FEL2013 - Classification: Long Wavelength FELs

Paper

Title

Page

Super-radiant Linac-based THz Sources in 2013

474

M. Gensch
HZDR, Dresden, Germany

There is a growing interest in THz and far infrared light sources for use in material studies. Both coherent radiative sources (CSR, COTR, etc.) and FEL sources have been developed in the last few years to address this need. This talk will describe recent developments in this growing field.

WEOBNO01

Design and Simulation of CAEP THz FEL

Y.H. Dou, X.J. Shu
Institute of Applied Physics and Computational Mathematics, People's Republic of China
X. Yang
CAEP/IAE, Mianyang, Sichuan, People's Republic of China

Funding: The work was supported by National Natural Science Foundation of China under Grant 11105019; National Major Instrumentation Special under Grant 2011YQ130018 and Project 2011B0401066 supported by CAEP.
Design and simulation of CAEP THz free electron laser (FEL) have been carried out using our 3D-OSIFEL code. The main work focuses on the optimization to different parameters of optical resonator and wiggler. The wiggler peak field strength and electron beam energy have been selected with eleven frequencies ranging from 1 THz to 3 THz. The gain and power of resonator are calculated, which are corresponding to eleven frequencies. It can be seen by simulation results that when facility works at 2.6 THz, the gain and output power are the highest in the frequency range from 1 THz to 3 THz. So the experiment may be conducted at this frequency first. When the device works toward 1 THz, the slippage length and the loss of optical resonator will increase due to the increase of the radiation wavelength, which results in the reduction of the gain and output power. So experiment running at 1THz could be challenged and the scheme of elliptical hole-coupling optical resonator is proposed to solve the problem. The simulation results show that the elliptical hole-coupling output is effective and applicable for the THz FEL and the output power can be increased by more than 30%.

WEOBNO02

Improving the Pulse Characteristics of Mid-IR FEL for Ultrafast Spectroscopy

T. Nakajima, T. Kii, H. Ohgaki, Y. Qin, X. Wang, H. Zen
Kyoto University, Institute for Advanced Energy, Kyoto, Japan

The FEL we have developed at Kyoto University, KU-FEL, is an oscillator-type FEL in the mid-infrared. The micropulse duration is 0.6 ps, the time interval between the successive micropulses is 350 ps, and the wavelength stability is 1.3% [1]. This pulse structure is not a problem at all for many applications in which the fluence of FEL itself plays a major role. However, for some applications, such a pulse structure spoils the time resolution. To improve the pulse characteristics of KU-FEL for ultrafast spectroscopy we have developed a simple two-stage optical system after the FEL output. In the first stage we introduce double plasma shutters for the ultrafast switching-on/off of a pulse train. In the second stage we introduce a bulk nonlinear material with several mm thickness to broaden the FEL spectrum through self-phase modulation so that we do not have to scan the FEL wavelength during the measurement. A combined use of this two-stage optical system and the single-shot spectral measurement technique we have recently developed for the mid-infrared wavelength region [2], we are now ready to perform single-shot ultrafast spectroscopy without scanning the FEL wavelength.
[1]Qin, Zen, Wang, Kii, Nakajima, and Ohgaki, Optics Letters 38, 1068 (2013).
[2]Wang, Nakajima, Zen, Kii, and Ohgaki, Optics Letters 37, 5148 (2012).

WEOBNO03

Intense Emission of Smith-Purcell Radiation at the Fundamental Frequency from a Grating Equipped with Sidewalls

477

J.T. Donohue
CENBG, Gradignan, France
J. Gardelle, P. Modin
CEA, LE BARP cedex, France

The two-dimensional theory of the Smith-Purcell free-electron laser predicts that coherent Smith-Purcell radiation can occur only at harmonics of the frequency of the evanescent wave that is resonant with the beam. Particle-in-cell simulations have shown that in a three-dimensional context, where the lamellar grating has sidewalls, coherent Smith-Purcell radiation can be copiously emitted at the fundamental frequency, for a well-defined range of beam energy. An experiment at microwave frequencies has confirmed this prediction . The power output is considerably greater than for the previously observed emission at the second harmonic, in agreement with three-dimensional simulations . The dependence of frequency on beam energy and emission angle is in good agreement with three-dimensional theory and simulations. Provided that a reduction in scale can be achieved, a path is open to coherent Smith-Purcell radiation at Terahertz frequencies.
(1) J. Gardelle, P. Modin and J.T. Donohue, Appl. Phys. Lett. 100, 131103 (2012).
(2) J. T. Donohue and J. Gardelle, Appl. Phys. Lett. 99, 161112 (2011).

