palaiseau, France . Elsevier Science Publishers B .V . (North-Holland)
energy spread as low as possible : these two requirements could also be better fulfilled with a low-frequency accelerator . Consequently, all the components of the accelerator chain have been developed at 433 MHz, i .e . 1/3 of the L-band frequency . A flexible tapered hybrid wiggler is required for high conversion efficiencies . A wiggler using the manual displacement of a permanent-magnet piece for the worse variations ( ± 200) of the magnetic field and pulsed coils for the on-line fine tuning (± 2%O) is being designed ; a field stability of the order of 10-3 is expected . Integrating these main components (a photo-injector, one 2-cell and two 3-cell cavities, a 2 m tapered wiggler coupled to an optical cavity), a near-infrared FEL (called ELSA) is under construction and will be used to study the main technical aspects and the physics issues involved in a high-power FEL . For the oscillator experiment, a unique 6 MW klystron would provide a macropulse average current between 100 and 200 mA bunch charge for 17-20 MeV electrons depending (1 to 10 nC) . According to our estimates, the efficiency for converting electron-beam energy to light should he between 4°k and 9% for a wavelength in the 15-25 ~Lm
on the
range.
We describe, first, the actual development status of the ELSA components and then we present the main expected performances of the FEL . Finally, the beam dynamics in the first part of the accelerator has been 11 . PROPOSED EXPE?IMENTS
210
R. Dei-Cas et al. / Lowfrequency photo-injector for ELSA
studied by means of five different codes, the results of which will be compared. 2. Photo-injector A general view of the photo-injector prototype is given in fig. 1 . It can be divided into three sections : the photocathode preparation chamber, the rf gun cavity and the electron diagnostic line. Not shown in this figure are the laser illuminating the photocathode and the rf generator. 2.1. Photocathode preparation chamber
According to the experience gained at Los Alamos [31, photoemitters should be convenient for electron production. The preparation system is now in operation and the first cathodes have been prepared; a good vacuum of 5 x 10 -11 Ton has been obtained in the preparation chamber previously baked at about 300 ° C. The quantum efficiency of the successive layers of Cs, K and Sb will be controlled under permanent mW laser irradiation, and the photoelectric current will L-e measured on-line. To increase homogeneity during the fabrication process, the photocathode holder is rotated, whereas the cesium (or potassium or antimony)'
evaporator is translated up and down. The homogeneity will be checl-ed at the end of the process by scanning the cathode surface with the same laser. The photocathode area (1-2 cm2 ) is defined by a mask aperture. 2.2. Photocathode laser
The laser chain consisting of a 72 MHz Quantronix mode-locked oscillator, an acousto-optic modulator (AOM) and a Quantel three-stage amplifier is now in operation. To minimize amplitude fluctuations during the macropulse, we use a preprogramming technique where the optimum Pockels cell voltage is deduced from previous macropulses by using a digital PID-like loop . As a preliminary result, fig. 2 shows the envelope of the macropulse and the voltage applied to the Pockels cell. Over 100 Rs, the fluctuation level is of the order of a few percent. This stability will be improved by using a more efficient filtering of the Pockels cell voltage. . The actual macopulse characteristics are the following: pulse energy of 10 WJ, pulse duration of 70 ps (FWHM), stability of about 1%, beam diameter at the exit of 2 mm and beam divergence of 125 wrad with a good angular stability. Assuming a photoemitter quantum efficiency of 1%, this laser beam would permit a charge of about 40 nC to he extracted, in agreement with the theoretical value Q = SE o E/d for S = 2 cm2.
Fig. 1 . Mechanical drawing of the photo-injector : photocathode preparation chamber (A), rf gun cavity (B) and electron diagnostic line (C).
R. Dei -Cas et al. / Lowfrequency photo-injector for ELSA Table 2 Klystron characteristics Rf peak power Average power Pulse duration Voltage Current Input power Gain Efficiency Spectral bandwidth
Laser signal
Pocke cell
6 MW 200 kW 200-400 lis 170-180 kV 61-75 A 40 W 51 .7 dB 57 .8` ±0.75 MHz
relative to the maximum surface field on the anode [1,21 . This cell is made of OFHC copper and welded by the electron-beam technique . A diamond tool was used for finishing the nose . The entire injector (see fig. 1) is now installed and the rf conditioning of the cavity has started at low power, as the main amplifier is still under tests . This amplifier uses a TH-526 tetrode and will deliver 2 .5 MW peak power during the 200 I.s micropulse.
Laser Fig. 2. Envelope of macropulse (72 MHz) and Pockels cell voltage (20 [Ls/division). The AOM selects 11N micropulses to run at lowrepetition rates with a large charge per bunch (for example, 10 nC at 14 MHz) or higher repetition rates with lower charges (72 MHz with 2 nC). One of the main goals of our experimental program is to find the optimal conditions to obtain the largest brightness for the electron beam; the laser pulse duration is a very important parameter as it governs the beam dynamics . By adding a laser pulse compressor, the bunch duration will be extended from 20 to 200 ps.
3. Accelerator chain
3.1 . Accelerating cavity The standing-wave rf cavity consists of three coupled cells operating at 433 MHz in the rr-mode . The cell shape was optimized to reduce the wake-field effects and to minimize the emittance growth while keeping the highest accelerating gradient, of the order of 7 MV /m ; the resulting shape has been presented previously [11 .
