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Characteristics of the NIJI-IV FEL at the electron-beam energy of 263 MeV
Nuclear Instruments and Methods in Physics Research A 407 (1998) 187—192
Characteristics of the NIJI-IV FEL at the electron-beam energy of 263 MeV N. Sei*, T. Yamazaki, K. Yamada, S. Sugiyama, T. Mikado, H. Ohgaki Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
Abstract Electron-beam qualities and characteristics of the FEL oscillation with the NIJI-IV FEL system were investigated at the low electron-beam energy of 263 MeV. The peak electron density at 263 MeV was suppressed due to instabilities in high electron-beam current, but it was about 40% higher than that at the normal electron-beam energy of 310 MeV below 4 mA. The maximum gain estimated from the measurements of the electron-beam qualities was about 1.5% at 4 mA. The measurement of the detuning curve suggests that the practical gain was about 0.8 times as low as the estimated value. Moreover, variation of the average FEL power versus the electron-beam current and the transverse profile was observed with a CCD camera system. The typical average power of the FEL was about 53 lW at 3.2 mA and we were able to observe up to the TEM mode. ( 1998 Elsevier Science B.V. All rights reserved. 03
1. Introduction Two experimental themes with the NIJI-IV FEL system have been studied at Electrotechnical Laboratory. One of them is the shortening wavelength and intensifying power of FELs with a fundamental emission from a 6.3 m optical klystron (ETLOKII). The operating point of the storage ring NIJI-IV has been modified to realize the higher-tune and lower-emittance [1], so that the wavelength of FEL oscillations has been shortened from 488 to 349 nm [2]. Moreover, the simple RF feedback system has contributed to shorten the bunch length [3]. Recently, a combination of two sextupole magnets and a quadrupole magnet has installed in one of the
* Corresponding author. Tel.: #81 298 54 5680; #81 298 54 5683; e-mail: [email protected].
fax:
short straight sections to suppress the head—tail instability. Experiments to realize FEL oscillations at around 300 nm are now under way. Another theme is application of higher harmonics, which includes FEL oscillations. The electron-beam energy is usually set to about 310 MeV in the case of FEL experiments with the fundamental emission. The maximum deflection parameter of the ETLOK-II is 2.26, and the wavelength of an FEL oscillation is shorter than about 350 nm. However, it is difficult to obtain the high reflectivity mirror under 200 nm. The electron-beam energy should be down in order to search the possibility that FEL oscillations with higher harmonics are achieved. It has accomplished to reduce the electron-beam energy down to 263 MeV in present operating point, and FEL experiments with the fundamental emission at around 488 nm have been carried out [4]. The FEL gain at 488 nm was
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 3 9 2 - 2
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estimated from the measurement of the peak electron density and confirmed by the observation of a detuning curve. In this article, the electron-beam qualities and characteristics of the NIJI-IV FEL at the low electron-beam energy of 263 MeV are described.
2. Electron-beam qualities In order to estimate the FEL gain and threshold current at 263 MeV, it is necessary to investigate the electron-beam qualities. Some beam instabilities in the single-bunch operation were observed in the NIJI-IV as the beam current increased [3]. Thus, we measured the dependence of the beam qualities upon the beam current. The bunch length pl was measured by using a streak camera with a resolution of 18 ps. As we reported before [3], the dependence of the bunch length upon the beam current had a large bump from 1 to 2 mA. In the case of 310 MeV, a similar bump also appeared in slightly higher beam current. A mode transition between dipole bunch oscillation to quadrupole one was observed in this beam current region. Such a bump on the bunch length curve and an extraordinary bunch lengthening above 6 mA had not been observed in the measurements before 1995 [2]. Recently, we detect traces of electric discharge on a surface of the RF cavity and repair an RF electrode which has a quite small leak. But we have not made the relation between these RF troubles and the bunch lengthening. The beam sizes were measured with a CCD camera which was focused on the center of the dispersive section. The beam profile was elliptical and the horizontal size p was 3—5 times as x long as the vertical one p . Generally, the y beam sizes, that is, the beam emittance on a storage ring does not vary in low beam current. However, the vertical size in the NIJI-IV increased below even a few mA as the beam current increased. We suppose this phenomenon to be caused by a microwave instability because the NIJI-IV has the large impedance of the vacuum chambers [5]. Moreover, the lifetime lengthening in higher beam current suggests that multiple Touschek
