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Visible oscillation of storage-ring free electron laser on TERAS
ce
Nuclear Instruments and Methods in Physics Research A318 (1992) 33-37 North-Holland
Visible oscillation of storage-ring free electron laser o K. Yamada, T. Yamazaki, S. Sugiyama, T. Tomimasu ', H. Ohgaki, T. Noguchi, T. Mikado, M. Chiwaki and R. Suzuki Electrotechnical Laboratom, 1-1-4 Umezono, Tsukuba City, Ibaraki 305. Japan
A visible laser oscillation has been achieved on the electron storage ring TERAS at 598 nm using a 1.47 m optical klystron. TERAS was operated in a three-bunch mode at a beam energy of 231 MeV with a beam current ranging from 3 to 10 mA per bunch. Coherent phase oscillations of the beam bunches were successfully suppressed by controlling a second harmonic RF cavity (Landau cavity) whose driving frequency is 343.24 MHz. The laser linewidth was observed to be 0.26 nm, including the detector resolution (0 .21 nm). The spectral intensity was about 104 times as large as that of the spontaneous emission . The peak laser power of the microtemporal pulse is roughly measured to be 10 mW at beam current 4 mA/bunch . l . Introduction
It is widely accepted that a storage ring is one of the most suitable accelerators for short-wavelength free electron laser (FEL) oscillation owing to its high energy with low emittance and small beam-energy spread . However, usual storage rings do not have sufficiently long straight sections to install long undulators . Consequently it is rather difficult to obtain the laser gain larger than the cavity loss. Since our storage ring TERAS was originally constructed for generation of synchrotron radiation and its applications, its low peak current in addition to short straight sections (1 .8 m) have made it difficult to achieve laser oscillation in spite of exploiting an optical klystron. We examined the characteristics of the beam quality and measured the FEL gain and cavity losses in order to assure visible laser oscillation with such a low gain system [1-4]. The gain was observed to be _ 1 x 10 -4 at a beam current of 1.6 mA/bunch . Although this value is just above the initial cavity loss, laser oscillation was difficult because of the cavity-mirror degradation by the undulator radiation. Recently the beam current was increased by a factor of 5 by modifying the radiofrequency knockout (RF-KO) method [1-4] to reduce the number of bunches . In addition, the beam quality has been improved successfully with careful tuning of the 2nd-harmonic Landau cavity (LC) [5] employed to suppress the coupled-bunch instability. These enabled us to achieve visible FEL oscillation on March 22, 1991 [6] and the data are being accumulated. In this paper Present address : FEL Research Institute, 2-7-4 Kyomachibori, Nishi-ku, Osaka City, 550 Japan.
we report the recent results of our FEL oscillation experiments. 2. Beam quality
The details of the storage ring TERAS [7] have been described in the previous reports [1-4]. TERAS is usually filled at an energy 300 MeV and its energy is ramped up or down for various applications . In case of FEL, the energy is ramped down to 230 - 240 MeV to obtain visible undulator radiations around 600 nm. In usual operations, 18 bunches are revolving in the ring. However, in FEL experiments only 3 bunches can be synchronized with an identical light pulse inside the laser cavity and the remainder does not contribute to the peak laser gain. Therefore, the redundant bunches must be eliminated to avoid mirror damage by the intense UV component in the undulator radiation. It is also known [4] that the beam quality is better with fewer number of bunches because of less intra-bunch interaction. For this purpose, a two-stage RF-KO method is used. At the first stage of this method, a RF signal with frequency 1/6 of the ring frequency is mixed with a signal of betatron frequency, amplified and fed to a rod-type perturbation electrode. 12 of 18 bunches are knocked out by selective excitation with six remaining bunches. Then the second stage RF-KO is performed with frequency 1/12 of the ring frequency mixed with betatron frequency, to reduce the number of bunches to the final value 3. The RF-KO used to be carried out after completing the electron injection and ramping down of the energy . During this procedure, however, a large number of electrons were lost. As a result, the final current was only - 3-4 mA per b;w%,
0168-9002/92/$05 .00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
11. NEW FELS
K Yainada et al. / Visible oscillation on TERAS
34
Fig. 1 . Typical spectrum-anah,-zer signals from a button-type beam monitor without (a) and with (b) LC tuning. Horizontal scale is 100 kHz/div. and vertical scale 10 dB/div .
