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Optical cavities for IR-FELs at the FELI
.__ I!3
J22J
Nuclear Instruments
and Methods in Physics Research A 375 ( 1996) IO-12
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH Section A
-5% i-.-
EISEVIER
Optical cavities for IR-FELs at the FELI K. Saeki”,*, S. Okuma”, E. Oshita”, K. Wakita”, A. Kobayashi’,
T. Tomimasu”
Abstract The g-parameters of two optical cavities whose length is 6.7 18 m are designed to be stable. The FEL beam is outcoupled from the upstream mirror with an aperture by hole output coupling. The cavity mirrors are spherical and gold-coated on copper substrate. The outcoupling mirror has a hole of 0.5 mm diameter in the center. Our alignment system concept is that the optical axis of two mirrors is adjusted to coincide with the electron beam position. The electron beam position in the beamduct of the undulator is detected using three OTR plates and images of OTR at the plates are observed by CCD cameras.
1. Introduction The Free Electron Laser Research Institute. Inc. (FELI) established in March 1991 as a user’s facility using linac-based FELs covering from FIR to UV range. We achieved first lasings at 5.5 pm using the FELI facility 1 on October 31. 1994 and at I .88 pm using the FELI facility 2 on February 27, 1995 [l,21. Each FELI facility consists of an optical cavity and an undulator. The type of the optical cavity is linear and the cavity consists of two mirrors being in alignment with a undulator since the alignment of this type cavity is easy. Characteristics of an optical cavity are fixed from the following input data. for example, micropulse separation, Rayleigh length, wavelength, etc. Furthermore, we should choose optimum parameters on optical beam waist, filling factor, 8 parameters, etc. We adopt so-called hole output coupling using broadband metal mirrors with an on-axis aperture in one of the mirrors. in order to operate over the broad spectral bandwidth. This is a very desirable technique since the system is simple and low damage metal mirrors can be used. In this paper, we present details of the optical cavity for the FELI facilities I and 2. Table 1 shows main parameters of the optical cavity for was
the FELI
facilities
2. Geometric
I and 2.
constraints
for optical cavity
As an optical cavity for FEL oscillations, * Corresponding
author.
016%9002/96/$15.00 Copyright s.SD! 0168-9002(95)01503-5
the Fabry-
Table I Main parameters of optical facilities I and 2
cavity
and transport
Facility
I
line for FELI
Facility
2
Optical cavity Type Length Rayleigh length
Small signal gain Total cavity loss Transport line Overall length Output beam diameter Transport pipe diameter Vacuum pressure Overall transmission Short term drift precision Short term angle precision
0 1996 Elsevier Science B.V. All rights reserved
optical mode 6.718 m I.0 m -0.93 -0.76 3.490 m 3.827 m Au on Cu
optical mode 6.718 m 0.87 m ~I.06 -0.72 3.267 m 3.902 m Au on Cu
0.5 mm
0.5 mm
22.7-4.X pm (achieved 20 -5 km) 25.6-72.84 I .7-2.4%
6.7-1.1 pm (achieved 3. I -1.8 pm) l4.8-72.3% 2. I -4.6%
80 m
70 m
27-103
mm
16-19
mm
160 mm 0.1 Torr >70%-
I60 mm 0.1 Torr >70%
(5
CO.2 mrad
CO.2 mrad
mm
K. Scleki et al. I Nucl. Iwir. and Meth. in Phu.
Perot type cavity which consists of two mirrors being in alignment with either end of the undulator is popular. The cavity length LC including the undulator, beam bending and focusing magnets is integral number times micropulse separation q,,,,,,. and is fixed to a minimum. L, = Nclq,,,,,,l2
,
(1)
where c is light speed and N is an integer. According to the electron beam pulse format of the FELI (6-10 ps micropulse, 5.6 ns/44.8 ns interpulse spacing, 24-ks long macropulse duration and IO-Hz repetition rate). Lc = 0.83975N = 6.718 m (N = 8) was selected. A distance LL, from the entrance of the undulator to the upstream mirror MI is 2.174 m, while a distance LL2 from the end of the undulator to the downstream mirror M2 is 2.544 m in the FELI facility I. A distance L<, = I .5 I9 m, LL2 = 2.199 m in the FELI facility 2. Although the undulator center position is not the cavity center position, the optical beam position is near center of the undulator so as to get a good interaction between the optical beam and the electron beam. The optical beam profile in the cavity is fixed from the cavity length Lt. the beam waist position, Rayleigh length Z, and the optical beam waist diameter 2~,,,. The optimum of the optical beam size is related to the filling factor. The tilling factor comes from the degree of the optical beam overlapped the electron beam, and it gives [4]. F, = I/{[1 +(1VJ2CJ]“-[l
+(VL.<,/2qV)2]“2}.