effect would also cause the growth of the beam emittance. The results of these measurements lead one of the important gain parameters o , a peak electron 1%!, density in m~3. o is given by 1%!, I % o " , 1%!, (2p)3@2 ef p p pl 3%7 x y
(1)
where I is electron-beam current, e is elementary % charge, and f is revolution frequency. Fig. 1 3%7 shows the current dependency of o in the 1%!, single-bunch operation at two different energies. As this figure shows, the peak density at 263 MeV was suppressed because of instabilities above 4 mA, but it was about 40% higher than that at 310 MeV below 4 mA. The reason for this relation is that the bunch length and beam emittance increase with the beam energy generally. Therefore, higher FEL gain can be expected at the low beam energy of 263 MeV though the beam current is comparatively low. The relative energy spread p /c was estimated by c measuring the modulation factor f of the spontaneous-emission spectrum versus the dispersive section gap. As the beam current increased, p /c also c became wider [3]. Actual lasing experiments at 263 MeV were performed with 5 mA per bunch or less, so that p /c did not go beyond 5]10~4 in c these experiments. An optimum N , the wave num$ ber of light passing over electrons in the dispersive
Fig. 1. Peak electron density versus the electron-beam current. Solid and open circle denote the peak density at 263 MeV and that at 310 MeV.
N. Sei et al. /Nucl. Instr. and Meth. in Phys. Res. A 407 (1998) 187—192
section, for the gain is given by 1 (2) (N#N )" $ 4p(p /c) c where N is the number of period in one undulator section. In the case of the ETLOK-II, N is 42 and N is not more than 110 with the undulator gaps of $ 36.25 mm. Thus, we can obtain the maximum gain when N -value is the maximum with the dispersive $ gap of 36 mm. However, the optimized N -value for $ the FEL power should be lower than that for the gain [6]. FEL gain G for the fundamental wavelength of 0 an optical klystron is described as the following equations: G "1.12]10~13j N2(N#N )K2[J (m)!J (m)]2 0 6 $ 1 0 ]fo F c~3, 1%!, f m"K2(4#2K2)~2, (3) where c is electron-beam energy, j is the period 6 length of an undulator, and F means the filling & factor in Ref. [7]. The FEL gain calculated with these formulae and experimental parameters is represented in Fig. 2. The calculated gain reaches up to about 1.5% at 4 mA, but it decreases at the higher beam current. This is the result of the decay of o and f for the beam current. We can estimate 1%!,
189
the threshold beam current for an FEL oscillation at 488 nm with Fig. 2. According to measurements of one round-trip cavity loss with an external cavity [8], the cavity mirrors were degraded by undulator radiation, and the typical round-trip cavity loss was valued to be about 600 ppm when the mirrors were exposed to the undulator radiation of 10 mA h. Therefore, an estimated value of the threshold beam current is only 0.12 mA per bunch.
3. FEL oscillations at 488 nm
3.1. Detuning curve The amplification of a resonated light pulse becomes weak as the synchronization of the resonated light pulse with an electron bunch shifts. This is explained by taking into account the gain distribution which is caused by longitudinal distribution of the electron bunch, f(z). We define the center of the electron bunch as the origin of z and the light-pulse displacement from the electron bunch per one round trip as d. Then the amplification as a function of d, that is to say a detuning curve F(d), can be calculated by summing the contributions over the electron position inside the electron bunch and over the round-trip number in the optical cavity [9]: F(d)" l :+=
Fig. 2. Gain at 488 nm estimated with the measurement of electron-beam qualities of the NIJI-IV. The undulator gaps and the dispersive gap are 36.25 and 36 mm, respectively.