Slides WEOBNO03 [11.891 MB]

WEOBNO04

Integration of Accelerator Based IR/THz Source for Pump Probe Experiments in the Infrastructure of the European XFEL

M.V. Yurkov, E. Schneidmiller
DESY, Hamburg, Germany
M. Gensch
HZDR, Dresden, Germany
M. Izquierdo
XFEL. EU, Hamburg, Germany
M. Krasilnikov, F. Stephan
DESY Zeuthen, Zeuthen, Germany

We analyze the scope of pump-probe experiments at the European XFEL involving IR/THz and x-ray pulses. Different experiments impose different requirements on the properties of the IR/THz source like wavelength range, spectral properties, intensity, pulse duration, polarization, synchronization capabilities, repetition rate, etc. Our previous analysis have shown that the most universal solution of the problem of the IR/THz radiation source is an accelerator based source. The electron accelerator is similar to that operating at the PITZ facility. It consists of an rf gun and a warm (or cold) accelerating section. Powerful, coherent radiation with prescribed temporal, spectral and polarization properties is generated in a set of radiators like undulators (using mechanism of SASE FEL for short wavelength and coherent undulator radiation for long wavelength), edge radiators, OTR foils, diffraction radiators. Location of the electron source close to the experiment would allow performing of pump-probe experiments involving also ultrashort electron pulses. In this paper we discuss problems of the integration of the accelerator based THz source into infrastructure of the European XFEL.

Slides WEOBNO04 [3.339 MB]

WEPSO14

Towards High Frequency Operation with a Multi-Grating Smith-Purcell FEL

525

J.T. Donohue
CENBG, Gradignan, France
J. Gardelle
CEA, LE BARP cedex, France

Three-dimensional simulations and experiments have shown that, for a grating equipped with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave is possible 1, 2. Since the underlying theory is scale invariant, the wavelength emitted is reduced in proportion to a uniform rescaling of the grating. In order to increase our 5 GHz to 100 GHz , the grating surface would be reduced by a factor of 400, which would lead to greatly reduced power. In addition, the required beam might be hard to generate. To avoid this, we propose to use several gratings in parallel with no overall reduction in the total width and the same beam as in our microwave experiment. For this scheme to succeed, it is essential that the bunching in the different gratings be coherent. . Simulations suggest that this occurs for as much as a ten-fold scale reduction. To test this idea, an experiment is using several gratings is being performed.
1. J. T. Donohue and J. Gardelle, Appl. Phys. Lett. 99, 161112 (2011).
2. J. Gardelle, P. Modin and J.T. Donohue, Appl. Phys. Lett. 100, 131103 (2012),.

WEPSO31

THz Radiation Source Potential of the R&D ERL at BNL

566

D. Kayran, I. Ben-Zvi, Y.C. Jing, B. Sheehy
BNL, Upton, Long Island, New York, USA

Funding: Work supported by BSA DOE, Contract No. DE-AC02-98CH10886
An ampere class 20 MeV superconducting Energy Recovery Linac (ERL) is under commissioning at Brookhaven National Laboratory (BNL) for testing concepts for high-energy electron cooling and electron-ion colliders. This ERL will be used as a test bed to study issues relevant for very high current ERLs. High repetition rate (9.5 MHz), CW operation and high performance of electron beam with some additional components make this ERL an excellent driver for high power coherent THz radiation source*. We discuss potential use of BNL ERL as a source of THz radiation and results of the beam dynamics simulation. We present the status and commissioning progress of the ERL.
*Ilan Ben-Zvi. et al. Coherent harmonic generation of THz radiation using wakefield bunching (presented at this conference)

WEPSO62

The IR and THz Free Electron Laser at the Fritz-Haber-Institut

657

W. Schöllkopf, W. Erlebach, S. Gewinner, G. Heyne, H. Junkes, A. Liedke, G. Meijer, V. Platschkowski, G. von Helden
FHI, Berlin, Germany
H. Bluem, D. Dowell, K. Jordan, R. Lange, J. Rathke, A.M.M. Todd, L.M. Young
AES, Princeton, New Jersey, USA
M.A. Davidsaver
BNL, Upton, New York, USA
S.C. Gottschalk
STI, Washington, USA
U. Lehnert, P. Michel, W. Seidel, R. Wünsch
HZDR, Dresden, Germany
H. Loos
SLAC, Menlo Park, California, USA