580
600
620 (nm)
which finally decayed down to less than 2 mA when the FEL experiments started . This is because the LC deteriorates the beam stability at higher total current, more than 80 mA . The final current has recently been increased up to 10 mA per bunch by performing the first-stage RF-KO during electron injection into the ring. With fewer bunches, each bunch can share a larger current even if the total current is limited. The laser gain was expected to be improved with the successful increase of the current . In addition, it has been proved that, when the total current is rather low, the LC works very effectively to
580 600 620 WAVELENGTH (nm)
Fig. 2. Spontaneous-emission spectra without (a) and with (b) the Landau-cavity tuning . Beam current is 8 .5 mA/bunch. 4000
t-
3000
W
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° 2000 LU J W
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1000 0 0.000
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(ns)
E
(ns)
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5.000
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âJ W
4500 3000 1500 0
TI
Fig . 3 . Bunch length without (a) and with (b) LC tuning . Beam current is 1 .4 mA/bunch .
K Yamada et al. / Visible osrillatiosi on
suppress the coherent phase oscillation and reduces the beam-energy spread by controlling finely the frequency tuner and phase shifter. Fig . I shows the frequency spectrum of signals from a button-type beam monitor without (fig. la) and with (fig. IN the LC tuning . The division of the vertical axis is according to a log scale. In fig . la, at least three sideband peaks, showing coherent phase oscillations, are observed on both sides of the central peak of 343 .24 MHz, the LC frequency . They are suppressed successfully by exciting the LC and controlling it, as shown in fig . lb. During the above tuning, the spontaneous-emission spectrum changes drastically . Figs. 2a and 2b show the typical spectra of spontaneous emission after passing through the output coupler of the laser cavity. for respective conditions in fig. l . In case (a), periodic fine structure peculiar to an optical klystron is not seen owing to the large energy spread. In case (b), however, a large modulation (f - 0.5) emerges, which means that the beam-energy spread was sufficiently reduced by adjusting the LC conditions. The LC tuning also had an influence on the bunch length in our experiments . Without the tuning, the bunch length is about 0.9 ns (FWHM) as shown in fig . 3a. On the other hand, it becomes shorter by a factor of two (fig. 3b) when the LC is tuned to suppress the sidebands. This increases the peak beam current by the same factor, which enhances the laser gain, together with the reduction of the energy spread. 3. Laser oscillation Actual laser oscillation experiments are conducted according to the following procedures . After injecting electrons into the ring in six bunches with the first-stage RF-KO, the beam energy is ramped down to around 230 MeV and then the number of bunches is reduced down to three by the second stage RF-ISO. Next the cavity mirrors are finely adjusted to make a proper TEM (N) optical mode inside the cavity by observing the lateral-mode profile of the spontaneous emission . The LC is tuned as mentioned in the previous section .