(4)
where l is the round-trip cavity loss and g is the # 1 peak gain in the considered wavelength. We can also consider F(d) as a function of l and g . This # 1 equation would be valid for low amplification where the bunch lengthening due to the interaction with the intense light pulse is ignored. The amplification of the resonated spontaneous emission was observed by a spectrum multi-channel analyzer which consisted of a photo-diode array with a monochromator. As an example, we show the experimental result of a detuning curve at the beam current of 0.64 mA in Fig. 3. The peak gain valued by Eq. (3) and the cavity loss measured by
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the above method are 3300 and 600 ppm, respectively. On the other hand, we calculate the value of F(d) for various value of l and g by a computer. As # 1 the result, it is noted that the curves which have same value of the difference g !l make similar 1 # behavior in low amplification (410). The bestfitting value of g !l is about 1900 ppm for this 1 # measurement. However, it is difficult to correctly estimate the value of l and g separately without # 1 considering the effect of the bunch heating. As Fig. 3 shows, g is roughly estimated to be 1
2500 ppm if the cavity loss corresponds to 600 ppm. This estimated value is about 0.8 times as large as the gain calculated from Eq. (3). The main reason for this gain reduction would be misalignment between the electron-beam axis and the magnetic centerline of the ETLOK-II. Including the gain reduction, we expect that the maximum peak gain at 488 mm was about 1.2%. The threshold beam current for the FEL oscillation was not measured accurately, but it was confirmed that the laser threshold occurred from 0.10 to 0.27 mA. This result is valid for the estimation from Eq. (3) as above. 3.2. Average FEL power
Fig. 3. Normalized laser power as a function of the mirror displacement from the tuned cavity length which exactly matched the electron round-trip in the ring.
The FEL average power out of the optical cavity was measured with a CCD camera. A schematic layout of the measurement is illustrated in Fig. 4. The CCD camera was adjusted to focus on the down-stream cavity mirror. In order to extract the FEL from the resonated spontaneous emission, a band-pass filter with the central wavelength of 488 nm and the bandwidth of 3 nm was placed in the optics. The efficiency of the CCD pixels was defined with an Ar` laser of which power was calibrated. The average power was determined by integrating the intensity over the whole CCD pixels. Though this measurement system is simple,
Fig. 4. Schematic layout of the measurement of the average power and the transverse profile. When the CCD pixels were calibrated with an Ar` laser, the cavity mirrors were taken out of this optics.
N. Sei et al. /Nucl. Instr. and Meth. in Phys. Res. A 407 (1998) 187—192
we can measure rather low average power and observe the distribution of the FEL intensity simultaneously. Fig. 5 shows an example of the change of the average power and the electron-beam current. The experiment was made in the imperfect single-bunch operation, and the ratio of the main bunch, which only contributed to the FEL oscillation, was about 0.5 when the oscillation started. This experiment was stopped at the total electron-beam current of 2.25 mA though the FEL still oscillated. The jumps observed in Fig. 5 were caused by the adjustment of the cavity length which probably drifted in the FEL operation. The ratio of the average power to the electron-beam current was roughly constant after the adjustment. The average FEL power was 34 times as high as the power of resonated spontaneous emission and the peak intensity of the FEL was 165 times as high as that of the resonated spontaneous emission at 488 nm. This ratio of the peak intensity agrees with the result of the detuning curve in Fig. 3. The maximum average power which we observed in this operation was 53 lW, so that the average intracavity power would be up to 1.77 W due to the mirror transmission of 30 ppm. 3.3. FEL transverse profile
191
With aligning the axis of the optical cavity to the electron-beam trajectory in the ETLOK-II, we obtained an almost perfect TEM mode at 488 nm. 00 The horizontal size and vertical size of the FEL profile which we measured on a cavity mirror were 1.72 and 1.75 mm, respectively. The theoretical beam size of the FEL on the cavity mirror, w , is R given as the following equation:
A B
jR 2 d w4 " , R p 2R!d
(5)
where j is the wavelength of the FEL, R is radius of the mirror, and d is the cavity length. By using parameters of the NIJI-IV FEL system to Eq. (5) [10], w is to be 1.62 mm. The observed R profile would be slightly wide due to the broad resonated spontaneous emission. When one of the cavity mirrors was rotated in the vertical plane from the precise alignment, pure TEM modes were observed [4]. In the case of the 0/ rotation in the horizontal plane, TEM modes /0 were not observed clearly because the horizontal electron-beam size was rather wide. Fig. 6 shows the intensity of each mode which is integrated over the horizontal direction on the down-stream mirror. We were able to measure up to TEM mode 03 and more higher modes were not observed due to the small mirror radius of 15 mm. Comparing the
Some transverse modes of the FEL were also observed with the CCD camera system in Fig. 4.
Fig. 5. An example of the average power and the electron-beam current.