A mid-infrared oscillator FEL with a design wavelength range from 4 to 50 μm has been commissioned at the Fritz-Haber-Institut in Berlin, Germany, for applications in molecular and cluster spectroscopy as well as surface science. The accelerator consists of a thermionic gridded electron gun, a subharmonic buncher and two S-band standing-wave copper structures. The device was designed to meet challenging specifications, including a final energy adjustable in the range of 15 to 50 MeV, low longitudinal emittance (< 50 keV-psec) and transverse emittance (< 20 π mm-mrad), at more than 200 pC bunch charge with aμpulse repetition rate of 1 GHz and a macro pulse length of up to 15 μs. Two isochronous achromatic 180 degree bends deliver the beam to the undulators, only one of which is presently installed, and to the beam dumps. Calculations of the FEL gain and IR-cavity losses predict that lasing will be possible in the wavelength range from less than 4 to more than 50 μm. First lasing was achieved at a wavelength of 16 μm in 2012*. We will describe the FEL system design and performance, provide examples of lasing, and touch on the first anticipated user experiments.
* W. Schöllkopf et al., MOOB01, Proc. FEL 2012.

WEPSO69

Optical Cavity Losses Calculation and Optimization of THz FEL with a Waveguide

689

P. Tan, Q. Fu, L. Li, B. Qin, K. Xiong, Y.Q. Xiong
HUST, Wuhan, People's Republic of China

Funding: the Fundamental Research Funds for the Central Universities，HUST：2012QN080
The optical cavity with waveguide is used in most long wavelength free electron lasers. In this paper, the losses, gains and modes of a terahertz FEL sources in Huazhong Univeristy of Science and Technology(HUST) are analysis. Then the radii of curvature of the optical mirrors and shapes of the waveguide are optimized.

WEPSO84

Present Status of Kyoto University Free Electron Laser

711

H. Zen, M. Inukai, T. Kii, R. Kinjo, K. Masuda, K. Mishima, H. Negm, H. Ohgaki, K. Okumura, M. Omer, K. Torgasin, K. Yoshida
Kyoto University, Institute for Advanced Energy, Kyoto, Japan

A mid-infrared FEL named as KU-FEL (Kyoto University FEL) has been developed for energy related sciences [1]. After the achievement of the first lasing and the power saturation in 2008 [2, 3], we have been working to extend the tunable range of the FEL [4]. By replacing the original 1.6-m undulator into a 1.8 m one, the tunable range was expanded from 10-13 to 5-15 μm in January 2012. Then we fabricated a new undulator duct to reduce the minimum undulator gap from 20 to 15 mm. At 15-mm gap, the FEL gain can be expected to be twice as high as that at 20 mm gap. Commissioning of the new duct will be done in the end of this April. In this presentation, we will report on the result of the commissioning such as tunable range of KU-FEL and the estimated FEL gain, which would be compared with a simulation.
[1] H. Zen, et al., Infrared Phys. Techn., 51, 382 (2008)
[2] H. Ohgaki, et al., Proc. of FEL08, 4 (2008)
[3] H. Ohgaki, et al., Proc. of FEL2009, 572 (2009)
[4] H. Zen, et al., Proc. of FEL2012

WEPSO89

Design of a Resonator for the CSU THz FEL

719

P.J.M. van der Slot
Mesa+, Enschede, The Netherlands
S. Biedron, S.V. Milton, P.J.M. van der Slot
CSU, Fort Collins, Colorado, USA

Funding: This research is support by Office of Naval Research Global, grant number N62909-10-1-7151
A 6-MeV L-band linac will be used to drive a planar, fixed gap, 2.5-cm period, hybrid undulator with parabolic pole faces. Consequently, this system is capable of generating wavelengths from 160 to 600 μm. In this paper we discuss the design of an optical resonator for this system. The resonator uses hole-coupled mirrors to allow for a straight electron beam line. The Rayleigh length, the position of the waist of the cold-cavity mode and the hole radii will be investigated to optimize the performance of the FEL.