VACUUM MIRROR MANIPULATOR
Finally the cavity length is exactly adjusted to one half of the distance between successive bunches, which is necessary because the laser gain is extremely low. When the cavity length comes close to the precise tuning, one or two peaks on the spectrum begin to oscillate irregularly and the laser starts to oscillate with the tuning within about 5 p.m. Typical arrangements for the FEL oscillation experiments arc shown in fig . 4. A 1 .47 m optical klystron is put inside the laser cavity composed of two dielectriccoated multilayer concave mirrors whose initial loss is 40 ppm including 30 ppm transmission . The cavity length is 5.238 m and the radius of curvature of the mirrors is 3 m. The cavity length and mirror angles around two axes perpendicular to the laser axis can be adjusted by two vacuum mirror manipulators driven by stepping motors and piezoelectric actuators with accuracy of 0.2 plm and 4 ti rad, respectively. The output light from the cavity is introduced to a monochromator combined with a highly sensitive photodiode array and the real-time spectrum is observed to detect the instantaneous change of the undulatorradiation spectrum, which must follow the onset of laser oscillation . The spectral data from the photodiode array is acquired in a microcomputer. Alternatively this monochromator is combined with a photomultiplier to observe macro-temporal structure of the laser. One half of the output light is divided by a beam splitter and is introduced to a streak camera through another monochromator to monitor the cavity length . A bandpass filter was also used in place of the monochromator. The cavity length can be roughly tuned by observing the pulse shape of the output light [8] by a streak camera [2-4j . Although the accuracy of this method depends on the cavity loss and the bunch length, it is better than ± 20 pm in our case. More precise tuning is made by observing the laser oscillation. The streak camera was used to measure the bunch length also. A calibrated PIN photodiode, with a bandpass filter in front of it, was put into the optical path to measure the output laser power. Figs. 5a-5c show the typical output spectra before oscillation, just on oscillation threshold and during
MONOCHROMATOR STREAK CAMERA
Fig . 4. Schematic arrangement for laser-oscillation experiment . II . NEW FELS
36
K. Yainacla et al. / Visible oscillation on TERAS $000 4000
C
3000 2000 Z
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Z
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6000
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iâ1
25080 m 20000
~ Z Z
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(CJ
20000 C 3 ~
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1s000
v }
10000
W
5000
the maximum-gain wavelength . Almost the same phenomenon was observed in the early ACO experiments [9] . The ACO group attributed this discrepancy to the multimode component in the spontaneous-emission spectrum. In our experiment, only the output light forming the TEM,, mode was observed even before oscillation. We think that there might be some other reasons for this phenomenon. In fig . 5c neutral-density filters were put in front of the monochromator to reduce the light intensity so as to observe only the lasing peak. The lasing wavelength is 598 nm and the linewidth is about 0.26 nm, including the spectral resolution of our detection system (0.21 nm). The spectral intensity of the lasing peak was increased by a factor of abo--:t 10 4 , compared with that of spontaneous emission. The peak power of the micro-temporal laser pulse is roughly measured to be - 10 mW at beam current 4 mA/bunch . When the cavity mirrors were new, the laser oscillation continued for at least 40 min at the beam current from 7.4 to 3.4 mA/bunch, although intermittently with several Hz. Fig . 6 shows the temporal behavior of the laser pulses when the laser is on, observed with a photomultiplier. The laser oscillates in a random-pulse manner, so called "chaos" [101. Each macro-temporal pulse is formed from micro-temporal pulses separated by 35 ns (electron-bunch period) . The pulse width of the macro pulses and the time difference between them was 2 - 5 ms -and 5 - 30 ms, respectively, as shown in fig . 6. According to the numerical analysis on the evolution of the laser intensity and beam energy spread [I I], slow gain fluctuation with a few tens of Hz is necessary to explain such chaotic oscillation . Actually, certain slow fluctuations are observed on the
Z 5750
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5950
WAVELENGTH
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Fig. 5. Output spectra before oscillation (a), just on oscillation threshold (b) and during oscillation (c). In case (c) neutral density filters are put in front of the monochromator to reduce the light intensity. oscillation . The ring current was - 8-5 mA/bunch . Fig . 5a corresponds to a central part of the spontaneous-emission spectrum, although modified by the transmission curve of the cavity mirrors as well as the light amplification . It should be noted that in fig . 5b the central peak tends to shift toward the longer-wavelength side with the light amplification, where the gain is larger . However, the amount of the shift is only 0 .17 times the spontaneous-emission fringe distance, which is smaller than the expected value (0.25) from
Fig. 6. Typical macrotemporal structure of the laser observed with a photomultiplier. Horizontal scale is 5 ms/div.