200
2.3. Rf cavity and rf generator
V (MV)
The shape of the cathode and anode noses was optimized to give the largest field on the photocathode Table 1 Attenuation of the first higher-order modes Mode
Monopoles TM010 TM011 Dipoles TM110 TM111
f [MHz]
Q without HOM coupler
Q with 1 coupler
Q with 2 couplers
440 .1 646 .4
23 700 25 300
23700 645
22 600 950
748.1 748 .5 931 .6 931 .8
37 400 32 800 27400 28 200
32 800 2200 230 28 300
18c0 150 1000
-200
-Inn ~
-600
L
-800 -200
0
200
400
600
t(ps1
Fig . 3 . Rf puls 11 . PROPOSED EXPERIMENTS
21 2
R Dei-Cas et al. / Lowfrequencyphoto-injector for ELSA
The iris between cells is not large enough for the rf power injected in the middle cell to propagate to adjacent cells and coupling slots must be added . A threecell mock-up has been built to measure the electrical parameters and to define the slot geometry. The transition between the R5 waveguide and the injection loop has been adjusted in this mock-up. The waveguide is terminated by a movable short circuit and a reduced waveguide HOM absorber working above cutoff for the 433 MHz frequency . The injection loop can be matched to the beam-loaded cavity by moving this shert circuit, and the coupling coefficient can be adjusted from 1 to 3. When an electron bunch travels through a cavity, it loses energy into all modes with fields on the axis and, if the bunch is off-centered, it will also excite transverse deflecting modes. These parasitic effects are responsible for bunch lengthening and energy losses. The spatial distribution and the frequencies and geometric impedances (R/Q) have been measured and calculated with SUPERFISH [4] for monopoles and with URMEL [5] for multipoles. To avoid beam instabilities, the impedances of the cavity at higher-order mode (HOM) frequencies must be sufficiently love; the highest impedances must be attenuated by orders of magnitude using damping systems (HOM absorbers). Different types of absorber have been tested. Two such couplers will be used for each cell, and their effect is summarized in table 1. 3.2. Klystron A 6 MW peak-power klystron [1] has been developed and tested by THOMSON . The main characteristics are summarized in table 2, while fig . 3 shows a 6 MW, 400 lis rf pulse. The klystron will be protected against reflected power by a specially developed circulator. 3.3. Modulator The ripple on the rf macropulse is due to the HV ripple ; indeed, in this klystron test, a delay-line modulator is used but, for the FEL application, we are developing a hard-tube modulator. A ripple lower than 3 x 10 -3 is expected by controlling the grid voltage of the TH-558 tetrode with a digitized wave function . . W iggler and optical cavity 4.1 . Wiggler structure As presented in ref . [1], we have designed a flexible hybrid tapered wiggler where the fine tuning is obtained by powering in a feedback loop, a small pulsed coil. Fig . 4 shows the magnet structure; the gap (1 .6 cm) is adjustable. The vacuum chamber (2.4 m in length) will
E E 0
Fig. 4. Magnei structure : L,,, = 3.2 cm, permanent magnet SmCos, gap =1.6 cm. be in massive Al alloy. It is machined in two pieces and assembled by electron beam welding. The feedback-loop response of the wiggler fine-tuning coils has been tested on a mock-up. The long-term B-field variation due to temperature drift as well as pulsed field variations induced by an external coil are corrected with a precision level better than 10-3 , as shown in fig. 5. 4.2. Optical cavity For the proof of principle experiment (see section 5), we will use a 10.37 m long optical cavity, which will be stabilized in length by an interferometer and in angle by detecting, with a laser beam and a quadrant detector, the angular variations of the mirrors . The 10 cm diame840 800
â E
760 .
m 720 . 4 U 680 . 640 . 0
2
4
6
8
10
91.9 99.8
12 time (hours)
1.10-3
91.7 91.6
c 91.5
-1 .10-3
91.4
9t3
0
2
4
6
6
10
12 time (hours)
Fig. 5. Feedback loop tests on the wiggler mock-up.
213
R. Dei -Cas et al. / Low frequency photo-injector for ELSA
Table 3 Main characteristics of the ELSA experiment
ter mirrors are mounted in a rigid frame suspended by three flexion bars, the positions of which are controlled by step motors; this mechanical support is similar to the one developed by Spectra Technology Inc. [61 for the BAC experiment. A prototype of the optical mirror mount has shown that prn positions and grad angles can be detected by such a system. A hole of a few mrâ in diameter in one of the mirrors is foreseen for the laser output coupling.