(2)
The FEL optical gain is proportional to the tilling factor F,. and the more F, is large. the more the size of the optical beam waist w,, close to the size of the electron beam CT. The minimum mode size for the lowest order TEM mode in an interaction length L, occurs when the Rayleigh length is Z, = L,/2 ~‘7 [4]. In the case of the FELI facility I. a Rayleigh length Z, equal to i\;‘i of the undulator length Lt,, this results in Z, = 0.58 m. In this case, the filling factor is greater than 50% in the wavelength range; A = 4.8-22.7 pm, but this is close to concentric (x,, = -I .048. g, = -0.846. gl,g, = 0.89) and it is severe that the optical axis from two mirrors coincide on the magnetic center axis. So we choose a somewhat large Rayleigh length of Z, = I m. The required curvature of the mirrors, R,, = 3.489 m and R, = 3.826 m, brings the configuration closer a little to confocal and improves the stability of the cavity ( gu = -0.925, g,, = -0.756. g(,g, = 0.70). Table 2 shows the characteristics of concentric and confocal cavities. If we take account into only gain. we should adopt the concentric cavity, although diffraction loss increases in the case severe
of the narrow
beamduct
in the concentric
and the mirror
alignment
is
cavity.
It is also important to define the angular tolerance for This is given in the case of a symmetric cavity by Brau [5]. the mirrors.
II
Rex. A -17-T(1996) IO-I-7
Table 2 Characteristics
of concentric
and confocal
cavities
Concentric
Confocal
Rayleigh length I: parameter Beam waist Filling factor Beam size on mirrors Diffraction loss Mirror alignment
small near - 1 small large large large severe
large near 0 large small small small easy
0, < (2hl?rlL~)“‘(l
- R)“‘( I + g)liJ
(3)
The tilt and drift of the cavity mirrors must be kept much smaller than this value. In the case of the FELI facility I. the parameters defining the system are, the Rayleigh length Z, = 1 m. the cavity length LL = 6.718 m. the wavelength A = 4.8-22.7 km. the g parameter g, = -0.925 and g,_ = -0.756, so the severest angular tolerance for this cavity must be much less than I I4 grad. The cavity mirror position controlled system is designed for a tolerance of IO prad. Fig. I shows s parameters chosen by the worldwide facilities achieved FEL oscillations with the FELI’s parameters. It is noted that many facilities design the g parameter near -0.5. but the FELI chooses the values near -I .O as the g parameters. Because we are able to control the cavity mirror for the required angular tolerance.
3. Optical cavity for the FELI facilities 1 and 2 Fig. 2 shows the upstream and downstream mirror mounts. The cavity mirror is held a solid gimbal mount which drives pan and tilt carried by a precision translation X, Y, Z stage that are able to use in vacuum. In order to 0.0
,
,
/
,
-0.5
-G
[r -I*
-1 .o
-1.5
-2.0
,‘,,,,I’,,,
-1.5
:l.O
,(,I, -0.5
0.0
g,(=l -L/R,) Fig. 1. g parameters of optical facilities achieved FEL oscillations.
cavities
used
in worldwide
I. FEL PRIZE TALK/NEW LASING
-7
3
I I
L!___
I
j
-
I
monitoring FEL power system. A Hewlett-Packard interferometer system is used to monitor the distance between the cavity mirrors. The interferometer beam is separated into two beams by a beamsplitter and is applied to the reflector part of the cavity mirror through the plane mirror interferometer and goes back to the receiver. This system are used continuously without shutting down. The resolution of this interferometer system is 0.5 pm. A HgCdTe detector is used for detecting the spontaneous emission and FEL beam. The power intensity is observed by the oscilloscope and the cavity length is changed by moving the cavity mirrors while monitoring the power intensity. The cavity mirror is moved with a resolution of 0.1 pm.