K. Yamada et al. / Visible oscillation on TE-RAS
beam profile which cannot be suppressed in spite of careful LC control . Although the reasons is not evident, ion trapping or coherent betatron oscillations induced by some transverse beam instability might give rise to the slow beam fluctuations . To achieve cw oscillation in our system, it will be necessary to suppress such slow fluctuations . Q-switching operation was tried mainly at 2.5 Hz. In this case the spectral intensity of the lasing peak was increased, but the laser oscillated less frequently than in normal operation . This suggests that the frequency of the slow beam fluctuation, mentioned above, is not so large compared with 2.5 Hz and it interferes the Q-switching operation . If the slow beam fluctuation can be well suppressed, a pulsed oscillation with higher peak power is also expected with the low-frequency Q-switching . 4. Conclusion A FEL oscillation has been achieved at a visible wavelength on the storage ring TERAS using an optical klystron . The laser gain was increased by the increase of the stored current. Fine tuning of the LC effectively suppressed the phase oscillation by moderating the coupled-bunch instability . This reduced the beam-energy spread and bunch length, both of which enhanced the laser gain. The laser oscillated in a chaotic manner, which is due to the slow beam fluctuations . The slow beam fluctuations must be suppressed in order to realize cw oscillation as well as higherpeak-power pulsed oscillation with the Q-switching in our low gain system. Acknowledgements The authors would like to thank Dr. Y. Miyahara of JAERI for his advice on the double RF system using
37
the Landau cavity and Mr. M. Kawai of Kawasaki heavy industries Ltd . for his assistance. They are also grateful to Drs. Y. Petroff, M. Billardon and M.E. Couprie of LURE for their suggestions on the visible FEL oscillation . This work has been supported by Special Coordination funds for the Promotion of Science and Technology until 1988, and Peaceful Utilization Technology of Nuclear Energy from 1989, both from the Science and Technology Agency. References [11 T. Yamazaki. in: Free Electron Lasers 11. ed. Y. Petroff. Proc. SPI E 1133 (1989) 62. [2] T. Yamazaki, K. Yamada, S. Sugiyama, T. Tomimasu . T. Mikado, M. Chiwaki . R. Suzuki and H. Ohgaki . Proc. 2nd Int . Symp. on Advanced Nuclear Energy Research. JAERI . 1990. p. 308 . [3] K. Yamada. T. Yamazaki. S. Sugiyama . T. Tomimasu. T. Mikado, M. Chiwaki, R. Suzuki and H . Ohgaki. Proc. Tokyo Int . Symp. '90 on FELs Tokyo. 1991. p. 30. [4] K. Yamada. Y. Yamazaki. S. Sugiyama. T. Tomimasu. T. Mikado. M. Chiwaki . R. Suzuki and H. Ohgaki . Nucl. Instr. and Meth. A-304 (1991) 86. [5] Y. Miyahara . S. Asaoka. G_ Isoyama, A. Mikuni . H. Nishimura . K. Soda and H. Kanzaki. Jpn . J. Appl. Phys. 22 (1983) L733: Y. Mivahara . S. Asaoka. A. Mikuni and K_ Soda. Nucl . Instr. and Meth. A260 (1987) 518. [6] T. Yamazaki, K. Yamada. S. Sugiyama. H. Ohgaki. T. Tomimasu. T. Noguchi. T. Mikado. M. Chiwaki and R_ Suzuki, Nucl. Instr. and Meth. A309 (1991) 343. [7] T. Tomimasu et al., TELL-TERAS Activity Report 1980-1986. Electrotech. Lab. (1987) p. 21 . [8] P. Elleaume, M. Velghe. M. Billardon and J.M. Ortega . Appl. Opt . 24 (1985) 2762. [9] M. Billardon, P. Elleaume. J.M. Ortega . C. Bazin. M. Bergher. M. Velghe, D.A.G. Deacon and Y. Petroff. IEEE J. Quantum Electron. QE-21 (1985) 805. [101 M. Billardon . Phys. Rev. Lett. 65 (1990) 713. [11] P. Elleaume, J. Phys. 45 (1984) 997.