Photo-injector
Cathode Energy Charge Pulse duration Repetition rate Peak current Normalized emittance Accelerator
5. The ELSA experiment 5.1 . Machine description
By integrating all the components under development, i.e. photo-injector, 433 MHz accelerator chain, wiggler and optical cavity, an IR-FEL (called ELSA, for "Etude d'un Laser Accordable"), will be built as a proof of principle experiment for studying high-quality beams and a high-peak power FEL . Fig . 6 shows the layout of the experiment. The main parameters are listed in table 3. The repetition rate will be limited to 20 Hz by the laser, by the installed electric power and by the necessary shielding (2 m thick walls). The accelerator and the rf generators are able to run at up to 150 Hz. The achromatic half-turn loop made of three magnets has been optimized up to the second order to minimize the emittance growth . According to PARMELA calculations, made for a beam with a low energy spread (0 E/E < 3 X 10-3 ) and containing 10 nC, the emittance growth due to space-charge forces is limited . By organizing the energy in the phase space diagram, the bunch can be compressed magnetically : the corn-
Energy Mean current Macropulse duration Repetition rate Peak power Macropulse mean power Laser
Wavelength Wiggler length Wiggler period Optical-cavity length Efficiency with tapering Peak power (micropulse) Mean power (macropulse)
CsK 2Sb 1-1 .5 MeV 1-10 nC 20-200 ps 72.22-14.44 MHz 50-200 A _-, 60% mm mrad 17-20 MeV 100-200 mA 200 ILs < 20 Hz 3 GW 3 MW 15-25 Wm 2m 3.2 cm 10.3774 m = (4-8)% = 30-50 MW =100 kW
pression factor of the half-turn loop is 1 .49 cm per percent of the energy spread, resulting in an increase by a factor of 2-3 in peak current intensity. Unfortunately, as pointed out by the LANL-FEL team and estimated by means of PARMELA, the emittance is expected to grow by a factor of 2-3; this behaviour is due to the transfer of correlated emittance to thermal emittance. Nevertheless, as indicated in table 5, it may be
R F .GUN
.433 MHz ACCELERATING CWTIES SPECTROMETER
. 2n, WIGGLER
OPTICAL CAIITY
Fig. 6. Layout of the ELSA proof of the principal experiment . II . PROPOSED EXPERIMENTS
R. Dei-Cas et al. / Lowfrequencyphoto-injector far ELSA
214
Table 4 Influence of space-charge forces on the laser performances for an untapered wiggler L,,, =1 in Lp AE/E Q [mm] [%l [nc] Gain Efficiency [%l 1 5 10
1 .5 15 16 .5
0.32 0.73 1.60
1.07 1 .33 1 .31
worthwhile to use magnetic compression since the lowlevel gain is larger for higher peak currents. 5.2 . Laser simulation
The simulations have been made using the ONDULA code [7] based on the KMR formalism, including selfconsistent calculations of the mean laser-radius and gain-time evolution . The evolution of the fundamental and harmonics modes of the laser beam have been obtained by solving the 2D Maxwell equations . The influence on the laser beam radius of the optical cavity and of the beam-guiding effects in the wiggler are described without any phenomenological parameter. The FEL. efficiency is strongly dependent on the bunch charge. However, space-charge effects limit this efficiency as they are responsible for emittance growth and for the increase in energy spread. These effects are shown in table 4 for two wiggler lengths with a constant magnetic field ; the saturation efficiency corresponds to 500 passes for the laser pulses in the optical cavity. The electron bunch parameters are the bunch charge; (Q), the bunch length (L p ) and the relative energy spread (® E/E ). The beam energy is E = 16 MeV, the beam radius r = 2 ntm, the laser wavelength n = 21 Rrn and the optical cavity has losses of 2%. When the charge is too small (Q ==1 nC), saturation is not possible for LW =1 m; it can be obtained with a 2 m wiggler or with a shorter pulse (higher electron density). For very large charges (Q = 40 nC, lasing is impossible because the bunch characteristics are very bad . For intermediate charges, the small signal gain is larger enough for a rapid start-up of the laser signal ; an efficiency of about 0.5% is possible for an ;3ntapered wiggler . For a given charge, Q = 8 .3 nC here, it is possible to increase the electron Table 5 Influence of pulse length aid energy spread for an untapered wiggler with L , =1 ta LP
AE`E
Gain
30 15 7 .5
0 .5 1 .0 2.0
1.31 1.50 1.63
[MM]
[al
Efficiency [%l 0.65 0.50 0.39
L ,=2m Gain
0 0.63 0.50
1 .63 3.32 2.13
Efficiency [%l 0.24 0.14 0.50
density, and then the efficiency, by shortening the bunch at the expense of the energy spread as shown in table 5. Fig . 7 shows, as an example, the influence on the efficiency, at saturation and averaged over 1000 passes, of the tapering (A BIB) over the 2 m wiggler for a 10 nC bunch charge, 100 ps pulse duration and 0 E/E = 2 x 10 -3 . The magnetic-field variation was optimized with a given parabolic shape. These calculations show that an extraction efficiency of about 10% could be obtained for A =15-25 p,m ; the laser peak power could be as large as 50 mW and the macropulse average power around 100 kW. 6. Beam dynamics Tae time evolution of electron bunches is simulated using five different codes: - two "ring and disk" codes, OAK (LAL-Orsay) and ATHOS (CEA- Bruyères -le-Châtel), - PARMELA (LANL), - two "Maxwell" codes, PRIAM (LAL-Orsay) and PIC-code (Ecole Polytechnic;ue).
t Efficiency
(%)
Lw _2m
10
at saturation
0
=10nC
t
= 100 PS
l!E/E = 2 .10-3
5 Mean efficiency (1000 passes)
0
50
E
= 17 MeV
a
=17pm
Parabolic profile
100
BB/B (°/â
Fig. 7. Optimized efficiency at saturation and averaged over 1000 passes as a function of wiggler tapering .
