- Email: [email protected]
Observation of self-amplified spontaneous emission in the infrared free electron laser “CLIO”
Nuclear Instruments and Methods in Physics Research A 393 (1997)326-331
NUCLEAR
INSTRUMENTS &MEmooS IN PHVSBCS RESEARCH Section A
ELSEVIER
Observation
of self-amplified spontaneous emission in the infrared free electron laser “CLIO”
R. Prazeres”, J.M. Berset, F. Glotin, D.A. Jaroszynski, 0. Marcouillk, J.M. Ortega LURE, b&t, 209d, Universith de Paris-Sud, 91405 Orsay cedex, France
Abstract A solution has been proposed about ten years ago to reach the X-ray range: the principle is to operate the FEL in the Self-Amplified Spontaneous Emission (SASE) configuration. In the high gain regime, the spontaneous emission is amplified along the undulator in a single pass configuration and without optical cavity. We report here the observation of SASE at the shortest wavelength, the mid-infrared range. A spectral analysis of the SASE has been carried-out in the start-up regime of SASE, far from the saturation level. Keywords:
FEL; SASE; Superradiance;
Coherent emission
1. Introduction There are many kinds of Free Electron Lasers (FEL), operating in various wavelength regions from millimeter waves to ultra-violet [ 11. In principle, the FEL is able to work in the X-ray special range. However, at present, the quality of the optical cavity mirrors and of the electron beam is not sufficient to allow the operation of the FEL at wavelengths shorter than 100 nm. The optical gain per pass may be improved by increasing the undulators length. However, the problems with the X-ray mirrors seems difficult to solve. Nevertheless, a solution has been proposed about ten years ago to reach the X-ray range [2]: the principle is to operate the FEL in a high gain regime, in single pass configuration, and without optical cavity. The “spontaneous emission”, produced by the electrons in the undulator, is amplified by the high FEL gain along the undulator, and reaches a saturation regime at the end of the undulator. This principle is called. “Self-Amplified Spontaneous Emission” (SASE). It requires a high quality electron beam (high peak current, low energy spread, small emittance), and a long undulator (tens of meters). The SASE action has already been observed in far-infrared [3], but not in X-ray. Indeed, in the infrared range, the electron beam requirements for observation of SASE are much more reason-
* Corresponding author.
able than for X-ray. The study of SASE in the infrared region is crucial to understand the process and to allow extrapolations leading to the future development of SASE sources in X-rays. We present here the shortest wavelength observation of SASE, in mid-infrared region at L = 5 and 10 lrn. These observations have been done with the “CLIO” FEL. The parameters of the experiment are displayed in Table 1. The SASE process is one aspect of the radiation which occurs in the FEL, and which involves spontaneous emission, coherent spontaneous emission, FEL gain, SASE and superradiance which is equivalent to SASE in short pulse regime [4]. These will be described below. Spontaneous emission (SE) is the radiation produced by a single electron, crossing an undulator of N magnetic periods. It peaks at the so-called resonance wavelength 1, = I, (1 + K2/2)/2yZ where i, and K are, respectively, the period and the “deflection parameter” of the undulator, and ymc’ is the electron beam energy. The single electron radiation is a wave-train of N periods, corresponding to a length of N1,. The spectral linewidth of the radiation is AU/W z l/N, and the angular aperture of emission is roughly estimated by ose z 2,/(21,/N&,), which represents es. = 6mrad in the case of CL10 with ymc2 = 50MeV. The spectral width Awlw g l/N becomes sharper as the electron bunch propagates through the undulator (i.e. as N increases). Considering a bunch of N, electrons, and supposing that the electrons’ density is uniformly distributed in the bunch (with bunch dimensions larger than the emitted wavelength A,), the
0168-9002/97/$17.00Copyright 0 1997 Elsevier Science B.V. All rights reserved SO168-9002(97)00503-2
PII
R. Prazeres et al. /Nucl. In.& and Meth. in Phys. Res. A 393 (1997) 326-331
321
Table I Electron beam
Time structure Undulator(s)
FEL
energy: ymc* peak current: i transverse section: cA macro-pulse: At micro-pulse: 6t No. of periods: N magnetic period: 1. Total length L = 2NL, Gain per pass: G Pierce parameter: p