11. NEW FELS
Nuclear Instruments and Methods in Physics Research A318 (1992) 33-37 North-Holland
Visible oscillation of storage-ring free electron laser o K. Yamada, T. Yamazaki, S. Sugiyama, T. Tomimasu ', H. Ohgaki, T. Noguchi, T. Mikado, M. Chiwaki and R. Suzuki Electrotechnical Laboratom, 1-1-4 Umezono, Tsukuba City, Ibaraki 305. Japan
A visible laser oscillation has been achieved on the electron storage ring TERAS at 598 nm using a 1.47 m optical klystron. TERAS was operated in a three-bunch mode at a beam energy of 231 MeV with a beam current ranging from 3 to 10 mA per bunch. Coherent phase oscillations of the beam bunches were successfully suppressed by controlling a second harmonic RF cavity (Landau cavity) whose driving frequency is 343.24 MHz. The laser linewidth was observed to be 0.26 nm, including the detector resolution (0 .21 nm). The spectral intensity was about 104 times as large as that of the spontaneous emission . The peak laser power of the microtemporal pulse is roughly measured to be 10 mW at beam current 4 mA/bunch . l . Introduction
It is widely accepted that a storage ring is one of the most suitable accelerators for short-wavelength free electron laser (FEL) oscillation owing to its high energy with low emittance and small beam-energy spread . However, usual storage rings do not have sufficiently long straight sections to install long undulators . Consequently it is rather difficult to obtain the laser gain larger than the cavity loss. Since our storage ring TERAS was originally constructed for generation of synchrotron radiation and its applications, its low peak current in addition to short straight sections (1 .8 m) have made it difficult to achieve laser oscillation in spite of exploiting an optical klystron. We examined the characteristics of the beam quality and measured the FEL gain and cavity losses in order to assure visible laser oscillation with such a low gain system [1-4]. The gain was observed to be _ 1 x 10 -4 at a beam current of 1.6 mA/bunch . Although this value is just above the initial cavity loss, laser oscillation was difficult because of the cavity-mirror degradation by the undulator radiation. Recently the beam current was increased by a factor of 5 by modifying the radiofrequency knockout (RF-KO) method [1-4] to reduce the number of bunches . In addition, the beam quality has been improved successfully with careful tuning of the 2nd-harmonic Landau cavity (LC) [5] employed to suppress the coupled-bunch instability. These enabled us to achieve visible FEL oscillation on March 22, 1991 [6] and the data are being accumulated. In this paper Present address : FEL Research Institute, 2-7-4 Kyomachibori, Nishi-ku, Osaka City, 550 Japan.
we report the recent results of our FEL oscillation experiments. 2. Beam quality
The details of the storage ring TERAS [7] have been described in the previous reports [1-4]. TERAS is usually filled at an energy 300 MeV and its energy is ramped up or down for various applications . In case of FEL, the energy is ramped down to 230 - 240 MeV to obtain visible undulator radiations around 600 nm. In usual operations, 18 bunches are revolving in the ring. However, in FEL experiments only 3 bunches can be synchronized with an identical light pulse inside the laser cavity and the remainder does not contribute to the peak laser gain. Therefore, the redundant bunches must be eliminated to avoid mirror damage by the intense UV component in the undulator radiation. It is also known [4] that the beam quality is better with fewer number of bunches because of less intra-bunch interaction. For this purpose, a two-stage RF-KO method is used. At the first stage of this method, a RF signal with frequency 1/6 of the ring frequency is mixed with a signal of betatron frequency, amplified and fed to a rod-type perturbation electrode. 12 of 18 bunches are knocked out by selective excitation with six remaining bunches. Then the second stage RF-KO is performed with frequency 1/12 of the ring frequency mixed with betatron frequency, to reduce the number of bunches to the final value 3. The RF-KO used to be carried out after completing the electron injection and ramping down of the energy . During this procedure, however, a large number of electrons were lost. As a result, the final current was only - 3-4 mA per b;w%,
0168-9002/92/$05 .00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
11. NEW FELS
K Yainada et al. / Visible oscillation on TERAS
34
Fig. 1 . Typical spectrum-anah,-zer signals from a button-type beam monitor without (a) and with (b) LC tuning. Horizontal scale is 100 kHz/div. and vertical scale 10 dB/div .
