A high power THz free electron laser (FEL) facility is under construction at China Academy of Engineering Physics (CAEP) since October, 2011. The radiation frequency of the FEL facility will be tuned in range of 1∼3 THz and the average output power will be about 10 W. The system mainly consists of a GaAs photoemission DC gun, superconductor accelerator, the hybrid wiggler, optical cavity. In this paper, the design considerations and first lasing are given.

First lasing of CAEP THz FEL facility

A high power THz free electron laser (FEL) facility is under construction at China Academy of Engineering Physics (CAEP) since October, 2011. The radiation frequency of the FEL facility will be tuned in range of 1∼3 THz and the average output power will be about 10 W. Much effort has been done for the THz FEL facility. The system mainly consists of a GAAS photoemission DC gun, superconductor accelerator, the hybrid wiggler, optical cavity. In this paper, the design considerations and present status are given.

Present status of CAEP THz FEL facility

To ensure the quality of high brightness electron beams needed by the terahertz FEL facility at China academy of engineering physics(CAEP),which aims to obtain 100 to 300 terahertz light, a feed-back control system is required to monitor the amplitude and phase jittering by measuring beam arrival time as well as bunch length at the site of the beam position monitor(BPM). In this paper, we make an idealized model of injector section and deduce analytic expressions of bunch arrival time and bunch length. In consideration of the space charge effect on bunch lengthening, bunch arrival time and bunch length as a function of DC gun voltage, buncher field amplitude and buncher phase is carefully calibrated by means of particle in cell (PIC) simulation. With the time and space resolution of the BPM, the control accuracy of phase is estimated to be 0.01 degree, while the amplitude is 0.04%. INTRODUCTION Owing to the challenge of advanced accelerator applications, the control accuracy of amplitude and phase is becoming stricter for control systems, under which circumstance emerges methods of beam-based feed-back control. Two representative examples are the fast feedback installed in Stanford Linear Collider(SLC) using BPM readings and fitted beams parameters to stabilize beams [1]; while a combination of RF and beam based feedback loops used at The Free Electron LASer at Hamburg(FLASH) has achieved regulation of ~10fs rms bunch arrival time jitter [2]. For the THz-FEL facility at CAEP [3], the main goal for the accelerator control systems is to achieve highly precise regulation of relative amplitude and absolute phase jitter below 0.01 % (rms) and 0.01 degrees (rms). KINEMATIC MODEL WITHOUT SPACE CHARGE In THz-FEL, the electron bunch has a length of ~30ps at the exit of the DC gun, and needs a bunching process through the downstream buncher to reach ~10ps length before entering the superconducting accelerating cavity. The BPM located close to the entrance of the accelerating cavity is used for beam position measurement. Here an ideal kinematic model without space charge is utilized to deduce analytical expressions of bunch arrival time and bunch length (compression ratio) at the site of BPM.

Beam Compression Dynamics and Associated Measurement Methods in Superconducting THz Source

The optical resonator of CAEP THz-FEL is optimized to ensure wavelength tunable in a wide range and high power operation. The FEL power strongly depends on the performance of the optical resonator including output efficiency, gain and round-trip loss. The optical resonator consists of metal-coated reflect mirror, the center-hole output mirror, waveguide. The influence of waveguide and Rayleigh length on the quality of optical cavity is evaluated by the 3D-OSIFEL code. The waveguide size, mirror curvature radius, output hole radius is optimized to different frequencies between 1 THz to 3 THz.

http://ieeexplore.ieee.org/iel7/7301993/7327382/07327642.pdf

Optical resonator optimization of CAEP THz-FEL

A high power THz free electron laser (FEL) facility is under construction at China Academy of Engineering Physics (CAEP) since October, 2011. The radiation frequency of the FEL facility is designed in range of 1~3 THz and the average output power is about 10 W. First lasing of the CAEP THz FEL (CTFEL) facility was obtained in August 29, 2017. Then the saturation operation is observed in September 20, 2017. The radiation frequency is sequentially as 1.99THz, 2.41THz and 2.92THz. And the average output power is larger than 10 W; the largest output power is about 20W. The peak power of micro-pulse is larger than 0.5MW. The optical resonator of CTFEL is optimized to ensure wavelength tunable in a wide range and high power operation. In this paper, the main designed and present status of the facility is given.

Lasing and saturation of CAEP THz FEL facility

To meet the requirement of material and biomedicine study, a Terahertz FEL user facility project was proposed by China Academy of Engineering Physics(CAEP). At present the project has been approved and the facility will be constructed within 5years. The facility will operate in the quasi CW mode and the average power is about 10W . The wavelength of the light can be adjusted between 100μm/3THz to 300μm/1THz according to the user needed by changing the electron energy and the magnetic field of the wiggler. The facility mainly consists of the electron source, the main accelerating structure, the hybrid wiggler, the optical oscillator cavity, the THz-ray transmission system and the detector. In order to achieve the high brightness beam, the photocathode DC gun will be used as the electron source. The electron will obtain the energy by passing through a superconducting accelerator, the electron energy after the accelerator is about 8MeV, which is suitable to obtain the terahertz light. The facility will be a useful tool to the science.

/pdf/the-terahertz-fel-facility-project-at-caep-5d04jlutmt.pdf

The terahertz fel facility project at caep

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The terahertz fel facility project at caep