electrons are incoherent sources, and the energy produced by SE scales as NN,. Coherent Spontaneous Emission (CSE) is observed if the dimensions 6x, 6y, 6z of the electron bunch are much smaller than the emitted wavelength. In this case, the individual sources (electrons) are completely coherent and the total emitted energy, integrated over wavelength and angle, is scaling as NNZ. The energy scales linearly with undulator length, and it scales as a square law with the electron current. This effect may occur only for large wavelength (millimeter range) FELs. In most part of FELs, including CLIO, the electron bunch length is larger than the emitted wavelength (6z>>1), and the CSE is, in principle, negligible. Nevertheless, a partial coherence still occurs if the electron bunch longitudinal density n(z) contains sharp edges, or more generally strong components at high frequency in the Fourier spectrum of n(z). In this case, a CSE component of intensity I,,,(o) adds to the SE intensity i,,(o). This CSE component is described by the diffraction theory which gives the coherent amplitude &,,(w) = A,(w). [FTn] (l/n) emitted by the bunch, at frequency o; where A,(w) is the amplitude emitted by one electron, [FTn] is the Fourier Transform of the longitudinal electron density n(z). The total intensity I,(w) = Icoh + I,, is the sum of coherent and incoherent components. The total radiated energy scales as N(N, + fNi), where f is a coefficient taking into account the degree of coherence. The magnitude of the coherent intensity is directly linked to the magnitude of the high frequency components in the Fourier spectrum of n(z). Note that, further on, “spontaneous emission” will represent both the SE and the CSE, SE representing its incoherent part and CSE its coherent part. The FEL gain is induced by an interaction between the electron bunch and the laser wave, which creates a periodical micro-bunching on the electron beam, and produces a strong Fourier component [FTn] (l/n) at the laser wavelength. This component adds to the initial laser wave and leads to the FEL gain. For large values of gain, the spontaneous emission which is produced in the entrance of the undulator can be amplified along the crossing of the undulator, as a single-pass process. This kind of
50 MeV 70 A 0.5 x 0.5 mm lOps@25Hz 10 ps @ 62 MHz 19 (for each undulator)
5.04cm 2 m (for 2 undulators) lW500% (for 2 undulators) 1.9 x 1o-3
radiation is called SASE. The analysis of this process has been done by several authors, and first by Kim in 1986 [5,6]. It gives the radiated power of SASE:
PsAsE = P Pbeam$exp(rl~ PWN,,, where P,,_, = (i) ymc2/e is the kinetic power of the is the electron beam, q = 8n (3/2) l/‘, N,, = nc(2n)‘/‘/a, number of electrons in one coherence length where n is the linear density of electrons. The coherence length is defined as the length of the wave train corresponding to a given spectral width (obtained by Fourier Transform) of CT,.The expression contains an important dimensionless parameter: ne2KZ
P=
C
[JJ]*
320,y3k~mc2E,
1 l/3
’
where [JJ] = [J,(t) - 5,(t)] is the parameter in the FEL equations which depends on the Bessel functions J,, and J, (close to unity with 5 = K*/4(1 + K*/2)), CT*is the cross-sectional area of the electron beam, k, = 27c/A,with i, the undulator period length. The power of SASE grows exponentially along the undulator (i.e. when N increases), and stops when the factor (l/9). exp(q . pN) 2 N,,. The saturation occurs when pN g 1, and the saturation power is pP,,,,. When the conditions are far from saturation (pN<
V. PRE-BUNCHING/SUPERRADIANCE
328
R. Prazeres et al. / Nucl. Imtr. and Meth. in Phys. Rex A 393 (1997) 326-331
responding to a time integration over the whole electron macro-pulse. The curve A exhibits clearly a non-linear behaviour (the curve B also appears but less obvious) which implies a coherence effect: CSE and/or SASE. The curve C corresponds to the curve B which has a magnitude multiplied by a factor of 2. Since the CSE (and the SE) scales linearly with the undulator length L, the difference between curves A and C is due to the presence of SASE which does not scale linearly with L. This shows that SASE is present, but it does not tell anything about the CSE. It is important to note that the spontaneous emission can be incoherent (such as SE), but also partially coherent (including the CSE). The SASE amplification acts on the