580
600
620 (nm)
which finally decayed down to less than 2 mA when the FEL experiments started . This is because the LC deteriorates the beam stability at higher total current, more than 80 mA . The final current has recently been increased up to 10 mA per bunch by performing the first-stage RF-KO during electron injection into the ring. With fewer bunches, each bunch can share a larger current even if the total current is limited. The laser gain was expected to be improved with the successful increase of the current . In addition, it has been proved that, when the total current is rather low, the LC works very effectively to
580 600 620 WAVELENGTH (nm)
Fig. 2. Spontaneous-emission spectra without (a) and with (b) the Landau-cavity tuning . Beam current is 8 .5 mA/bunch. 4000
t-
3000
W
1Z
° 2000 LU J W
CIC
1000 0 0.000
1 .000
2.000
3.000
4 .000
TIME
(ns)
E
(ns)
5.000
5.000
7.000
6000
W IZ
âJ W
4500 3000 1500 0
TI
Fig . 3 . Bunch length without (a) and with (b) LC tuning . Beam current is 1 .4 mA/bunch .
K Yamada et al. / Visible osrillatiosi on
suppress the coherent phase oscillation and reduces the beam-energy spread by controlling finely the frequency tuner and phase shifter. Fig . I shows the frequency spectrum of signals from a button-type beam monitor without (fig. la) and with (fig. IN the LC tuning . The division of the vertical axis is according to a log scale. In fig . la, at least three sideband peaks, showing coherent phase oscillations, are observed on both sides of the central peak of 343 .24 MHz, the LC frequency . They are suppressed successfully by exciting the LC and controlling it, as shown in fig . lb. During the above tuning, the spontaneous-emission spectrum changes drastically . Figs. 2a and 2b show the typical spectra of spontaneous emission after passing through the output coupler of the laser cavity. for respective conditions in fig. l . In case (a), periodic fine structure peculiar to an optical klystron is not seen owing to the large energy spread. In case (b), however, a large modulation (f - 0.5) emerges, which means that the beam-energy spread was sufficiently reduced by adjusting the LC conditions. The LC tuning also had an influence on the bunch length in our experiments . Without the tuning, the bunch length is about 0.9 ns (FWHM) as shown in fig . 3a. On the other hand, it becomes shorter by a factor of two (fig. 3b) when the LC is tuned to suppress the sidebands. This increases the peak beam current by the same factor, which enhances the laser gain, together with the reduction of the energy spread. 3. Laser oscillation Actual laser oscillation experiments are conducted according to the following procedures . After injecting electrons into the ring in six bunches with the first-stage RF-KO, the beam energy is ramped down to around 230 MeV and then the number of bunches is reduced down to three by the second stage RF-ISO. Next the cavity mirrors are finely adjusted to make a proper TEM (N) optical mode inside the cavity by observing the lateral-mode profile of the spontaneous emission . The LC is tuned as mentioned in the previous section .