1-
Fig. 7. Mirror mount for FELl cavity. 4. Conclusions achieve optical beam stability. we adopt a concept that the optical axis of mirrors and the FEL beam are adjusted to coincide with the electron beam position [6l. The cavity mirrors are spherical and gold coated on copper substrate and the diameter is 60 mm. the thickness is I2 mm. Gold is a good reflective material from near visible to far infrared and the reflection is near 99%. At the same time. it has sufticient strength and resistance for radiation damage. An aperture of 0.5 mm in the upstream mirror provides outcoupling of a fraction of the optical radiation. In the case of the facility I. the optical beam waist diameter 2n.,> is 2.5-5.4 mm and the spotsize on the upstream mirror is 8-l 8 mm for A = 4.8-23.7 pm. So, the extraction ratio of outcoupling is 0.08-0.75%. The total cavity loss is I .7-2.3%. In the case of the facility 2, ~vv~,is 1.I -2.7 mm and the spotsize on the upstream mirror is S-12 mm for A = I. I-6.7 pm. The aperture of an extraction mirror is 0.5 mm, so the extraction ratio of outcoupling is 0.5%3.0% The total cavity loss is 2.14.6%. We have prepared two methods to control the cavity length. one is the interferometer system an the other is the
The main features of the optical cavity for the undulator
I and transport system are summarized as follows. ( I) In order to improve stability of the cavity. we choose the Rayleigh length which equals to f of the undulator length, and the configuration of the cavity closes to concentric. (2) The extraction ratio of outcouplinp is 0.08-0.75% for the wavelength h, = 4.8-22.7 pm. So, we estimate the intracavity peak power is about 0.4-3.8 GW.
References [ll T. Tomimasu et al., PAC’95. Dallas, May l-5. FAA30. [2] E. Oshna et al., PAC’9.5, Dallas. May l-5. TAQ35. [3] M. Billardon et al.. IEEE J. Quantum Electron QE-?I ( 1985) 805. 141 S. Benson, Nucl. Instr. and Meth. A 304 (1991) 773. 151 C. Brau. Free-electron Laser (Academic Press. San Diego. CA, 1990) p. 32X. [6] E. Nishimura et al.. Nucl. lnstr. and Meth. A 341 (1994) 39.
J22J
Nuclear Instruments
and Methods in Physics Research A 375 ( 1996) IO-12
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH Section A
-5% i-.-
EISEVIER
Optical cavities for IR-FELs at the FELI K. Saeki”,*, S. Okuma”, E. Oshita”, K. Wakita”, A. Kobayashi’,
T. Tomimasu”
Abstract The g-parameters of two optical cavities whose length is 6.7 18 m are designed to be stable. The FEL beam is outcoupled from the upstream mirror with an aperture by hole output coupling. The cavity mirrors are spherical and gold-coated on copper substrate. The outcoupling mirror has a hole of 0.5 mm diameter in the center. Our alignment system concept is that the optical axis of two mirrors is adjusted to coincide with the electron beam position. The electron beam position in the beamduct of the undulator is detected using three OTR plates and images of OTR at the plates are observed by CCD cameras.
1. Introduction The Free Electron Laser Research Institute. Inc. (FELI) established in March 1991 as a user’s facility using linac-based FELs covering from FIR to UV range. We achieved first lasings at 5.5 pm using the FELI facility 1 on October 31. 1994 and at I .88 pm using the FELI facility 2 on February 27, 1995 [l,21. Each FELI facility consists of an optical cavity and an undulator. The type of the optical cavity is linear and the cavity consists of two mirrors being in alignment with a undulator since the alignment of this type cavity is easy. Characteristics of an optical cavity are fixed from the following input data. for example, micropulse separation, Rayleigh length, wavelength, etc. Furthermore, we should choose optimum parameters on optical beam waist, filling factor, 8 parameters, etc. We adopt so-called hole output coupling using broadband metal mirrors with an on-axis aperture in one of the mirrors. in order to operate over the broad spectral bandwidth. This is a very desirable technique since the system is simple and low damage metal mirrors can be used. In this paper, we present details of the optical cavity for the FELI facilities I and 2. Table 1 shows main parameters of the optical cavity for was
the FELI
facilities
2. Geometric
I and 2.
constraints
for optical cavity
As an optical cavity for FEL oscillations, * Corresponding
author.