Paper	Title	Page
WEIBNO01	Super-radiant Linac-based THz Sources in 2013	474
	M. Gensch HZDR, Dresden, Germany
	There is a growing interest in THz and far infrared light sources for use in material studies. Both coherent radiative sources (CSR, COTR, etc.) and FEL sources have been developed in the last few years to address this need. This talk will describe recent developments in this growing field.

WEOBNO01	Design and Simulation of CAEP THz FEL
	Y.H. Dou, X.J. Shu Institute of Applied Physics and Computational Mathematics, People's Republic of China X. Yang CAEP/IAE, Mianyang, Sichuan, People's Republic of China
	Funding: The work was supported by National Natural Science Foundation of China under Grant 11105019; National Major Instrumentation Special under Grant 2011YQ130018 and Project 2011B0401066 supported by CAEP. Design and simulation of CAEP THz free electron laser (FEL) have been carried out using our 3D-OSIFEL code. The main work focuses on the optimization to different parameters of optical resonator and wiggler. The wiggler peak field strength and electron beam energy have been selected with eleven frequencies ranging from 1 THz to 3 THz. The gain and power of resonator are calculated, which are corresponding to eleven frequencies. It can be seen by simulation results that when facility works at 2.6 THz, the gain and output power are the highest in the frequency range from 1 THz to 3 THz. So the experiment may be conducted at this frequency first. When the device works toward 1 THz, the slippage length and the loss of optical resonator will increase due to the increase of the radiation wavelength, which results in the reduction of the gain and output power. So experiment running at 1THz could be challenged and the scheme of elliptical hole-coupling optical resonator is proposed to solve the problem. The simulation results show that the elliptical hole-coupling output is effective and applicable for the THz FEL and the output power can be increased by more than 30%.

WEOBNO02	Improving the Pulse Characteristics of Mid-IR FEL for Ultrafast Spectroscopy
	T. Nakajima, T. Kii, H. Ohgaki, Y. Qin, X. Wang, H. Zen Kyoto University, Institute for Advanced Energy, Kyoto, Japan
	The FEL we have developed at Kyoto University, KU-FEL, is an oscillator-type FEL in the mid-infrared. The micropulse duration is 0.6 ps, the time interval between the successive micropulses is 350 ps, and the wavelength stability is 1.3% [1]. This pulse structure is not a problem at all for many applications in which the fluence of FEL itself plays a major role. However, for some applications, such a pulse structure spoils the time resolution. To improve the pulse characteristics of KU-FEL for ultrafast spectroscopy we have developed a simple two-stage optical system after the FEL output. In the first stage we introduce double plasma shutters for the ultrafast switching-on/off of a pulse train. In the second stage we introduce a bulk nonlinear material with several mm thickness to broaden the FEL spectrum through self-phase modulation so that we do not have to scan the FEL wavelength during the measurement. A combined use of this two-stage optical system and the single-shot spectral measurement technique we have recently developed for the mid-infrared wavelength region [2], we are now ready to perform single-shot ultrafast spectroscopy without scanning the FEL wavelength. [1]Qin, Zen, Wang, Kii, Nakajima, and Ohgaki, Optics Letters 38, 1068 (2013). [2]Wang, Nakajima, Zen, Kii, and Ohgaki, Optics Letters 37, 5148 (2012).

WEOBNO03	Intense Emission of Smith-Purcell Radiation at the Fundamental Frequency from a Grating Equipped with Sidewalls	477
	J.T. Donohue CENBG, Gradignan, France J. Gardelle, P. Modin CEA, LE BARP cedex, France
	The two-dimensional theory of the Smith-Purcell free-electron laser predicts that coherent Smith-Purcell radiation can occur only at harmonics of the frequency of the evanescent wave that is resonant with the beam. Particle-in-cell simulations have shown that in a three-dimensional context, where the lamellar grating has sidewalls, coherent Smith-Purcell radiation can be copiously emitted at the fundamental frequency, for a well-defined range of beam energy. An experiment at microwave frequencies has confirmed this prediction . The power output is considerably greater than for the previously observed emission at the second harmonic, in agreement with three-dimensional simulations . The dependence of frequency on beam energy and emission angle is in good agreement with three-dimensional theory and simulations. Provided that a reduction in scale can be achieved, a path is open to coherent Smith-Purcell radiation at Terahertz frequencies. (1) J. Gardelle, P. Modin and J.T. Donohue, Appl. Phys. Lett. 100, 131103 (2012). (2) J. T. Donohue and J. Gardelle, Appl. Phys. Lett. 99, 161112 (2011).
	Slides WEOBNO03 [11.891 MB]