Fig. 6. Average FEL intensity integrated over the horizontal direction on the down-stream mirror. Each TEM mode was 0/ made by the rotation of the down-stream mirror of 160 lrad (TEM ), 240 lrad (TEM ) and 280 lrad (TEM ). Power ratio 01 02 03 of each mode to the TEM is presented in the box in this figure. 00
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average power of the higher modes with that of the TEM mode, we find an interesting fact that the 00 power of each mode except the TEM mode was 03 almost constant. This conservation of the average power suggests that saturation of the FEL occurred even in the higher mode in spite of the reduction of the filling factor [11]. The diffraction loss would be the main reason why the power of the TEM 03 mode is lower than the other modes. We also observed the shift of the wavelength of the FEL oscillation when the transverse profile changed. 4. Conclusions Electron-beam qualities and characteristics of the NIJI-IV FEL at the low electron-beam energy of 263 MeV have been measured. The peak electron density at 263 MeV was suppressed due to instabilities in high beam current, but it was about 40% higher than that at 310 MeV below 4 mA. The gain calculated by using the measured beam qualities reaches up to about 1.5% at 4 mA. The estimation from the observed detuning curve suggests that the practical gain is about 0.8 times as large as the calculated one. The typical average power of the FEL was about 53 lW at 3.2 mA. The ratio of the average power to the electron-beam current was roughly constant in the high gain region if the
cavity length did not drift. We have observed up to the TEM mode and found that saturation of the 03 FEL occurred even in the higher mode in spite of the reduction of the filling factor. We have also measured the temporal structure of the FEL, which are presented in other paper [4]. In order to estimate the FEL gain with the third-harmonic emission at wavelength above 200 nm, it is intended to reduce the electron-beam energy below 237 MeV and to investigate the electron-beam qualities in the next step.
References [1] T. Yamazaki et al., Nucl. Instr. and Meth. A 341 (1994) ABS 3. [2] T. Yamazaki et al., Nucl. Instr. and Meth. A 358 (1995) 353. [3] N. Sei et al., Nucl. Instr. and Meth. A 393 (1997) 38. [4] T. Yamazaki et al., in: G. Dattoli, A. Renieri (Eds.), Free Electron Lasers 1996, North-Holland, Amsterdam, 1997, p. II-7. [5] M. Yokoyama et al., Nucl. Instr. and Meth. A 375 (1996) 53. [6] M.E. Couprie et al., Nucl. Instr. and Meth. A 272 (1988) 166. [7] W.B. Colson et al., Appl. Phys. B 29 (1982) 101. [8] K. Yamada et al., Nucl. Instr. and Meth. A 358 (1995) 392. [9] P. Elleaume et al., J. Phys. (Paris) 45 (1984) 997. [10] T. Yamazaki et al., Nucl. Instr. and Meth. A 331 (1993) 27. [11] M.E. Couprie et al., Phys. Rev. A 44 (1991) 1301.
Characteristics of the NIJI-IV FEL at the electron-beam energy of 263 MeV N. Sei*, T. Yamazaki, K. Yamada, S. Sugiyama, T. Mikado, H. Ohgaki Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
Abstract Electron-beam qualities and characteristics of the FEL oscillation with the NIJI-IV FEL system were investigated at the low electron-beam energy of 263 MeV. The peak electron density at 263 MeV was suppressed due to instabilities in high electron-beam current, but it was about 40% higher than that at the normal electron-beam energy of 310 MeV below 4 mA. The maximum gain estimated from the measurements of the electron-beam qualities was about 1.5% at 4 mA. The measurement of the detuning curve suggests that the practical gain was about 0.8 times as low as the estimated value. Moreover, variation of the average FEL power versus the electron-beam current and the transverse profile was observed with a CCD camera system. The typical average power of the FEL was about 53 lW at 3.2 mA and we were able to observe up to the TEM mode. ( 1998 Elsevier Science B.V. All rights reserved. 03
1. Introduction Two experimental themes with the NIJI-IV FEL system have been studied at Electrotechnical Laboratory. One of them is the shortening wavelength and intensifying power of FELs with a fundamental emission from a 6.3 m optical klystron (ETLOKII). The operating point of the storage ring NIJI-IV has been modified to realize the higher-tune and lower-emittance [1], so that the wavelength of FEL oscillations has been shortened from 488 to 349 nm [2]. Moreover, the simple RF feedback system has contributed to shorten the bunch length [3]. Recently, a combination of two sextupole magnets and a quadrupole magnet has installed in one of the
* Corresponding author. Tel.: #81 298 54 5680; #81 298 54 5683; e-mail: [email protected].