R. Dei -Cas et aL / Lowfrequency photo-injector for ELSA
Table 6 Normalized emittance as obtained with different codes for Em =15 MV/m = 3) OAK [ ,ff mm mradi 10 nC- 25 ps 125 10 nC- 50 ps 110 5 nC- 50 ps 90 10 nC-100 ps E (y
PARATHOS PIC- PRIAM MELA Code 104 120 74 108 116 112 64 82 86 54 55 85 50 82
[keV]
OAK
10 nC- 25 ps 10 nC- 50 ps 5 nC- 50 ps 10 nC-100 ps 1 nC- 50 ps
154 105 75 50
PARMELA 170 130 80 20
ATHOS 175 124 78 82 25
PICCode 138 115 95
PRIAM 100 92 60 78
The same entry parameters have been used for these codes : the normalized emittance at the photo-injector exit (with no magnetic field from the focusing coil) and the energy spread are presented in tables 6 and 7, respectively, for different bunch charges and laser pulse durations. By powering the focusing coil, the radial beam expansion is strongly limited as observed in fig. 8, where the bunch dimensions are plotted for different focusing fields .
Anode
700 Gs
~
_ _
.. . 1000 Gs RF 11 9a
-nM-
-
--.
_=
1500 Gs b M un
Table 7 Energy spread for Em =15 MV/m AE
Cathode
215
mm
Fig. 8. Bunch evolution in the rf gap of the photo-injector for different focusing fields (PRIAM simulation) . Accelerating the bunches at about 5 MeV (after the two-cell cavity at 433 MHz, see fig. 6), the normalized emittance is still significantly reduced as shown in fig. 9. This beneficial effect of a focusing coil on bunches, where a strong correlation persists between energy and phase, was first pointed out by Carlsten j8J. With a good phase matching, of the 433 MHz accelerating cavity, the energy spread is strongly reduced, as shown -. fig. 10. At 5 MeV, the relative energy spread (FWHM) is 5 X 10-3, i.e. of the order of 2 X 10-3 at 17 MeV . 7. Conclusion Precise electron-beam dynamics calculatic~-, are still needed to take into account all the effects the electron
-v v E E K
W
2 cells of 433 MHz
Fig. 9 . Emittance and rae :us envelopes with a two-cell cavity, with exit energy = 5 MeV. PARMELA simulation . The 93% emittance is four tinges E (rms): 10 nC, 50 ps. Il . PROPOSED EXPERIMENTS
R. Dei -Cas et al. / Low frequency photo-injector for ELSA
216
Experiments will be performed with the photo-injector prototype from October 1989 to mid-1990 to investigate the electron bunch characteristics as a function of rf gun and photocathode illuminating laser parameters . The ELSA components will then be assembled and the first free-electron laser beam is expected by the beginning of 1991 .
References
0
Energy spectrum 29 58
87
116
200 100
Y w
-100 -200
Fig. 10. Energy spread at the exit (5 MeV) of the first cavity . PARMELA simulation . bunches are subjected to, particularly the wake fields, and to optimize ,he overall FEL machine. Real beam characteristics also have to be included in the simulation codes .
[1] R. Dei -Cas et al ., Proc . 10th Int . Free Electron Laser Conf., Jerusalem, 1988, Nucl . Instr. and Meth . A285 (1989) 320. [2] S. Joly et al ., Proc. Int. Conf. on Optical Science and Engineering, Paris, 24-28 April 1989, Proc. SPIE 1133 (1989) 54. [3] J .S . Fraser and R.L. Sheffield, IEEE J . Quantum Electron. QE-23 (1987) 1489 ; J .S . Fraser et al., 1987 IEEE Particle Accelerator Conf., Washington, DC (IEEE Publishing, New York, 1987) p . 1705 . [4] K . Halbach and R.F . Holsinger, Particle Accelerators 7 (1976) 213 . [5] T . Weiland, Nucl . Instr . and Meth . 216 (1983) 329 . [6] D.M . Schemwell et al ., Nucl . Instr. and Meth. A259 (1987) 56 . [7] D. Iracane, these Proceedings (11th Free Electron Laser Conf., Naples, FL, USA, 1989) Nucl . Instr. and Meth . A296 (1990) 417 . [8] B.L . Carlsten, Proc . 10th Int . Free Electron Laser Conf., Jerusalem, Israel, 1988, Nucl . Instr. and Meth. A285 (1989) 313 .
209
Section II. Proposed experiments STATUS REPORT ON THE LOW-FREQUENCY PHOTO-INJECTOR AND ON FEL EXPERIMENT (ELSA)
EI
R. DEI-CAS, P. BALLEYGUIER, A. BERTIN, M.A . BEUVE, A. BINET, A. BLOQUET, R. BOIS, C. BONETTI, J.P. DE BRION, F. COÇU, J. DI CRESCENZO, P. FOURDIN, J. FREHAUT, M. GUILLOUD, G. HAOUAT, A. HERSCOVICI, D. IRACANE, R. JOLY, S. .OLY, J.C . JOUYS, J.P. LAGET, C. LASPALLES, H. LEBOUTET, J.G. MARMOUGET, D. MASSERON, Y. DE PENQUER, Y. PRANAL, J. SIGAUD, S. STRIBY, D. VÉRON, J . VOUILLARMET, J. AUCOUTURIER *, A. BENSUSSAN *, M. SIMON *, A. DUBROVIN +, G. LE MEUR +, J.C. ADAM § and A. HÉRON §
Commissariat à l'Energie Atomique, Service PTN, Centre d'Etudes de Bruyères-le-Châtel, B.P. no 12, 91680 Bruyères-le-Châtel, France
The photo-injector presented at the FEL-88 Conference is now completely assembled. Performances of the main components will be outlined. The photo-injector beam dynamics has been simulated by using different codes and the results will be compared . By integrating the main components under development (photo-injector, 433 MHz rf cavity, rf generator, wiggler and optical cavity) a FEL experiment (called ELSA), presently under construction, will be performed in the 20 Wm range. The expected performances will be presented.