total spontaneous emission: SE + CSE. The growing of curves A and B is due to a mixing of both phenomena: SASE and CSE, because both are non-linear with respect the electron beam current. The amplification with a 2N periods undulator, which leads to SASE in curve A, acts also in the first undulator, and is responsible for the small non-linearity of curve B. In this case the coherence degree of the spontaneous emission is weak (CSE<< SE) or non-existant (CSE = 0). On the other hand, if we assume that the SASE amplification does not occur in the first undulator, but only in the second one, in this case the small non-linearity of curve B is due to the presence of CSE. In conclusion, these measurements show a rather strong SASE effect but they do not allow to deduce the degree of CSE. The linac must be carefully tuned in order to obtain SASE; as well as for operation of the FEL. There is a strong influence of the focusing quadrupoles (i.e. electron transverse section). On the other hand, the electron
2. Measurements
Spontaneous emission has been measured from the CL10 free electron laser in the following conditions. The radiation is picked-up by a retractable mirror at 45”, which is placed before the extraction mirror of the optical cavity. The radiation is sent directly to the detector (without passing through the telescope which is used to modify the laser mode and decrease the optical divergence). The detector is in the experiment room, which is about 15m away from the undulator. A focusing lens, with a diameter of about @ g 20mm, is installed in front of the detector, making an angular aperture for detection of ede, = 1.3mrad. This is to compare with the angular aperture of the SE which can be roughly estimated by: 0,,~2Jm=6 mrad for 1= 10 pm and for the undulator length N1, = 2 m. The undulator of CL10 is divided in two equal parts of N = 19 periods, for which each gap is independently adjustable. This allows to run the FEL in a two colour operation [7]. This feature is very useful in the SASE measurements, because it allows SASE radiation with a N = 19 periods undulator or one with 2N = 38 periods. This allows a discrimination between CSE and SASE, because CSE scales linearly with the undulator length, whereas SASE scales exponentially. The electron beam current can also be varied, by controlling the aperture of a beam slit on the linac. Fig. 1 shows the spontaneous emission, measured as a function of the electron beam average current, for a 2N periods undulator (curve A) and N periods undulator (curve B). The signal was detected with a nitrogen cooled InSb semiconductor, with time constant about 10 KScor-
0
5
10 e-
15 beam
20 current
25
30
35
(mA)
Fig. 1. Intensity of SASE versus electron beam current, curves A and B respectively with 2N = 38 and N = 19 periods undulator. Curve C is Curve B with doubling of the magnitude.
r
R. Prazeres et al. /Nucl.
In&r. and Meth. in Phys. Rex A 393 (1997) 326-331
329
SASE on,-
current
25 time Fig. 2. Influence
of the RF phase of the linac on the SASE intensity
(ps) (top curves).
The bottom curves represent the electron beam current
(which remains quite constant).
beam trajectory does not act strongly: because the optical cavity, which fixes the longitudinal axis, is not present in this configuration. The axis is only determined by the criterion of linearity of the beam trajectory along the undulator. There is another parameter of the linac which influences strongly both SASE/CSE and FEL: the phase of the RF wave according the electron bunches, in the pre-buncher RF cavity. A misalignment of this RF phase induces a spread of the electron bunch which makes a modification of the longitudinal distribution, increases the bunch length and therefore decreases the peak current. Nevertheless, the phase does not influence the electron beam energy, because the pre-buncher RF cavity is placed before the accelerator section, which works independently and transmits always the same energy of 50 MeV to the electrons. The influence of this RF phase (i.e. of the electron bunch peak current) on the SASE is shown in Fig. 2. This figure displays the time evolution, within the macro-pulse scale, of both the SASE intensity (top curves) and the electron beam current (bottom curves). A small modification of the phase of the linac is sufficient to reduce the SASE intensity by a factor of 15, whereas the electron beam average current remains more or less constant. This is characteristic of SASE (or gain), which is strongly dependant on the electron beam quality. Measurements of the SASE have been done for different values of the linac RF phase. These measurements are shown in the set of Figs. 3. For each phase, the