WEOBNO04	Integration of Accelerator Based IR/THz Source for Pump Probe Experiments in the Infrastructure of the European XFEL
	M.V. Yurkov, E. Schneidmiller DESY, Hamburg, Germany M. Gensch HZDR, Dresden, Germany M. Izquierdo XFEL. EU, Hamburg, Germany M. Krasilnikov, F. Stephan DESY Zeuthen, Zeuthen, Germany
	We analyze the scope of pump-probe experiments at the European XFEL involving IR/THz and x-ray pulses. Different experiments impose different requirements on the properties of the IR/THz source like wavelength range, spectral properties, intensity, pulse duration, polarization, synchronization capabilities, repetition rate, etc. Our previous analysis have shown that the most universal solution of the problem of the IR/THz radiation source is an accelerator based source. The electron accelerator is similar to that operating at the PITZ facility. It consists of an rf gun and a warm (or cold) accelerating section. Powerful, coherent radiation with prescribed temporal, spectral and polarization properties is generated in a set of radiators like undulators (using mechanism of SASE FEL for short wavelength and coherent undulator radiation for long wavelength), edge radiators, OTR foils, diffraction radiators. Location of the electron source close to the experiment would allow performing of pump-probe experiments involving also ultrashort electron pulses. In this paper we discuss problems of the integration of the accelerator based THz source into infrastructure of the European XFEL.
	Slides WEOBNO04 [3.339 MB]

WEPSO14	Towards High Frequency Operation with a Multi-Grating Smith-Purcell FEL	525
	J.T. Donohue CENBG, Gradignan, France J. Gardelle CEA, LE BARP cedex, France
	Three-dimensional simulations and experiments have shown that, for a grating equipped with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave is possible 1, 2. Since the underlying theory is scale invariant, the wavelength emitted is reduced in proportion to a uniform rescaling of the grating. In order to increase our 5 GHz to 100 GHz , the grating surface would be reduced by a factor of 400, which would lead to greatly reduced power. In addition, the required beam might be hard to generate. To avoid this, we propose to use several gratings in parallel with no overall reduction in the total width and the same beam as in our microwave experiment. For this scheme to succeed, it is essential that the bunching in the different gratings be coherent. . Simulations suggest that this occurs for as much as a ten-fold scale reduction. To test this idea, an experiment is using several gratings is being performed. 1. J. T. Donohue and J. Gardelle, Appl. Phys. Lett. 99, 161112 (2011). 2. J. Gardelle, P. Modin and J.T. Donohue, Appl. Phys. Lett. 100, 131103 (2012),.

WEPSO31	THz Radiation Source Potential of the R&D ERL at BNL	566
	D. Kayran, I. Ben-Zvi, Y.C. Jing, B. Sheehy BNL, Upton, Long Island, New York, USA
	Funding: Work supported by BSA DOE, Contract No. DE-AC02-98CH10886 An ampere class 20 MeV superconducting Energy Recovery Linac (ERL) is under commissioning at Brookhaven National Laboratory (BNL) for testing concepts for high-energy electron cooling and electron-ion colliders. This ERL will be used as a test bed to study issues relevant for very high current ERLs. High repetition rate (9.5 MHz), CW operation and high performance of electron beam with some additional components make this ERL an excellent driver for high power coherent THz radiation source. We discuss potential use of BNL ERL as a source of THz radiation and results of the beam dynamics simulation. We present the status and commissioning progress of the ERL. Ilan Ben-Zvi. et al. Coherent harmonic generation of THz radiation using wakefield bunching (presented at this conference)