VACUUM MIRROR MANIPULATOR
Finally the cavity length is exactly adjusted to one half of the distance between successive bunches, which is necessary because the laser gain is extremely low. When the cavity length comes close to the precise tuning, one or two peaks on the spectrum begin to oscillate irregularly and the laser starts to oscillate with the tuning within about 5 p.m. Typical arrangements for the FEL oscillation experiments arc shown in fig . 4. A 1 .47 m optical klystron is put inside the laser cavity composed of two dielectriccoated multilayer concave mirrors whose initial loss is 40 ppm including 30 ppm transmission . The cavity length is 5.238 m and the radius of curvature of the mirrors is 3 m. The cavity length and mirror angles around two axes perpendicular to the laser axis can be adjusted by two vacuum mirror manipulators driven by stepping motors and piezoelectric actuators with accuracy of 0.2 plm and 4 ti rad, respectively. The output light from the cavity is introduced to a monochromator combined with a highly sensitive photodiode array and the real-time spectrum is observed to detect the instantaneous change of the undulatorradiation spectrum, which must follow the onset of laser oscillation . The spectral data from the photodiode array is acquired in a microcomputer. Alternatively this monochromator is combined with a photomultiplier to observe macro-temporal structure of the laser. One half of the output light is divided by a beam splitter and is introduced to a streak camera through another monochromator to monitor the cavity length . A bandpass filter was also used in place of the monochromator. The cavity length can be roughly tuned by observing the pulse shape of the output light [8] by a streak camera [2-4j . Although the accuracy of this method depends on the cavity loss and the bunch length, it is better than ± 20 pm in our case. More precise tuning is made by observing the laser oscillation. The streak camera was used to measure the bunch length also. A calibrated PIN photodiode, with a bandpass filter in front of it, was put into the optical path to measure the output laser power. Figs. 5a-5c show the typical output spectra before oscillation, just on oscillation threshold and during
MONOCHROMATOR STREAK CAMERA
Fig . 4. Schematic arrangement for laser-oscillation experiment . II . NEW FELS
36
K. Yainacla et al. / Visible oscillation on TERAS $000 4000
C
3000 2000 Z
1000
Z
5750
5800
5850
5900
595 :
6000
WAVELENGTHi
6050
6100
6250
6200
iâ1
25080 m 20000
~ Z Z
10000 5000
5750
5800
5850
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5950
WAVELENGTH
6000
6050
6100
6150
( CA)
25000
(CJ
20000 C 3 ~
6200
1s000
v }
10000
W
5000
the maximum-gain wavelength . Almost the same phenomenon was observed in the early ACO experiments [9] . The ACO group attributed this discrepancy to the multimode component in the spontaneous-emission spectrum. In our experiment, only the output light forming the TEM,, mode was observed even before oscillation. We think that there might be some other reasons for this phenomenon. In fig . 5c neutral-density filters were put in front of the monochromator to reduce the light intensity so as to observe only the lasing peak. The lasing wavelength is 598 nm and the linewidth is about 0.26 nm, including the spectral resolution of our detection system (0.21 nm). The spectral intensity of the lasing peak was increased by a factor of abo--:t 10 4 , compared with that of spontaneous emission. The peak power of the micro-temporal laser pulse is roughly measured to be - 10 mW at beam current 4 mA/bunch . When the cavity mirrors were new, the laser oscillation continued for at least 40 min at the beam current from 7.4 to 3.4 mA/bunch, although intermittently with several Hz. Fig . 6 shows the temporal behavior of the laser pulses when the laser is on, observed with a photomultiplier. The laser oscillates in a random-pulse manner, so called "chaos" [101. Each macro-temporal pulse is formed from micro-temporal pulses separated by 35 ns (electron-bunch period) . The pulse width of the macro pulses and the time difference between them was 2 - 5 ms -and 5 - 30 ms, respectively, as shown in fig . 6. According to the numerical analysis on the evolution of the laser intensity and beam energy spread [I I], slow gain fluctuation with a few tens of Hz is necessary to explain such chaotic oscillation . Actually, certain slow fluctuations are observed on the
Z 5750
5800
5850
5900
5950
WAVELENGTH
6000
6050
6100
6150
6200
fâl
Fig. 5. Output spectra before oscillation (a), just on oscillation threshold (b) and during oscillation (c). In case (c) neutral density filters are put in front of the monochromator to reduce the light intensity. oscillation . The ring current was - 8-5 mA/bunch . Fig . 5a corresponds to a central part of the spontaneous-emission spectrum, although modified by the transmission curve of the cavity mirrors as well as the light amplification . It should be noted that in fig . 5b the central peak tends to shift toward the longer-wavelength side with the light amplification, where the gain is larger . However, the amount of the shift is only 0 .17 times the spontaneous-emission fringe distance, which is smaller than the expected value (0.25) from
Fig. 6. Typical macrotemporal structure of the laser observed with a photomultiplier. Horizontal scale is 5 ms/div.
