A CW normal-conductive RF gun for free electron laser and energy recovery linac applications

A CW normal-conductive RF gun for free electron laser and energy recovery linac applications K. Baptiste a , J. Corlett a , S. Kwiatkowski a , S. Lidia a , J. Qiang a , F. Sannibale a,∗ , K. Sonnad a , J. Staples a , S. Virostek a , R. Wells a a Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, US Abstract Currently proposed energy recovery linac and high average power free electron laser projects require electron beam sources that can generate up to ∼ 1 nC bunch charges with less than 1 mm mrad normalized emittance at high repetition rates (greater than ∼ 1 MHz). Proposed sources are based around either high voltage DC or microwave RF guns, each with its particular set of technological limits and system complications. We propose an approach for a gun fully based on mature RF and mechanical technology that greatly diminishes many of such complications. The concepts for such a source as well as the present RF and mechanical design are described. Simulations that demonstrate the beam quality preservation and transport capability of an injector scheme based on such a gun are also presented. Key words: electron source, low emittance, high brightness, ERL-FEL Injector PACS: 29.25.Bx, 29.27.Bd, 29.27.Eg, 41.60.Cr 1. Introduction A number of proposed projects in energy recovery linacs (ERL) and linac-based soft-xray-VUV free elec- tron lasers (FEL) requires injection of 100 pC - 1 nC electron bunches at repetition rates from kHz to hun- dreds of MHz [1, 2, 3, 4, 5, 6, 7]. For both ERL and FEL applications, production of high-brightness elec- tron beams is critical in achieving the desired x-ray per- formance and normalized beam emittances lower than 1 mm mrad are required. This places particular burdens on the cathode, gun and injector systems. Limiting early emittance growth requires minimum accelerating elec- tric ﬁelds in the gun of ∼ 20 MV/m and gun voltages of ∼ 500 kV or higher [8]. Obtaining these ﬁelds and voltages in a DC gun requires a signiﬁcant eﬀort and expertise, especially in preventing insulator breakdown. Voltages of up to ∼ 350 kV have been already demon- strated [9] and there is a group trying to extend such a value up to ∼ 750 kV [10]. Superconducting, high-frequency cavities are likely candidates for high repetition rate photoinjectors, and an intense RD however, the average power density in the cavity structure limits the practical repetition rate to the kHz range [15]. By decreasing the RF frequency, the size of the cavities increases with a beneﬁcial reduc- tion of the power density on the structure walls. In this situation, the repetition rate of the system can increase and continuous wave (CW) operation can be achieved. The Boeing gun has achieved 25% duty cycle operation at 433 MHz [16], a 700 MHz CW normal conducting gun has been proposed [17], and a group at Los Alamos is completing the construction of a 700 MHz normal- conducting RF gun where a sophisticated and state of the art cooling system has been designed to operate in CW mode [18, 19]. In our approach [20, 21, 22], based on an idea of J. Staples and developed at the Lawrence Berkeley Na- tional Laboratory in the framework of the activities March 5, 2009 author. Tel: +1 510 486 5924; Fax: +1 510 486 Email addresses: fsannibale@lbl.gov (F. Sannibale) Preprint submitted to Nuclear Instruments and Methods A