l. Introduction To reach a high-efficiency regime in a high-reakpower free electron laser, a specific injector delivering high-quality electron beams, a low-frequency rf linac and a flexible tapered wiggler have to be developed. A research and development program has been initiated in 1987 and was described in refs . [1,2]. Compared to a conventional thermionic gun, a pho-
tocathode illuminated by an adequate pulsed laser is able to produce high-brightness bunched beams [3]. For high-intensity bunches, space-charge forces will result in emittance growth. This effect, together with rf dynamics forces due to E-field time variation, can be minimized by using long bunches and, consequently, a lowfrequency injector . For technical reasons, a frequency of 144 MHz was chosen for the rf gun cavity with the drawback of a maximum azcelerated E-field limited to 15-20 MV /m. For bunches containing 5-10 nC, the optimized cell geometry, with respect to beam qualities, seems to be still an open question . To get the highest efficiency in converting the electron-beam energy into photon energy in the wiggler, the peak current should be as large as possible and the * Present address: Thomson-CSF, Bagneux, France . + Present address : LAL, Orsay, France . § Present address : Ecole Polytechnique, Palaiseau, France . Elsevier Science Publishers B .V . (North-Holland)
energy spread as low as possible : these two requirements could also be better fulfilled with a low-frequency accelerator . Consequently, all the components of the accelerator chain have been developed at 433 MHz, i .e . 1/3 of the L-band frequency . A flexible tapered hybrid wiggler is required for high conversion efficiencies . A wiggler using the manual displacement of a permanent-magnet piece for the worse variations ( ± 200) of the magnetic field and pulsed coils for the on-line fine tuning (± 2%O) is being designed ; a field stability of the order of 10-3 is expected . Integrating these main components (a photo-injector, one 2-cell and two 3-cell cavities, a 2 m tapered wiggler coupled to an optical cavity), a near-infrared FEL (called ELSA) is under construction and will be used to study the main technical aspects and the physics issues involved in a high-power FEL . For the oscillator experiment, a unique 6 MW klystron would provide a macropulse average current between 100 and 200 mA bunch charge for 17-20 MeV electrons depending (1 to 10 nC) . According to our estimates, the efficiency for converting electron-beam energy to light should he between 4°k and 9% for a wavelength in the 15-25 ~Lm
on the
range.
We describe, first, the actual development status of the ELSA components and then we present the main expected performances of the FEL . Finally, the beam dynamics in the first part of the accelerator has been 11 . PROPOSED EXPE?IMENTS
210
R. Dei-Cas et al. / Lowfrequency photo-injector for ELSA
studied by means of five different codes, the results of which will be compared. 2. Photo-injector A general view of the photo-injector prototype is given in fig. 1 . It can be divided into three sections : the photocathode preparation chamber, the rf gun cavity and the electron diagnostic line. Not shown in this figure are the laser illuminating the photocathode and the rf generator. 2.1. Photocathode preparation chamber
According to the experience gained at Los Alamos [31, photoemitters should be convenient for electron production. The preparation system is now in operation and the first cathodes have been prepared; a good vacuum of 5 x 10 -11 Ton has been obtained in the preparation chamber previously baked at about 300 ° C. The quantum efficiency of the successive layers of Cs, K and Sb will be controlled under permanent mW laser irradiation, and the photoelectric current will L-e measured on-line. To increase homogeneity during the fabrication process, the photocathode holder is rotated, whereas the cesium (or potassium or antimony)'
evaporator is translated up and down. The homogeneity will be checl-ed at the end of the process by scanning the cathode surface with the same laser. The photocathode area (1-2 cm2 ) is defined by a mask aperture. 2.2. Photocathode laser
The laser chain consisting of a 72 MHz Quantronix mode-locked oscillator, an acousto-optic modulator (AOM) and a Quantel three-stage amplifier is now in operation. To minimize amplitude fluctuations during the macropulse, we use a preprogramming technique where the optimum Pockels cell voltage is deduced from previous macropulses by using a digital PID-like loop . As a preliminary result, fig. 2 shows the envelope of the macropulse and the voltage applied to the Pockels cell. Over 100 Rs, the fluctuation level is of the order of a few percent. This stability will be improved by using a more efficient filtering of the Pockels cell voltage. . The actual macopulse characteristics are the following: pulse energy of 10 WJ, pulse duration of 70 ps (FWHM), stability of about 1%, beam diameter at the exit of 2 mm and beam divergence of 125 wrad with a good angular stability. Assuming a photoemitter quantum efficiency of 1%, this laser beam would permit a charge of about 40 nC to he extracted, in agreement with the theoretical value Q = SE o E/d for S = 2 cm2.
Fig. 1 . Mechanical drawing of the photo-injector : photocathode preparation chamber (A), rf gun cavity (B) and electron diagnostic line (C).