016%9002/96/$15.00 Copyright s.SD! 0168-9002(95)01503-5
the Fabry-
Table I Main parameters of optical facilities I and 2
cavity
and transport
Facility
I
line for FELI
Facility
2
Optical cavity Type Length Rayleigh length
Small signal gain Total cavity loss Transport line Overall length Output beam diameter Transport pipe diameter Vacuum pressure Overall transmission Short term drift precision Short term angle precision
0 1996 Elsevier Science B.V. All rights reserved
optical mode 6.718 m I.0 m -0.93 -0.76 3.490 m 3.827 m Au on Cu
optical mode 6.718 m 0.87 m ~I.06 -0.72 3.267 m 3.902 m Au on Cu
0.5 mm
0.5 mm
22.7-4.X pm (achieved 20 -5 km) 25.6-72.84 I .7-2.4%
6.7-1.1 pm (achieved 3. I -1.8 pm) l4.8-72.3% 2. I -4.6%
80 m
70 m
27-103
mm
16-19
mm
160 mm 0.1 Torr >70%-
I60 mm 0.1 Torr >70%
(5
CO.2 mrad
CO.2 mrad
mm
K. Scleki et al. I Nucl. Iwir. and Meth. in Phu.
Perot type cavity which consists of two mirrors being in alignment with either end of the undulator is popular. The cavity length LC including the undulator, beam bending and focusing magnets is integral number times micropulse separation q,,,,,,. and is fixed to a minimum. L, = Nclq,,,,,,l2
,
(1)
where c is light speed and N is an integer. According to the electron beam pulse format of the FELI (6-10 ps micropulse, 5.6 ns/44.8 ns interpulse spacing, 24-ks long macropulse duration and IO-Hz repetition rate). Lc = 0.83975N = 6.718 m (N = 8) was selected. A distance LL, from the entrance of the undulator to the upstream mirror MI is 2.174 m, while a distance LL2 from the end of the undulator to the downstream mirror M2 is 2.544 m in the FELI facility I. A distance L<, = I .5 I9 m, LL2 = 2.199 m in the FELI facility 2. Although the undulator center position is not the cavity center position, the optical beam position is near center of the undulator so as to get a good interaction between the optical beam and the electron beam. The optical beam profile in the cavity is fixed from the cavity length Lt. the beam waist position, Rayleigh length Z, and the optical beam waist diameter 2~,,,. The optimum of the optical beam size is related to the filling factor. The tilling factor comes from the degree of the optical beam overlapped the electron beam, and it gives [4]. F, = I/{[1 +(1VJ2CJ]“-[l
+(VL.<,/2qV)2]“2}.
(2)
The FEL optical gain is proportional to the tilling factor F,. and the more F, is large. the more the size of the optical beam waist w,, close to the size of the electron beam CT. The minimum mode size for the lowest order TEM mode in an interaction length L, occurs when the Rayleigh length is Z, = L,/2 ~‘7 [4]. In the case of the FELI facility I. a Rayleigh length Z, equal to i\;‘i of the undulator length Lt,, this results in Z, = 0.58 m. In this case, the filling factor is greater than 50% in the wavelength range; A = 4.8-22.7 pm, but this is close to concentric (x,, = -I .048. g, = -0.846. gl,g, = 0.89) and it is severe that the optical axis from two mirrors coincide on the magnetic center axis. So we choose a somewhat large Rayleigh length of Z, = I m. The required curvature of the mirrors, R,, = 3.489 m and R, = 3.826 m, brings the configuration closer a little to confocal and improves the stability of the cavity ( gu = -0.925, g,, = -0.756. g(,g, = 0.70). Table 2 shows the characteristics of concentric and confocal cavities. If we take account into only gain. we should adopt the concentric cavity, although diffraction loss increases in the case severe
of the narrow
beamduct
in the concentric
and the mirror
alignment
is
cavity.
It is also important to define the angular tolerance for This is given in the case of a symmetric cavity by Brau [5]. the mirrors.