wavelength spectrum is displayed in Fig. 3(a) and the electron beam current versus time (macro-pulse scale) is displayed in Fig. 3(b). The intensity of SASE (in a.u.) for each curve is displayed in a table in Fig. (3a). When SASE intensity increases, Fig. 3(a) exhibits an increasing of the spectrum linewidth and a shift of the central wavelength toward large values. This effect has always been observed during our experiments. The larger spectrum in Fig. 3(a), corresponding to the larger SASE intensity, is centered at 3, = 11.5 urn, and has a linewidth of A1 = 2.7 urn corresponding to AI/2/1= 23%. The smaller spectrum is centered at 1 = 10 pm, and has a linewidth of AI = 1.8 urn corresponding to AI/I = 18%. Measurements of the SE with zero SASE was not possible because the signal was too small. Nevertheless, the theoretical value of SE linewidth is AA/jn= l/(38 periods) = 2.6%. The angular distribution of the SE increases the linewidth up to 3% or 4%, but not much more. In theory, a modification on the RF phase does not affect the electron beam trajectory or energy. A resonance wavelength shift from L = 10 to 11.5 pm represents a variation of 15%, and corresponds to an angular error of 0 = Jm = 8 mrad or to an energy variation of Ay/y = 7%. Such angular misalignment is not realistic because it represents a translation of 12 cm at the detector distance. The hypothesis of an energy variation is also not realistic because the energy acceptance of the linac is only Ay/y = 2.5%, due to an energy selection slit installed at the entrance of the FEL. For a 2N
V. PRE-BUNCHING/SUPERRADlANCE
R. Prazeres et al. /Nucl. Instr. and Meth. in Phys. Res. A 393 (1997) 326-331
330
10
j i
8
h .:: 2 h .Ia
6
4
7
8
9
11
10
wavelength
(al
0
5
12
Ib)
14
15
(pm)
15
10 time
13
20
2
(ps)
Fig. 3(a). Spectra of SASE for various amplification efficiencies (varying the RF phase of the linac). (b) Electron beam current versus time, for various RF phases of the linac (corresponding to Fig. 3(a)).
periods undulator, the SASE theory gives a,/o = experimental The (9p/(2~,/3[2N]))“~ =0.6%. increasing of the linewidth up to 23% is not in agreement with the SASE theory, which predicts a narrowing of the spectrum as compared to the SE. However, the SASE theory, giving the expression of a&, is only valid during the exponential growing of intensity and not during the start-up regime of SASE like in these measurements where the radiation and the gain are evolving at the same time. Within the Fourier limit conditions, a linewidth of An/1 = 23% corresponds to
a wave train of about AZ z 40 pm = 41. This value is 10 times shorter than the length N1 = 400pm of the SE wave train. It corresponds to a short pulse regime which corresponds to a “spiky” behaviour of the SASE or CSE process.
3. Conclusion The high gain, which can be obtained with the CL10 infrared free electron laser, allows to observe SASE in the
R. Prazeres et al. /Nucl.
Instr. and Meth. in Ph.vs. Res. A 393 (1997) 326-331
start-up regime, at 1 = 5 and 10 urn, with an undulator 2m long and 38 magnetic periods. The SASE intensity has been measured as a function of the electron beam current, and wavelength spectra have been determined for different values of SASE efficiency. The possibility to use either the full length (2N = 38 periods) or half length (N = 19 periods) of the undulator, allowed us to study the behavior of SASE with regards to the undulator length. The wavelength spectra exhibit a strong enlargement of the linewidth and a large shift of the central wavelength which are not explained by the theory of SASE.
331
References [l] [Z] [3] [4]
[S] [6] [7]
Proceedings of FEL’95 conference, Nucl. Instr. and Meth. A 375 (1996). B. Bonifacio et al., Opt. Commun. 50 (1984) 373. D. Bocek, et al., Nucl. Instr. and Meth. A 375 (1996) p. 13; D. A. Kirkpatrick, Nucl. Instr. and Meth. A 285 (1989) p. 43. D. A. Jaroszynski et al., these Proceedings (18th Free Electron Laser Conf., Rome, Italy, 1996) Nucl. Instr. and Meth. A 393 (1997) 332. K.-J. Kim, Phys. Rev. Lett. 57 (15) (1986) 1871. B. Bonifacio et al., Phys. Rev. A 44(6) (1991) 3441. D. A. Jaroszynski et al., Phys. Rev. Lett. 72(15) (1994) 2387.