R. Dei -Cas et al. / Lowfrequency photo-injector for ELSA Table 2 Klystron characteristics Rf peak power Average power Pulse duration Voltage Current Input power Gain Efficiency Spectral bandwidth
Laser signal
Pocke cell
6 MW 200 kW 200-400 lis 170-180 kV 61-75 A 40 W 51 .7 dB 57 .8` ±0.75 MHz
relative to the maximum surface field on the anode [1,21 . This cell is made of OFHC copper and welded by the electron-beam technique . A diamond tool was used for finishing the nose . The entire injector (see fig. 1) is now installed and the rf conditioning of the cavity has started at low power, as the main amplifier is still under tests . This amplifier uses a TH-526 tetrode and will deliver 2 .5 MW peak power during the 200 I.s micropulse.
Laser Fig. 2. Envelope of macropulse (72 MHz) and Pockels cell voltage (20 [Ls/division). The AOM selects 11N micropulses to run at lowrepetition rates with a large charge per bunch (for example, 10 nC at 14 MHz) or higher repetition rates with lower charges (72 MHz with 2 nC). One of the main goals of our experimental program is to find the optimal conditions to obtain the largest brightness for the electron beam; the laser pulse duration is a very important parameter as it governs the beam dynamics . By adding a laser pulse compressor, the bunch duration will be extended from 20 to 200 ps.
3. Accelerator chain
3.1 . Accelerating cavity The standing-wave rf cavity consists of three coupled cells operating at 433 MHz in the rr-mode . The cell shape was optimized to reduce the wake-field effects and to minimize the emittance growth while keeping the highest accelerating gradient, of the order of 7 MV /m ; the resulting shape has been presented previously [11 .
200
2.3. Rf cavity and rf generator
V (MV)
The shape of the cathode and anode noses was optimized to give the largest field on the photocathode Table 1 Attenuation of the first higher-order modes Mode
Monopoles TM010 TM011 Dipoles TM110 TM111
f [MHz]
Q without HOM coupler
Q with 1 coupler
Q with 2 couplers
440 .1 646 .4
23 700 25 300
23700 645
22 600 950
748.1 748 .5 931 .6 931 .8
37 400 32 800 27400 28 200
32 800 2200 230 28 300
18c0 150 1000
-200
-Inn ~
-600
L
-800 -200
0
200
400
600
t(ps1
Fig . 3 . Rf puls 11 . PROPOSED EXPERIMENTS
21 2
R Dei-Cas et al. / Lowfrequencyphoto-injector for ELSA
The iris between cells is not large enough for the rf power injected in the middle cell to propagate to adjacent cells and coupling slots must be added . A threecell mock-up has been built to measure the electrical parameters and to define the slot geometry. The transition between the R5 waveguide and the injection loop has been adjusted in this mock-up. The waveguide is terminated by a movable short circuit and a reduced waveguide HOM absorber working above cutoff for the 433 MHz frequency . The injection loop can be matched to the beam-loaded cavity by moving this shert circuit, and the coupling coefficient can be adjusted from 1 to 3. When an electron bunch travels through a cavity, it loses energy into all modes with fields on the axis and, if the bunch is off-centered, it will also excite transverse deflecting modes. These parasitic effects are responsible for bunch lengthening and energy losses. The spatial distribution and the frequencies and geometric impedances (R/Q) have been measured and calculated with SUPERFISH [4] for monopoles and with URMEL [5] for multipoles. To avoid beam instabilities, the impedances of the cavity at higher-order mode (HOM) frequencies must be sufficiently love; the highest impedances must be attenuated by orders of magnitude using damping systems (HOM absorbers). Different types of absorber have been tested. Two such couplers will be used for each cell, and their effect is summarized in table 1. 3.2. Klystron A 6 MW peak-power klystron [1] has been developed and tested by THOMSON . The main characteristics are summarized in table 2, while fig . 3 shows a 6 MW, 400 lis rf pulse. The klystron will be protected against reflected power by a specially developed circulator. 3.3. Modulator The ripple on the rf macropulse is due to the HV ripple ; indeed, in this klystron test, a delay-line modulator is used but, for the FEL application, we are developing a hard-tube modulator. A ripple lower than 3 x 10 -3 is expected by controlling the grid voltage of the TH-558 tetrode with a digitized wave function . . W iggler and optical cavity 4.1 . Wiggler structure As presented in ref . [1], we have designed a flexible hybrid tapered wiggler where the fine tuning is obtained by powering in a feedback loop, a small pulsed coil. Fig . 4 shows the magnet structure; the gap (1 .6 cm) is adjustable. The vacuum chamber (2.4 m in length) will
E E 0
Fig. 4. Magnei structure : L,,, = 3.2 cm, permanent magnet SmCos, gap =1.6 cm. be in massive Al alloy. It is machined in two pieces and assembled by electron beam welding. The feedback-loop response of the wiggler fine-tuning coils has been tested on a mock-up. The long-term B-field variation due to temperature drift as well as pulsed field variations induced by an external coil are corrected with a precision level better than 10-3 , as shown in fig. 5. 4.2. Optical cavity For the proof of principle experiment (see section 5), we will use a 10.37 m long optical cavity, which will be stabilized in length by an interferometer and in angle by detecting, with a laser beam and a quadrant detector, the angular variations of the mirrors . The 10 cm diame840 800
â E
760 .
m 720 . 4 U 680 . 640 . 0
2
4
6
8
10
91.9 99.8
12 time (hours)
1.10-3
91.7 91.6
c 91.5
-1 .10-3
91.4
9t3
0
2
4
6
6
10
12 time (hours)
Fig. 5. Feedback loop tests on the wiggler mock-up.