K. Yamada et al. / Visible oscillation on TE-RAS
beam profile which cannot be suppressed in spite of careful LC control . Although the reasons is not evident, ion trapping or coherent betatron oscillations induced by some transverse beam instability might give rise to the slow beam fluctuations . To achieve cw oscillation in our system, it will be necessary to suppress such slow fluctuations . Q-switching operation was tried mainly at 2.5 Hz. In this case the spectral intensity of the lasing peak was increased, but the laser oscillated less frequently than in normal operation . This suggests that the frequency of the slow beam fluctuation, mentioned above, is not so large compared with 2.5 Hz and it interferes the Q-switching operation . If the slow beam fluctuation can be well suppressed, a pulsed oscillation with higher peak power is also expected with the low-frequency Q-switching . 4. Conclusion A FEL oscillation has been achieved at a visible wavelength on the storage ring TERAS using an optical klystron . The laser gain was increased by the increase of the stored current. Fine tuning of the LC effectively suppressed the phase oscillation by moderating the coupled-bunch instability . This reduced the beam-energy spread and bunch length, both of which enhanced the laser gain. The laser oscillated in a chaotic manner, which is due to the slow beam fluctuations . The slow beam fluctuations must be suppressed in order to realize cw oscillation as well as higherpeak-power pulsed oscillation with the Q-switching in our low gain system. Acknowledgements The authors would like to thank Dr. Y. Miyahara of JAERI for his advice on the double RF system using
37
the Landau cavity and Mr. M. Kawai of Kawasaki heavy industries Ltd . for his assistance. They are also grateful to Drs. Y. Petroff, M. Billardon and M.E. Couprie of LURE for their suggestions on the visible FEL oscillation . This work has been supported by Special Coordination funds for the Promotion of Science and Technology until 1988, and Peaceful Utilization Technology of Nuclear Energy from 1989, both from the Science and Technology Agency. References [11 T. Yamazaki. in: Free Electron Lasers 11. ed. Y. Petroff. Proc. SPI E 1133 (1989) 62. [2] T. Yamazaki, K. Yamada, S. Sugiyama, T. Tomimasu . T. Mikado, M. Chiwaki . R. Suzuki and H. Ohgaki . Proc. 2nd Int . Symp. on Advanced Nuclear Energy Research. JAERI . 1990. p. 308 . [3] K. Yamada. T. Yamazaki. S. Sugiyama . T. Tomimasu. T. Mikado, M. Chiwaki, R. Suzuki and H . Ohgaki. Proc. Tokyo Int . Symp. '90 on FELs Tokyo. 1991. p. 30. [4] K. Yamada. Y. Yamazaki. S. Sugiyama. T. Tomimasu. T. Mikado. M. Chiwaki . R. Suzuki and H. Ohgaki . Nucl. Instr. and Meth. A-304 (1991) 86. [5] Y. Miyahara . S. Asaoka. G_ Isoyama, A. Mikuni . H. Nishimura . K. Soda and H. Kanzaki. Jpn . J. Appl. Phys. 22 (1983) L733: Y. Mivahara . S. Asaoka. A. Mikuni and K_ Soda. Nucl . Instr. and Meth. A260 (1987) 518. [6] T. Yamazaki, K. Yamada. S. Sugiyama. H. Ohgaki. T. Tomimasu. T. Noguchi. T. Mikado. M. Chiwaki and R_ Suzuki, Nucl. Instr. and Meth. A309 (1991) 343. [7] T. Tomimasu et al., TELL-TERAS Activity Report 1980-1986. Electrotech. Lab. (1987) p. 21 . [8] P. Elleaume, M. Velghe. M. Billardon and J.M. Ortega . Appl. Opt . 24 (1985) 2762. [9] M. Billardon, P. Elleaume. J.M. Ortega . C. Bazin. M. Bergher. M. Velghe, D.A.G. Deacon and Y. Petroff. IEEE J. Quantum Electron. QE-21 (1985) 805. [101 M. Billardon . Phys. Rev. Lett. 65 (1990) 713. [11] P. Elleaume, J. Phys. 45 (1984) 997.
11. NEW FELS