V. PRE-BUNCHINGJSUPERRADIANCE
NUCLEAR
INSTRUMENTS &MEmooS IN PHVSBCS RESEARCH Section A
ELSEVIER
Observation
of self-amplified spontaneous emission in the infrared free electron laser “CLIO”
R. Prazeres”, J.M. Berset, F. Glotin, D.A. Jaroszynski, 0. Marcouillk, J.M. Ortega LURE, b&t, 209d, Universith de Paris-Sud, 91405 Orsay cedex, France
Abstract A solution has been proposed about ten years ago to reach the X-ray range: the principle is to operate the FEL in the Self-Amplified Spontaneous Emission (SASE) configuration. In the high gain regime, the spontaneous emission is amplified along the undulator in a single pass configuration and without optical cavity. We report here the observation of SASE at the shortest wavelength, the mid-infrared range. A spectral analysis of the SASE has been carried-out in the start-up regime of SASE, far from the saturation level. Keywords:
FEL; SASE; Superradiance;
Coherent emission
1. Introduction There are many kinds of Free Electron Lasers (FEL), operating in various wavelength regions from millimeter waves to ultra-violet [ 11. In principle, the FEL is able to work in the X-ray special range. However, at present, the quality of the optical cavity mirrors and of the electron beam is not sufficient to allow the operation of the FEL at wavelengths shorter than 100 nm. The optical gain per pass may be improved by increasing the undulators length. However, the problems with the X-ray mirrors seems difficult to solve. Nevertheless, a solution has been proposed about ten years ago to reach the X-ray range [2]: the principle is to operate the FEL in a high gain regime, in single pass configuration, and without optical cavity. The “spontaneous emission”, produced by the electrons in the undulator, is amplified by the high FEL gain along the undulator, and reaches a saturation regime at the end of the undulator. This principle is called. “Self-Amplified Spontaneous Emission” (SASE). It requires a high quality electron beam (high peak current, low energy spread, small emittance), and a long undulator (tens of meters). The SASE action has already been observed in far-infrared [3], but not in X-ray. Indeed, in the infrared range, the electron beam requirements for observation of SASE are much more reason-
* Corresponding author.
able than for X-ray. The study of SASE in the infrared region is crucial to understand the process and to allow extrapolations leading to the future development of SASE sources in X-rays. We present here the shortest wavelength observation of SASE, in mid-infrared region at L = 5 and 10 lrn. These observations have been done with the “CLIO” FEL. The parameters of the experiment are displayed in Table 1. The SASE process is one aspect of the radiation which occurs in the FEL, and which involves spontaneous emission, coherent spontaneous emission, FEL gain, SASE and superradiance which is equivalent to SASE in short pulse regime [4]. These will be described below. Spontaneous emission (SE) is the radiation produced by a single electron, crossing an undulator of N magnetic periods. It peaks at the so-called resonance wavelength 1, = I, (1 + K2/2)/2yZ where i, and K are, respectively, the period and the “deflection parameter” of the undulator, and ymc’ is the electron beam energy. The single electron radiation is a wave-train of N periods, corresponding to a length of N1,. The spectral linewidth of the radiation is AU/W z l/N, and the angular aperture of emission is roughly estimated by ose z 2,/(21,/N&,), which represents es. = 6mrad in the case of CL10 with ymc2 = 50MeV. The spectral width Awlw g l/N becomes sharper as the electron bunch propagates through the undulator (i.e. as N increases). Considering a bunch of N, electrons, and supposing that the electrons’ density is uniformly distributed in the bunch (with bunch dimensions larger than the emitted wavelength A,), the
0168-9002/97/$17.00Copyright 0 1997 Elsevier Science B.V. All rights reserved SO168-9002(97)00503-2
PII
R. Prazeres et al. /Nucl. In.& and Meth. in Phys. Res. A 393 (1997) 326-331
321
Table I Electron beam
Time structure Undulator(s)
FEL
energy: ymc* peak current: i transverse section: cA macro-pulse: At micro-pulse: 6t No. of periods: N magnetic period: 1. Total length L = 2NL, Gain per pass: G Pierce parameter: p