213
R. Dei -Cas et al. / Low frequency photo-injector for ELSA
Table 3 Main characteristics of the ELSA experiment
ter mirrors are mounted in a rigid frame suspended by three flexion bars, the positions of which are controlled by step motors; this mechanical support is similar to the one developed by Spectra Technology Inc. [61 for the BAC experiment. A prototype of the optical mirror mount has shown that prn positions and grad angles can be detected by such a system. A hole of a few mrâ in diameter in one of the mirrors is foreseen for the laser output coupling.
Photo-injector
Cathode Energy Charge Pulse duration Repetition rate Peak current Normalized emittance Accelerator
5. The ELSA experiment 5.1 . Machine description
By integrating all the components under development, i.e. photo-injector, 433 MHz accelerator chain, wiggler and optical cavity, an IR-FEL (called ELSA, for "Etude d'un Laser Accordable"), will be built as a proof of principle experiment for studying high-quality beams and a high-peak power FEL . Fig . 6 shows the layout of the experiment. The main parameters are listed in table 3. The repetition rate will be limited to 20 Hz by the laser, by the installed electric power and by the necessary shielding (2 m thick walls). The accelerator and the rf generators are able to run at up to 150 Hz. The achromatic half-turn loop made of three magnets has been optimized up to the second order to minimize the emittance growth . According to PARMELA calculations, made for a beam with a low energy spread (0 E/E < 3 X 10-3 ) and containing 10 nC, the emittance growth due to space-charge forces is limited . By organizing the energy in the phase space diagram, the bunch can be compressed magnetically : the corn-
Energy Mean current Macropulse duration Repetition rate Peak power Macropulse mean power Laser
Wavelength Wiggler length Wiggler period Optical-cavity length Efficiency with tapering Peak power (micropulse) Mean power (macropulse)
CsK 2Sb 1-1 .5 MeV 1-10 nC 20-200 ps 72.22-14.44 MHz 50-200 A _-, 60% mm mrad 17-20 MeV 100-200 mA 200 ILs < 20 Hz 3 GW 3 MW 15-25 Wm 2m 3.2 cm 10.3774 m = (4-8)% = 30-50 MW =100 kW
pression factor of the half-turn loop is 1 .49 cm per percent of the energy spread, resulting in an increase by a factor of 2-3 in peak current intensity. Unfortunately, as pointed out by the LANL-FEL team and estimated by means of PARMELA, the emittance is expected to grow by a factor of 2-3; this behaviour is due to the transfer of correlated emittance to thermal emittance. Nevertheless, as indicated in table 5, it may be
R F .GUN
.433 MHz ACCELERATING CWTIES SPECTROMETER
. 2n, WIGGLER
OPTICAL CAIITY
Fig. 6. Layout of the ELSA proof of the principal experiment . II . PROPOSED EXPERIMENTS
R. Dei-Cas et al. / Lowfrequencyphoto-injector far ELSA
214
Table 4 Influence of space-charge forces on the laser performances for an untapered wiggler L,,, =1 in Lp AE/E Q [mm] [%l [nc] Gain Efficiency [%l 1 5 10
1 .5 15 16 .5
0.32 0.73 1.60
1.07 1 .33 1 .31
worthwhile to use magnetic compression since the lowlevel gain is larger for higher peak currents. 5.2 . Laser simulation
The simulations have been made using the ONDULA code [7] based on the KMR formalism, including selfconsistent calculations of the mean laser-radius and gain-time evolution . The evolution of the fundamental and harmonics modes of the laser beam have been obtained by solving the 2D Maxwell equations . The influence on the laser beam radius of the optical cavity and of the beam-guiding effects in the wiggler are described without any phenomenological parameter. The FEL. efficiency is strongly dependent on the bunch charge. However, space-charge effects limit this efficiency as they are responsible for emittance growth and for the increase in energy spread. These effects are shown in table 4 for two wiggler lengths with a constant magnetic field ; the saturation efficiency corresponds to 500 passes for the laser pulses in the optical cavity. The electron bunch parameters are the bunch charge; (Q), the bunch length (L p ) and the relative energy spread (® E/E ). The beam energy is E = 16 MeV, the beam radius r = 2 ntm, the laser wavelength n = 21 Rrn and the optical cavity has losses of 2%. When the charge is too small (Q ==1 nC), saturation is not possible for LW =1 m; it can be obtained with a 2 m wiggler or with a shorter pulse (higher electron density). For very large charges (Q = 40 nC, lasing is impossible because the bunch characteristics are very bad . For intermediate charges, the small signal gain is larger enough for a rapid start-up of the laser signal ; an efficiency of about 0.5% is possible for an ;3ntapered wiggler . For a given charge, Q = 8 .3 nC here, it is possible to increase the electron Table 5 Influence of pulse length aid energy spread for an untapered wiggler with L , =1 ta LP
AE`E
Gain
30 15 7 .5
0 .5 1 .0 2.0
1.31 1.50 1.63
[MM]
[al
Efficiency [%l 0.65 0.50 0.39
L ,=2m Gain
0 0.63 0.50
1 .63 3.32 2.13
Efficiency [%l 0.24 0.14 0.50
density, and then the efficiency, by shortening the bunch at the expense of the energy spread as shown in table 5. Fig . 7 shows, as an example, the influence on the efficiency, at saturation and averaged over 1000 passes, of the tapering (A BIB) over the 2 m wiggler for a 10 nC bunch charge, 100 ps pulse duration and 0 E/E = 2 x 10 -3 . The magnetic-field variation was optimized with a given parabolic shape. These calculations show that an extraction efficiency of about 10% could be obtained for A =15-25 p,m ; the laser peak power could be as large as 50 mW and the macropulse average power around 100 kW. 6. Beam dynamics Tae time evolution of electron bunches is simulated using five different codes: - two "ring and disk" codes, OAK (LAL-Orsay) and ATHOS (CEA- Bruyères -le-Châtel), - PARMELA (LANL), - two "Maxwell" codes, PRIAM (LAL-Orsay) and PIC-code (Ecole Polytechnic;ue).