fax:
short straight sections to suppress the head—tail instability. Experiments to realize FEL oscillations at around 300 nm are now under way. Another theme is application of higher harmonics, which includes FEL oscillations. The electron-beam energy is usually set to about 310 MeV in the case of FEL experiments with the fundamental emission. The maximum deflection parameter of the ETLOK-II is 2.26, and the wavelength of an FEL oscillation is shorter than about 350 nm. However, it is difficult to obtain the high reflectivity mirror under 200 nm. The electron-beam energy should be down in order to search the possibility that FEL oscillations with higher harmonics are achieved. It has accomplished to reduce the electron-beam energy down to 263 MeV in present operating point, and FEL experiments with the fundamental emission at around 488 nm have been carried out [4]. The FEL gain at 488 nm was
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 3 9 2 - 2
III. EXPERIMENTS
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N. Sei et al. /Nucl. Instr. and Meth. in Phys. Res. A 407 (1998) 187—192
estimated from the measurement of the peak electron density and confirmed by the observation of a detuning curve. In this article, the electron-beam qualities and characteristics of the NIJI-IV FEL at the low electron-beam energy of 263 MeV are described.
2. Electron-beam qualities In order to estimate the FEL gain and threshold current at 263 MeV, it is necessary to investigate the electron-beam qualities. Some beam instabilities in the single-bunch operation were observed in the NIJI-IV as the beam current increased [3]. Thus, we measured the dependence of the beam qualities upon the beam current. The bunch length pl was measured by using a streak camera with a resolution of 18 ps. As we reported before [3], the dependence of the bunch length upon the beam current had a large bump from 1 to 2 mA. In the case of 310 MeV, a similar bump also appeared in slightly higher beam current. A mode transition between dipole bunch oscillation to quadrupole one was observed in this beam current region. Such a bump on the bunch length curve and an extraordinary bunch lengthening above 6 mA had not been observed in the measurements before 1995 [2]. Recently, we detect traces of electric discharge on a surface of the RF cavity and repair an RF electrode which has a quite small leak. But we have not made the relation between these RF troubles and the bunch lengthening. The beam sizes were measured with a CCD camera which was focused on the center of the dispersive section. The beam profile was elliptical and the horizontal size p was 3—5 times as x long as the vertical one p . Generally, the y beam sizes, that is, the beam emittance on a storage ring does not vary in low beam current. However, the vertical size in the NIJI-IV increased below even a few mA as the beam current increased. We suppose this phenomenon to be caused by a microwave instability because the NIJI-IV has the large impedance of the vacuum chambers [5]. Moreover, the lifetime lengthening in higher beam current suggests that multiple Touschek
effect would also cause the growth of the beam emittance. The results of these measurements lead one of the important gain parameters o , a peak electron 1%!, density in m~3. o is given by 1%!, I % o " , 1%!, (2p)3@2 ef p p pl 3%7 x y
(1)
where I is electron-beam current, e is elementary % charge, and f is revolution frequency. Fig. 1 3%7 shows the current dependency of o in the 1%!, single-bunch operation at two different energies. As this figure shows, the peak density at 263 MeV was suppressed because of instabilities above 4 mA, but it was about 40% higher than that at 310 MeV below 4 mA. The reason for this relation is that the bunch length and beam emittance increase with the beam energy generally. Therefore, higher FEL gain can be expected at the low beam energy of 263 MeV though the beam current is comparatively low. The relative energy spread p /c was estimated by c measuring the modulation factor f of the spontaneous-emission spectrum versus the dispersive section gap. As the beam current increased, p /c also c became wider [3]. Actual lasing experiments at 263 MeV were performed with 5 mA per bunch or less, so that p /c did not go beyond 5]10~4 in c these experiments. An optimum N , the wave num$ ber of light passing over electrons in the dispersive
Fig. 1. Peak electron density versus the electron-beam current. Solid and open circle denote the peak density at 263 MeV and that at 310 MeV.