electrons are incoherent sources, and the energy produced by SE scales as NN,. Coherent Spontaneous Emission (CSE) is observed if the dimensions 6x, 6y, 6z of the electron bunch are much smaller than the emitted wavelength. In this case, the individual sources (electrons) are completely coherent and the total emitted energy, integrated over wavelength and angle, is scaling as NNZ. The energy scales linearly with undulator length, and it scales as a square law with the electron current. This effect may occur only for large wavelength (millimeter range) FELs. In most part of FELs, including CLIO, the electron bunch length is larger than the emitted wavelength (6z>>1), and the CSE is, in principle, negligible. Nevertheless, a partial coherence still occurs if the electron bunch longitudinal density n(z) contains sharp edges, or more generally strong components at high frequency in the Fourier spectrum of n(z). In this case, a CSE component of intensity I,,,(o) adds to the SE intensity i,,(o). This CSE component is described by the diffraction theory which gives the coherent amplitude &,,(w) = A,(w). [FTn] (l/n) emitted by the bunch, at frequency o; where A,(w) is the amplitude emitted by one electron, [FTn] is the Fourier Transform of the longitudinal electron density n(z). The total intensity I,(w) = Icoh + I,, is the sum of coherent and incoherent components. The total radiated energy scales as N(N, + fNi), where f is a coefficient taking into account the degree of coherence. The magnitude of the coherent intensity is directly linked to the magnitude of the high frequency components in the Fourier spectrum of n(z). Note that, further on, “spontaneous emission” will represent both the SE and the CSE, SE representing its incoherent part and CSE its coherent part. The FEL gain is induced by an interaction between the electron bunch and the laser wave, which creates a periodical micro-bunching on the electron beam, and produces a strong Fourier component [FTn] (l/n) at the laser wavelength. This component adds to the initial laser wave and leads to the FEL gain. For large values of gain, the spontaneous emission which is produced in the entrance of the undulator can be amplified along the crossing of the undulator, as a single-pass process. This kind of
50 MeV 70 A 0.5 x 0.5 mm lOps@25Hz 10 ps @ 62 MHz 19 (for each undulator)
5.04cm 2 m (for 2 undulators) lW500% (for 2 undulators) 1.9 x 1o-3
radiation is called SASE. The analysis of this process has been done by several authors, and first by Kim in 1986 [5,6]. It gives the radiated power of SASE:
PsAsE = P Pbeam$exp(rl~ PWN,,, where P,,_, = (i) ymc2/e is the kinetic power of the is the electron beam, q = 8n (3/2) l/‘, N,, = nc(2n)‘/‘/a, number of electrons in one coherence length where n is the linear density of electrons. The coherence length is defined as the length of the wave train corresponding to a given spectral width (obtained by Fourier Transform) of CT,.The expression contains an important dimensionless parameter: ne2KZ
P=
C
[JJ]*
320,y3k~mc2E,
1 l/3
’
where [JJ] = [J,(t) - 5,(t)] is the parameter in the FEL equations which depends on the Bessel functions J,, and J, (close to unity with 5 = K*/4(1 + K*/2)), CT*is the cross-sectional area of the electron beam, k, = 27c/A,with i, the undulator period length. The power of SASE grows exponentially along the undulator (i.e. when N increases), and stops when the factor (l/9). exp(q . pN) 2 N,,. The saturation occurs when pN g 1, and the saturation power is pP,,,,. When the conditions are far from saturation (pN<
V. PRE-BUNCHING/SUPERRADIANCE
328
R. Prazeres et al. / Nucl. Imtr. and Meth. in Phys. Rex A 393 (1997) 326-331
responding to a time integration over the whole electron macro-pulse. The curve A exhibits clearly a non-linear behaviour (the curve B also appears but less obvious) which implies a coherence effect: CSE and/or SASE. The curve C corresponds to the curve B which has a magnitude multiplied by a factor of 2. Since the CSE (and the SE) scales linearly with the undulator length L, the difference between curves A and C is due to the presence of SASE which does not scale linearly with L. This shows that SASE is present, but it does not tell anything about the CSE. It is important to note that the spontaneous emission can be incoherent (such as SE), but also partially coherent (including the CSE). The SASE amplification acts on the total spontaneous emission: SE + CSE. The growing of curves A and B is due to a mixing of both