t Efficiency
(%)
Lw _2m
10
at saturation
0
=10nC
t
= 100 PS
l!E/E = 2 .10-3
5 Mean efficiency (1000 passes)
0
50
E
= 17 MeV
a
=17pm
Parabolic profile
100
BB/B (°/â
Fig. 7. Optimized efficiency at saturation and averaged over 1000 passes as a function of wiggler tapering .
R. Dei -Cas et aL / Lowfrequency photo-injector for ELSA
Table 6 Normalized emittance as obtained with different codes for Em =15 MV/m = 3) OAK [ ,ff mm mradi 10 nC- 25 ps 125 10 nC- 50 ps 110 5 nC- 50 ps 90 10 nC-100 ps E (y
PARATHOS PIC- PRIAM MELA Code 104 120 74 108 116 112 64 82 86 54 55 85 50 82
[keV]
OAK
10 nC- 25 ps 10 nC- 50 ps 5 nC- 50 ps 10 nC-100 ps 1 nC- 50 ps
154 105 75 50
PARMELA 170 130 80 20
ATHOS 175 124 78 82 25
PICCode 138 115 95
PRIAM 100 92 60 78
The same entry parameters have been used for these codes : the normalized emittance at the photo-injector exit (with no magnetic field from the focusing coil) and the energy spread are presented in tables 6 and 7, respectively, for different bunch charges and laser pulse durations. By powering the focusing coil, the radial beam expansion is strongly limited as observed in fig. 8, where the bunch dimensions are plotted for different focusing fields .
Anode
700 Gs
~
_ _
.. . 1000 Gs RF 11 9a
-nM-
-
--.
_=
1500 Gs b M un
Table 7 Energy spread for Em =15 MV/m AE
Cathode
215
mm
Fig. 8. Bunch evolution in the rf gap of the photo-injector for different focusing fields (PRIAM simulation) . Accelerating the bunches at about 5 MeV (after the two-cell cavity at 433 MHz, see fig. 6), the normalized emittance is still significantly reduced as shown in fig. 9. This beneficial effect of a focusing coil on bunches, where a strong correlation persists between energy and phase, was first pointed out by Carlsten j8J. With a good phase matching, of the 433 MHz accelerating cavity, the energy spread is strongly reduced, as shown -. fig. 10. At 5 MeV, the relative energy spread (FWHM) is 5 X 10-3, i.e. of the order of 2 X 10-3 at 17 MeV . 7. Conclusion Precise electron-beam dynamics calculatic~-, are still needed to take into account all the effects the electron
-v v E E K
W
2 cells of 433 MHz
Fig. 9 . Emittance and rae :us envelopes with a two-cell cavity, with exit energy = 5 MeV. PARMELA simulation . The 93% emittance is four tinges E (rms): 10 nC, 50 ps. Il . PROPOSED EXPERIMENTS
R. Dei -Cas et al. / Low frequency photo-injector for ELSA
216
Experiments will be performed with the photo-injector prototype from October 1989 to mid-1990 to investigate the electron bunch characteristics as a function of rf gun and photocathode illuminating laser parameters . The ELSA components will then be assembled and the first free-electron laser beam is expected by the beginning of 1991 .
References
0
Energy spectrum 29 58
87
116
200 100
Y w
-100 -200
Fig. 10. Energy spread at the exit (5 MeV) of the first cavity . PARMELA simulation . bunches are subjected to, particularly the wake fields, and to optimize ,he overall FEL machine. Real beam characteristics also have to be included in the simulation codes .
[1] R. Dei -Cas et al ., Proc . 10th Int . Free Electron Laser Conf., Jerusalem, 1988, Nucl . Instr. and Meth . A285 (1989) 320. [2] S. Joly et al ., Proc. Int. Conf. on Optical Science and Engineering, Paris, 24-28 April 1989, Proc. SPIE 1133 (1989) 54. [3] J .S . Fraser and R.L. Sheffield, IEEE J . Quantum Electron. QE-23 (1987) 1489 ; J .S . Fraser et al., 1987 IEEE Particle Accelerator Conf., Washington, DC (IEEE Publishing, New York, 1987) p . 1705 . [4] K . Halbach and R.F . Holsinger, Particle Accelerators 7 (1976) 213 . [5] T . Weiland, Nucl . Instr . and Meth . 216 (1983) 329 . [6] D.M . Schemwell et al ., Nucl . Instr. and Meth. A259 (1987) 56 . [7] D. Iracane, these Proceedings (11th Free Electron Laser Conf., Naples, FL, USA, 1989) Nucl . Instr. and Meth . A296 (1990) 417 . [8] B.L . Carlsten, Proc . 10th Int . Free Electron Laser Conf., Jerusalem, Israel, 1988, Nucl . Instr. and Meth. A285 (1989) 313 .