N. Sei et al. /Nucl. Instr. and Meth. in Phys. Res. A 407 (1998) 187—192
section, for the gain is given by 1 (2) (N#N )" $ 4p(p /c) c where N is the number of period in one undulator section. In the case of the ETLOK-II, N is 42 and N is not more than 110 with the undulator gaps of $ 36.25 mm. Thus, we can obtain the maximum gain when N -value is the maximum with the dispersive $ gap of 36 mm. However, the optimized N -value for $ the FEL power should be lower than that for the gain [6]. FEL gain G for the fundamental wavelength of 0 an optical klystron is described as the following equations: G "1.12]10~13j N2(N#N )K2[J (m)!J (m)]2 0 6 $ 1 0 ]fo F c~3, 1%!, f m"K2(4#2K2)~2, (3) where c is electron-beam energy, j is the period 6 length of an undulator, and F means the filling & factor in Ref. [7]. The FEL gain calculated with these formulae and experimental parameters is represented in Fig. 2. The calculated gain reaches up to about 1.5% at 4 mA, but it decreases at the higher beam current. This is the result of the decay of o and f for the beam current. We can estimate 1%!,
189
the threshold beam current for an FEL oscillation at 488 nm with Fig. 2. According to measurements of one round-trip cavity loss with an external cavity [8], the cavity mirrors were degraded by undulator radiation, and the typical round-trip cavity loss was valued to be about 600 ppm when the mirrors were exposed to the undulator radiation of 10 mA h. Therefore, an estimated value of the threshold beam current is only 0.12 mA per bunch.
3. FEL oscillations at 488 nm
3.1. Detuning curve The amplification of a resonated light pulse becomes weak as the synchronization of the resonated light pulse with an electron bunch shifts. This is explained by taking into account the gain distribution which is caused by longitudinal distribution of the electron bunch, f(z). We define the center of the electron bunch as the origin of z and the light-pulse displacement from the electron bunch per one round trip as d. Then the amplification as a function of d, that is to say a detuning curve F(d), can be calculated by summing the contributions over the electron position inside the electron bunch and over the round-trip number in the optical cavity [9]: F(d)" l :+=
Fig. 2. Gain at 488 nm estimated with the measurement of electron-beam qualities of the NIJI-IV. The undulator gaps and the dispersive gap are 36.25 and 36 mm, respectively.
(4)
where l is the round-trip cavity loss and g is the # 1 peak gain in the considered wavelength. We can also consider F(d) as a function of l and g . This # 1 equation would be valid for low amplification where the bunch lengthening due to the interaction with the intense light pulse is ignored. The amplification of the resonated spontaneous emission was observed by a spectrum multi-channel analyzer which consisted of a photo-diode array with a monochromator. As an example, we show the experimental result of a detuning curve at the beam current of 0.64 mA in Fig. 3. The peak gain valued by Eq. (3) and the cavity loss measured by
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the above method are 3300 and 600 ppm, respectively. On the other hand, we calculate the value of F(d) for various value of l and g by a computer. As # 1 the result, it is noted that the curves which have same value of the difference g !l make similar 1 # behavior in low amplification (410). The bestfitting value of g !l is about 1900 ppm for this 1 # measurement. However, it is difficult to correctly estimate the value of l and g separately without # 1 considering the effect of the bunch heating. As Fig. 3 shows, g is roughly estimated to be 1
2500 ppm if the cavity loss corresponds to 600 ppm. This estimated value is about 0.8 times as large as the gain calculated from Eq. (3). The main reason for this gain reduction would be misalignment between the electron-beam axis and the magnetic centerline of the ETLOK-II. Including the gain reduction, we expect that the maximum peak gain at 488 mm was about 1.2%. The threshold beam current for the FEL oscillation was not measured accurately, but it was confirmed that the laser threshold occurred from 0.10 to 0.27 mA. This result is valid for the estimation from Eq. (3) as above. 3.2. Average FEL power
Fig. 3. Normalized laser power as a function of the mirror displacement from the tuned cavity length which exactly matched the electron round-trip in the ring.
The FEL average power out of the optical cavity was measured with a CCD camera. A schematic layout of the measurement is illustrated in Fig. 4. The CCD camera was adjusted to focus on the down-stream cavity mirror. In order to extract the FEL from the resonated spontaneous emission, a band-pass filter with the central wavelength of 488 nm and the bandwidth of 3 nm was placed in the optics. The efficiency of the CCD pixels was defined with an Ar` laser of which power was calibrated. The average power was determined by integrating the intensity over the whole CCD pixels. Though this measurement system is simple,
Fig. 4. Schematic layout of the measurement of the average power and the transverse profile. When the CCD pixels were calibrated with an Ar` laser, the cavity mirrors were taken out of this optics.