WEPSO62	The IR and THz Free Electron Laser at the Fritz-Haber-Institut	657
	W. Schöllkopf, W. Erlebach, S. Gewinner, G. Heyne, H. Junkes, A. Liedke, G. Meijer, V. Platschkowski, G. von Helden FHI, Berlin, Germany H. Bluem, D. Dowell, K. Jordan, R. Lange, J. Rathke, A.M.M. Todd, L.M. Young AES, Princeton, New Jersey, USA M.A. Davidsaver BNL, Upton, New York, USA S.C. Gottschalk STI, Washington, USA U. Lehnert, P. Michel, W. Seidel, R. Wünsch HZDR, Dresden, Germany H. Loos SLAC, Menlo Park, California, USA
	A mid-infrared oscillator FEL with a design wavelength range from 4 to 50 μm has been commissioned at the Fritz-Haber-Institut in Berlin, Germany, for applications in molecular and cluster spectroscopy as well as surface science. The accelerator consists of a thermionic gridded electron gun, a subharmonic buncher and two S-band standing-wave copper structures. The device was designed to meet challenging specifications, including a final energy adjustable in the range of 15 to 50 MeV, low longitudinal emittance (< 50 keV-psec) and transverse emittance (< 20 π mm-mrad), at more than 200 pC bunch charge with aμpulse repetition rate of 1 GHz and a macro pulse length of up to 15 μs. Two isochronous achromatic 180 degree bends deliver the beam to the undulators, only one of which is presently installed, and to the beam dumps. Calculations of the FEL gain and IR-cavity losses predict that lasing will be possible in the wavelength range from less than 4 to more than 50 μm. First lasing was achieved at a wavelength of 16 μm in 2012. We will describe the FEL system design and performance, provide examples of lasing, and touch on the first anticipated user experiments. W. Schöllkopf et al., MOOB01, Proc. FEL 2012.

WEPSO69	Optical Cavity Losses Calculation and Optimization of THz FEL with a Waveguide	689
	P. Tan, Q. Fu, L. Li, B. Qin, K. Xiong, Y.Q. Xiong HUST, Wuhan, People's Republic of China
	Funding: the Fundamental Research Funds for the Central Universities，HUST：2012QN080 The optical cavity with waveguide is used in most long wavelength free electron lasers. In this paper, the losses, gains and modes of a terahertz FEL sources in Huazhong Univeristy of Science and Technology(HUST) are analysis. Then the radii of curvature of the optical mirrors and shapes of the waveguide are optimized.

WEPSO84	Present Status of Kyoto University Free Electron Laser	711
	H. Zen, M. Inukai, T. Kii, R. Kinjo, K. Masuda, K. Mishima, H. Negm, H. Ohgaki, K. Okumura, M. Omer, K. Torgasin, K. Yoshida Kyoto University, Institute for Advanced Energy, Kyoto, Japan
	A mid-infrared FEL named as KU-FEL (Kyoto University FEL) has been developed for energy related sciences [1]. After the achievement of the first lasing and the power saturation in 2008 [2, 3], we have been working to extend the tunable range of the FEL [4]. By replacing the original 1.6-m undulator into a 1.8 m one, the tunable range was expanded from 10-13 to 5-15 μm in January 2012. Then we fabricated a new undulator duct to reduce the minimum undulator gap from 20 to 15 mm. At 15-mm gap, the FEL gain can be expected to be twice as high as that at 20 mm gap. Commissioning of the new duct will be done in the end of this April. In this presentation, we will report on the result of the commissioning such as tunable range of KU-FEL and the estimated FEL gain, which would be compared with a simulation. [1] H. Zen, et al., Infrared Phys. Techn., 51, 382 (2008) [2] H. Ohgaki, et al., Proc. of FEL08, 4 (2008) [3] H. Ohgaki, et al., Proc. of FEL2009, 572 (2009) [4] H. Zen, et al., Proc. of FEL2012

WEPSO89	Design of a Resonator for the CSU THz FEL	719
	P.J.M. van der Slot Mesa+, Enschede, The Netherlands S. Biedron, S.V. Milton, P.J.M. van der Slot CSU, Fort Collins, Colorado, USA
	Funding: This research is support by Office of Naval Research Global, grant number N62909-10-1-7151 A 6-MeV L-band linac will be used to drive a planar, fixed gap, 2.5-cm period, hybrid undulator with parabolic pole faces. Consequently, this system is capable of generating wavelengths from 160 to 600 μm. In this paper we discuss the design of an optical resonator for this system. The resonator uses hole-coupled mirrors to allow for a straight electron beam line. The Rayleigh length, the position of the waist of the cold-cavity mode and the hole radii will be investigated to optimize the performance of the FEL.