II
Rex. A -17-T(1996) IO-I-7
Table 2 Characteristics
of concentric
and confocal
cavities
Concentric
Confocal
Rayleigh length I: parameter Beam waist Filling factor Beam size on mirrors Diffraction loss Mirror alignment
small near - 1 small large large large severe
large near 0 large small small small easy
0, < (2hl?rlL~)“‘(l
- R)“‘( I + g)liJ
(3)
The tilt and drift of the cavity mirrors must be kept much smaller than this value. In the case of the FELI facility I. the parameters defining the system are, the Rayleigh length Z, = 1 m. the cavity length LL = 6.718 m. the wavelength A = 4.8-22.7 km. the g parameter g, = -0.925 and g,_ = -0.756, so the severest angular tolerance for this cavity must be much less than I I4 grad. The cavity mirror position controlled system is designed for a tolerance of IO prad. Fig. I shows s parameters chosen by the worldwide facilities achieved FEL oscillations with the FELI’s parameters. It is noted that many facilities design the g parameter near -0.5. but the FELI chooses the values near -I .O as the g parameters. Because we are able to control the cavity mirror for the required angular tolerance.
3. Optical cavity for the FELI facilities 1 and 2 Fig. 2 shows the upstream and downstream mirror mounts. The cavity mirror is held a solid gimbal mount which drives pan and tilt carried by a precision translation X, Y, Z stage that are able to use in vacuum. In order to 0.0
,
,
/
,
-0.5
-G
[r -I*
-1 .o
-1.5
-2.0
,‘,,,,I’,,,
-1.5
:l.O
,(,I, -0.5
0.0
g,(=l -L/R,) Fig. 1. g parameters of optical facilities achieved FEL oscillations.
cavities
used
in worldwide
I. FEL PRIZE TALK/NEW LASING
-7
3
I I
L!___
I
j
-
I
monitoring FEL power system. A Hewlett-Packard interferometer system is used to monitor the distance between the cavity mirrors. The interferometer beam is separated into two beams by a beamsplitter and is applied to the reflector part of the cavity mirror through the plane mirror interferometer and goes back to the receiver. This system are used continuously without shutting down. The resolution of this interferometer system is 0.5 pm. A HgCdTe detector is used for detecting the spontaneous emission and FEL beam. The power intensity is observed by the oscilloscope and the cavity length is changed by moving the cavity mirrors while monitoring the power intensity. The cavity mirror is moved with a resolution of 0.1 pm.
1-
Fig. 7. Mirror mount for FELl cavity. 4. Conclusions achieve optical beam stability. we adopt a concept that the optical axis of mirrors and the FEL beam are adjusted to coincide with the electron beam position [6l. The cavity mirrors are spherical and gold coated on copper substrate and the diameter is 60 mm. the thickness is I2 mm. Gold is a good reflective material from near visible to far infrared and the reflection is near 99%. At the same time. it has sufticient strength and resistance for radiation damage. An aperture of 0.5 mm in the upstream mirror provides outcoupling of a fraction of the optical radiation. In the case of the facility I. the optical beam waist diameter 2n.,> is 2.5-5.4 mm and the spotsize on the upstream mirror is 8-l 8 mm for A = 4.8-23.7 pm. So, the extraction ratio of outcoupling is 0.08-0.75%. The total cavity loss is I .7-2.3%. In the case of the facility 2, ~vv~,is 1.I -2.7 mm and the spotsize on the upstream mirror is S-12 mm for A = I. I-6.7 pm. The aperture of an extraction mirror is 0.5 mm, so the extraction ratio of outcoupling is 0.5%3.0% The total cavity loss is 2.14.6%. We have prepared two methods to control the cavity length. one is the interferometer system an the other is the
The main features of the optical cavity for the undulator
I and transport system are summarized as follows. ( I) In order to improve stability of the cavity. we choose the Rayleigh length which equals to f of the undulator length, and the configuration of the cavity closes to concentric. (2) The extraction ratio of outcouplinp is 0.08-0.75% for the wavelength h, = 4.8-22.7 pm. So, we estimate the intracavity peak power is about 0.4-3.8 GW.
References [ll T. Tomimasu et al., PAC’95. Dallas, May l-5. FAA30. [2] E. Oshna et al., PAC’9.5, Dallas. May l-5. TAQ35. [3] M. Billardon et al.. IEEE J. Quantum Electron QE-?I ( 1985) 805. 141 S. Benson, Nucl. Instr. and Meth. A 304 (1991) 773. 151 C. Brau. Free-electron Laser (Academic Press. San Diego. CA, 1990) p. 32X. [6] E. Nishimura et al.. Nucl. lnstr. and Meth. A 341 (1994) 39.