phenomena: SASE and CSE, because both are non-linear with respect the electron beam current. The amplification with a 2N periods undulator, which leads to SASE in curve A, acts also in the first undulator, and is responsible for the small non-linearity of curve B. In this case the coherence degree of the spontaneous emission is weak (CSE<< SE) or non-existant (CSE = 0). On the other hand, if we assume that the SASE amplification does not occur in the first undulator, but only in the second one, in this case the small non-linearity of curve B is due to the presence of CSE. In conclusion, these measurements show a rather strong SASE effect but they do not allow to deduce the degree of CSE. The linac must be carefully tuned in order to obtain SASE; as well as for operation of the FEL. There is a strong influence of the focusing quadrupoles (i.e. electron transverse section). On the other hand, the electron
2. Measurements
Spontaneous emission has been measured from the CL10 free electron laser in the following conditions. The radiation is picked-up by a retractable mirror at 45”, which is placed before the extraction mirror of the optical cavity. The radiation is sent directly to the detector (without passing through the telescope which is used to modify the laser mode and decrease the optical divergence). The detector is in the experiment room, which is about 15m away from the undulator. A focusing lens, with a diameter of about @ g 20mm, is installed in front of the detector, making an angular aperture for detection of ede, = 1.3mrad. This is to compare with the angular aperture of the SE which can be roughly estimated by: 0,,~2Jm=6 mrad for 1= 10 pm and for the undulator length N1, = 2 m. The undulator of CL10 is divided in two equal parts of N = 19 periods, for which each gap is independently adjustable. This allows to run the FEL in a two colour operation [7]. This feature is very useful in the SASE measurements, because it allows SASE radiation with a N = 19 periods undulator or one with 2N = 38 periods. This allows a discrimination between CSE and SASE, because CSE scales linearly with the undulator length, whereas SASE scales exponentially. The electron beam current can also be varied, by controlling the aperture of a beam slit on the linac. Fig. 1 shows the spontaneous emission, measured as a function of the electron beam average current, for a 2N periods undulator (curve A) and N periods undulator (curve B). The signal was detected with a nitrogen cooled InSb semiconductor, with time constant about 10 KScor-
0
5
10 e-
15 beam
20 current
25
30
35
(mA)
Fig. 1. Intensity of SASE versus electron beam current, curves A and B respectively with 2N = 38 and N = 19 periods undulator. Curve C is Curve B with doubling of the magnitude.
r
R. Prazeres et al. /Nucl.
In&r. and Meth. in Phys. Rex A 393 (1997) 326-331
329
SASE on,-
current
25 time Fig. 2. Influence
of the RF phase of the linac on the SASE intensity
(ps) (top curves).
The bottom curves represent the electron beam current
(which remains quite constant).
beam trajectory does not act strongly: because the optical cavity, which fixes the longitudinal axis, is not present in this configuration. The axis is only determined by the criterion of linearity of the beam trajectory along the undulator. There is another parameter of the linac which influences strongly both SASE/CSE and FEL: the phase of the RF wave according the electron bunches, in the pre-buncher RF cavity. A misalignment of this RF phase induces a spread of the electron bunch which makes a modification of the longitudinal distribution, increases the bunch length and therefore decreases the peak current. Nevertheless, the phase does not influence the electron beam energy, because the pre-buncher RF cavity is placed before the accelerator section, which works independently and transmits always the same energy of 50 MeV to the electrons. The influence of this RF phase (i.e. of the electron bunch peak current) on the SASE is shown in Fig. 2. This figure displays the time evolution, within the macro-pulse scale, of both the SASE intensity (top curves) and the electron beam current (bottom curves). A small modification of the phase of the linac is sufficient to reduce the SASE intensity by a factor of 15, whereas the electron beam average current remains more or less constant. This is characteristic of SASE (or gain), which is strongly dependant on the electron beam quality. Measurements of the SASE have been done for different values of the linac RF phase. These measurements are shown in the set of Figs. 3. For each phase, the