N. Sei et al. /Nucl. Instr. and Meth. in Phys. Res. A 407 (1998) 187—192
we can measure rather low average power and observe the distribution of the FEL intensity simultaneously. Fig. 5 shows an example of the change of the average power and the electron-beam current. The experiment was made in the imperfect single-bunch operation, and the ratio of the main bunch, which only contributed to the FEL oscillation, was about 0.5 when the oscillation started. This experiment was stopped at the total electron-beam current of 2.25 mA though the FEL still oscillated. The jumps observed in Fig. 5 were caused by the adjustment of the cavity length which probably drifted in the FEL operation. The ratio of the average power to the electron-beam current was roughly constant after the adjustment. The average FEL power was 34 times as high as the power of resonated spontaneous emission and the peak intensity of the FEL was 165 times as high as that of the resonated spontaneous emission at 488 nm. This ratio of the peak intensity agrees with the result of the detuning curve in Fig. 3. The maximum average power which we observed in this operation was 53 lW, so that the average intracavity power would be up to 1.77 W due to the mirror transmission of 30 ppm. 3.3. FEL transverse profile
191
With aligning the axis of the optical cavity to the electron-beam trajectory in the ETLOK-II, we obtained an almost perfect TEM mode at 488 nm. 00 The horizontal size and vertical size of the FEL profile which we measured on a cavity mirror were 1.72 and 1.75 mm, respectively. The theoretical beam size of the FEL on the cavity mirror, w , is R given as the following equation:
A B
jR 2 d w4 " , R p 2R!d
(5)
where j is the wavelength of the FEL, R is radius of the mirror, and d is the cavity length. By using parameters of the NIJI-IV FEL system to Eq. (5) [10], w is to be 1.62 mm. The observed R profile would be slightly wide due to the broad resonated spontaneous emission. When one of the cavity mirrors was rotated in the vertical plane from the precise alignment, pure TEM modes were observed [4]. In the case of the 0/ rotation in the horizontal plane, TEM modes /0 were not observed clearly because the horizontal electron-beam size was rather wide. Fig. 6 shows the intensity of each mode which is integrated over the horizontal direction on the down-stream mirror. We were able to measure up to TEM mode 03 and more higher modes were not observed due to the small mirror radius of 15 mm. Comparing the
Some transverse modes of the FEL were also observed with the CCD camera system in Fig. 4.
Fig. 5. An example of the average power and the electron-beam current.
Fig. 6. Average FEL intensity integrated over the horizontal direction on the down-stream mirror. Each TEM mode was 0/ made by the rotation of the down-stream mirror of 160 lrad (TEM ), 240 lrad (TEM ) and 280 lrad (TEM ). Power ratio 01 02 03 of each mode to the TEM is presented in the box in this figure. 00
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average power of the higher modes with that of the TEM mode, we find an interesting fact that the 00 power of each mode except the TEM mode was 03 almost constant. This conservation of the average power suggests that saturation of the FEL occurred even in the higher mode in spite of the reduction of the filling factor [11]. The diffraction loss would be the main reason why the power of the TEM 03 mode is lower than the other modes. We also observed the shift of the wavelength of the FEL oscillation when the transverse profile changed. 4. Conclusions Electron-beam qualities and characteristics of the NIJI-IV FEL at the low electron-beam energy of 263 MeV have been measured. The peak electron density at 263 MeV was suppressed due to instabilities in high beam current, but it was about 40% higher than that at 310 MeV below 4 mA. The gain calculated by using the measured beam qualities reaches up to about 1.5% at 4 mA. The estimation from the observed detuning curve suggests that the practical gain is about 0.8 times as large as the calculated one. The typical average power of the FEL was about 53 lW at 3.2 mA. The ratio of the average power to the electron-beam current was roughly constant in the high gain region if the
cavity length did not drift. We have observed up to the TEM mode and found that saturation of the 03 FEL occurred even in the higher mode in spite of the reduction of the filling factor. We have also measured the temporal structure of the FEL, which are presented in other paper [4]. In order to estimate the FEL gain with the third-harmonic emission at wavelength above 200 nm, it is intended to reduce the electron-beam energy below 237 MeV and to investigate the electron-beam qualities in the next step.
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