Basic Design of a 20K C-band 2.6-cell Photocathode RF Gun

A cryo-cooled C-band photocathode RF gun that operates at 20K is under development. The RF gun is a 2.6-cell pillbox-type -mode cavity with the resonant frequency of 5712 MHz. Using high-purity Oxygen-free copper, a high accelerating efficiency is expected from a theoretical prediction of the anomalous skin effect at low temperatures. Considering the cooling capacity, the RF gun system is designed to operate at the RF duty factor of 0.01%. After machining of the basic test cavity for low power and low temperature tests, the RF properties of the cavity have been measured at room temperature prior to assembling at KEK by means of diffusion bonding technique. Since it is intended for basic understanding and measurements of low temperature RF properties, the cavity has not been equipped with structures for the photocathode assembling or the RF input coupler. The measured properties of the cavity have shown good agreement with the results of Superfish simulation. INTRODUCTION A C-band photocathode RF gun is under development for future use in the compact X-ray linac developed at KEK [1]. The electron beam is extracted by using a cryocooled copper RF cavity, which can be driven with considerably low RF power compared to those in room temperature RF guns. As a basic design the RF gun cavity consists of 2.6-cell pillbox-type -mode structure and operates at 5712 MHz under low temperature of 20K. A low power test cavity has been designed with Poisson Superfish [2] for basic understanding of the RF properties at low temperature. The low temperature experiment on the test cavity is under preparation. This paper reports the result of the RF properties measured at room temperature before diffusion bonding. SPECIFICATIONS OF THE RF GUN The surface resistance of the cavity material and resultant RF power efficiency of beam acceleration depend on the cavity temperature. The behavior of the surface resistance at 5712 MHz has been calculated on the basis of the theory of the anomalous skin effect [3], which is shown in Fig. 1 together with the result of calculation of the normal skin effect for comparison. In the calculation the copper material with a residual resistance ratio (RRR) of 3000 was assumed corresponding to the purity of more than 99.9999% (6N) that was specified for the fabrication of the cavity. Figure 1: Temperature dependence of theoretical surface resistances of RRR=3000 copper cavity at 5712 MHz. Solid line: result of the anomalous skin effect, dashed line: the normal skin effect. As seen in Fig. 1, the surface resistance of the cavity at 20K is reduced to approximately 18% of that at room temperature, which means that the quality factor of the cavity is 5.5 times as large as that at room temperature. From the result of the calculation, the design specifications for the cryo-cooled copper RF gun cavity has been determined as listed in Table 1. The Q0 and the shunt impedance expected at room temperature by ___________________________________________ * Work supported by the Photon and Quantum Basic Research Coordinated Development Program of the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT). # tanaka@lebra.nihon-u.ac. jp Table 1: Specifications for the 2.6-Cell 20K Cryocooled Photocathode RF Gun RF frequency 5712 MHz Source peak RF power 4 MW Q0 60000 Shunt impedance 550 M/m Coupling coefficient 20 Cavity length 68.2 mm RF pulse duration 2 s Maximum field on axis 95 MV/m Maximum peak RF loss 0.73 MW Output beam energy 3 MeV RF duty factor 0.01 % Maximum beam charge 0.5 nC/bunch Laser pulse repetition rate 357 MHz Laser pulse length 10-20 ps 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-MOPRI030