wavelength spectrum is displayed in Fig. 3(a) and the electron beam current versus time (macro-pulse scale) is displayed in Fig. 3(b). The intensity of SASE (in a.u.) for each curve is displayed in a table in Fig. (3a). When SASE intensity increases, Fig. 3(a) exhibits an increasing of the spectrum linewidth and a shift of the central wavelength toward large values. This effect has always been observed during our experiments. The larger spectrum in Fig. 3(a), corresponding to the larger SASE intensity, is centered at 3, = 11.5 urn, and has a linewidth of A1 = 2.7 urn corresponding to AI/2/1= 23%. The smaller spectrum is centered at 1 = 10 pm, and has a linewidth of AI = 1.8 urn corresponding to AI/I = 18%. Measurements of the SE with zero SASE was not possible because the signal was too small. Nevertheless, the theoretical value of SE linewidth is AA/jn= l/(38 periods) = 2.6%. The angular distribution of the SE increases the linewidth up to 3% or 4%, but not much more. In theory, a modification on the RF phase does not affect the electron beam trajectory or energy. A resonance wavelength shift from L = 10 to 11.5 pm represents a variation of 15%, and corresponds to an angular error of 0 = Jm = 8 mrad or to an energy variation of Ay/y = 7%. Such angular misalignment is not realistic because it represents a translation of 12 cm at the detector distance. The hypothesis of an energy variation is also not realistic because the energy acceptance of the linac is only Ay/y = 2.5%, due to an energy selection slit installed at the entrance of the FEL. For a 2N
V. PRE-BUNCHING/SUPERRADlANCE
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330
10
j i
8
h .:: 2 h .Ia
6
4
7
8
9
11
10
wavelength
(al
0
5
12
Ib)
14
15
(pm)
15
10 time
13
20
2
(ps)
Fig. 3(a). Spectra of SASE for various amplification efficiencies (varying the RF phase of the linac). (b) Electron beam current versus time, for various RF phases of the linac (corresponding to Fig. 3(a)).
periods undulator, the SASE theory gives a,/o = experimental The (9p/(2~,/3[2N]))“~ =0.6%. increasing of the linewidth up to 23% is not in agreement with the SASE theory, which predicts a narrowing of the spectrum as compared to the SE. However, the SASE theory, giving the expression of a&, is only valid during the exponential growing of intensity and not during the start-up regime of SASE like in these measurements where the radiation and the gain are evolving at the same time. Within the Fourier limit conditions, a linewidth of An/1 = 23% corresponds to
a wave train of about AZ z 40 pm = 41. This value is 10 times shorter than the length N1 = 400pm of the SE wave train. It corresponds to a short pulse regime which corresponds to a “spiky” behaviour of the SASE or CSE process.
3. Conclusion The high gain, which can be obtained with the CL10 infrared free electron laser, allows to observe SASE in the
R. Prazeres et al. /Nucl.
Instr. and Meth. in Ph.vs. Res. A 393 (1997) 326-331
start-up regime, at 1 = 5 and 10 urn, with an undulator 2m long and 38 magnetic periods. The SASE intensity has been measured as a function of the electron beam current, and wavelength spectra have been determined for different values of SASE efficiency. The possibility to use either the full length (2N = 38 periods) or half length (N = 19 periods) of the undulator, allowed us to study the behavior of SASE with regards to the undulator length. The wavelength spectra exhibit a strong enlargement of the linewidth and a large shift of the central wavelength which are not explained by the theory of SASE.
331
References [l] [Z] [3] [4]
[S] [6] [7]
Proceedings of FEL’95 conference, Nucl. Instr. and Meth. A 375 (1996). B. Bonifacio et al., Opt. Commun. 50 (1984) 373. D. Bocek, et al., Nucl. Instr. and Meth. A 375 (1996) p. 13; D. A. Kirkpatrick, Nucl. Instr. and Meth. A 285 (1989) p. 43. D. A. Jaroszynski et al., these Proceedings (18th Free Electron Laser Conf., Rome, Italy, 1996) Nucl. Instr. and Meth. A 393 (1997) 332. K.-J. Kim, Phys. Rev. Lett. 57 (15) (1986) 1871. B. Bonifacio et al., Phys. Rev. A 44(6) (1991) 3441. D. A. Jaroszynski et al., Phys. Rev. Lett. 72(15) (1994) 2387.
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