Abstract
We developed an electron gun system capable of delivering high brightness beams to drive soft x-ray free-electron lasers (SXFEL). We modified the conventional radio-frequency (rf) gun concept by using a 50-kV gun with a gridded thermionic cathode connected with a 238-MHz acceleration cavity. We achieved a low target emittance by applying the following two methods: (1) to obtain a parallel beam after the control grid, we optimized the grid voltage to compensate for the electric potential distortion near the grid (i.e., the lens effect); and (2) to avoid emittance growth due to the space charge effect at low energy regions, the 50-kV gun was connected to a 238-MHz rf cavity with a minimum distance, which immediately accelerated the beam energy to 500 keV. We built a gun test stand to verify the high beam performance, i.e., a normalized emittance of less than 4 mm mrad with a bunch charge of 1 nC and a pulse length shorter than 0.7 ns, as predicted by cst and parmela codes. Our proof-of-performance experiments demonstrated that a projection emittance of 1.7 mm mrad was obtained as a core part, representing 60% of all extracted electrons and the measured emittance agreed well with the simulation value of 2.0 mm mrad. This paper presents an overview of the electron gun system and our proof-of-performance experimental results.
Highlights
X-ray and soft x-ray free-electron lasers (XFEL [1,2,3,4,5] and SXFEL [6,7,8]) have long served as powerful tools for exploring innovative science over a wide variety of academic fields, including chemistry, materials science, biology, nonlinear optics, and many other domains
We developed an electron gun system capable of delivering high brightness beams to drive soft x-ray free-electron lasers (SXFEL)
We achieved a low target emittance by applying the following two methods: (1) to obtain a parallel beam after the control grid, we optimized the grid voltage to compensate for the electric potential distortion near the grid; and (2) to avoid emittance growth due to the space charge effect at low energy regions, the 50-kV gun was connected to a 238-MHz rf cavity with a minimum distance, which immediately accelerated the beam energy to 500 keV
Summary
X-ray and soft x-ray free-electron lasers (XFEL [1,2,3,4,5] and SXFEL [6,7,8]) have long served as powerful tools for exploring innovative science over a wide variety of academic fields, including chemistry, materials science, biology, nonlinear optics, and many other domains. As a third type of FEL electron source, we developed an rf gun using a gridded thermionic cathode, which provides a sufficiently small beam emittance with pulse length shorter than 0.7 ns and offers the prime advantage of the thermionic gun (ease of maintenance) while not requiring the complex high voltage pulse modulator and chopper systems. Our system features a 50-kV thermionic gun connected to a 238-MHz rf cavity in order to immediately increase the beam energy to 500 keV or even higher This system uses a commercially available gridded cathode that provides an electron pulse shorter than 1 ns, which assures high capture efficiency for the subsequent rf acceleration cavity. IV summarizes the beam performance measurements at the gun test stand and compares our experimental results with particle tracking simulations
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