The rf gun for the siberian circular light source “skif”

The Siberian Circular Light Source is a new mediumenergy high brightness synchrotron light facility which is under construction on the Budker Institute of Nuclear Physics (BINP) in Russia, Novosibirsk [1]. The accelerator facility for convenience is divided into three components; a 3 GeV storage ring, a full-energy booster synchrotron and a 200 MeV injector linac with a thermionic gridded RF gun electron source. This paper describes the RF gun design and plans for operation. INTRODUCTION The prototype of the RF gun presented in this paper is the RF gun constructed for Novosibirsk Free Electron Laser facility [2] and the RF gun for VNIIEF [3] that have been extensively tested and operated at BINP and Sarov. The differences are the pulse regime of work instead the continuous one and the higher beam energy (800 keV). It has an oxide thermionic gridded cathode mounted into the RF gun cavity. A trigger pulse with 1 ns width came to the cathode-grid gap at the moment when RF phase voltage in the cavity becomes 40 degrees in order to emit an electron bunch. This bunch is accelerated by the cavity field up to 800 keV. This energy is less than those maximal one that occurred at the launch phase of 70 degrees. So, the 40 degrees bunch has the specific energy distribution of particles that led to bunch longitudinal compress of up to 20 ps at the end of some drift space distance. In order to get the desired 4 ps width bunch, there must be additionally used a special third harmonic cavity. Hereby, the RF gun generates an electron beam suitable for velocity bunching of it down by 250 times on a 3-metrelong drift space containing a third harmonic cavity, a preaccelerator, and a diagnostic and focusing equipment. Respective cavity resonance frequencies are 178, 534 and 2856 MHz. All works in a pulse regime with 1 Hz repetition frequency. There is the possibility to use a higher repetition frequency, e.g., of 20 Hz, to make a conditioning of the cavities. The main requirements to the RF gun are the phase and amplitude field stability of about 1° and 1% respectively. This is followed from the requirement for the electron beam at the linac exit to have no more than 1% of energy spread and energy deviation suitable for injection through the linac-to-buster transfer line into the booster synchrotron. All these cavities together with the linac accelerating structures must be accurate synchronized within 2 psec by the Libera electronic system equipment with master generator frequency of 178 MHz. The temperature of all cavities is stabilized accurate within 0.1 K. Their resonance frequencies, except the pre-accelerator one, are controlled by a precision feedback control system equipped with piezoelectric tuners. Two modes of operation are planed; short-pulse mode, in which a single 1 nC electron bunch is injected into one buster RF bucket and long-pulse mode in which a train of single bunches with the total charge of 16.5 nC is injected into the booster working at an operational frequency of 356 MHz. Such value of charges leads to a beam loading effect those results in voltage drop of cavity acceleration. Simulations show that the maximum voltage drop is less than 1%. Furthermore, a possible phase shift of the voltage due to the sound or thermal impact of the RF power pulse into the cavity is estimated as slow enough to compensate the both by the vector modulator equipped feedback control system. The amplitude and phase of the RF master generator signal is controlled by the vector modulator and drives a solid-state preamplifier. A second 0.7 MW solid state amplifier equipped by built-in circulators, amplifies the signal which is connected to the RF gun by a 6 1/8” coaxial feeder and an RF power coupler. The trigger pulses with 1 ns width and the variable 0÷150 V amplitude are formed by a special transistor scheme displaced into a mobile block inserted into the RF gun and connected to the cathode-grid assembly through a set of collets. The trigger signal is phase controlled by the reference signal of the master generator through electronic equipment made by BINP. The detailed RF parameters of the RF gun are listed in Table 1. The RF gun now is manufactured on the BINP experimental production that will be lasted for 1 year and then will be tested at a special stand-prototype consisting from the third harmonic cavity, pre-accelerator, the first accelerating section of the linac, and the diagnostic equipment in order to test the system during a building structure for the SKIF facility is constructed and made. RF GUN DESIGN The RF gun schematic view is presented in Figs. 1 and 2. ___________________________________________ † v.n.volkov@inp.nsk.su 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB149 MC3: Novel Particle Sources and Acceleration Techniques T02 Electron Sources WEPAB149 2965 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I