INITIAL PERFORMANCE OF A 500 kV, CHOPPER PREBUNCHER INJECTION SYSTEM FOR A 17 GHz LINAC*

The design features of a 17 GHz chopper prebuncher injection system are described, and initial test results operating with a 500 kV low emittance electron gun and single section 17 GHz linac are presented. The 17 GHz traveling wave relativistic klystron that provides RF power to the linac structure, and the 500 kV electron gun that forms the low emittance (βγ ε = 3π mm-mradian) source for the injection system, are pulsed simultaneously using the MIT Plasma Science and Fusion Center HV modulator [1]. The HV injection system is designed to produce RF bunches of 15° to 20° for injection into the linac structure, with subsequent compression to approximately 1 degree to provide sub-picosecond bunch operation over an energy range of 10 to 20 MeV with steady-state pulse currents of up to 250 mA. The use of an economical, on-line diagnostic system having the capability of accurately measuring the short RF bunch performance of the 17 GHz linac would be highly advantageous; and a simple technique of accomplishing this, using higher order mode 17 GHz cavities, is discussed. 1 INJECTION SYSTEM DESCRIPTION The design and beam optics characteristics of the injector and 17 GHz linac have been presented elsewhere [2,3], and the system is briefly summarized as follows. Three low aberration thin lens assemblies enable the beam to be transported from a 500 kV 1.2A low emittance source through a 17 GHz chopper cavity and a high gradient, velocity modulating prebuncher cavity, a φ 2 mm chopping collimator and a small injection aperture at the entry of the linac structure. A dc magnetic dipole, located at the chopper cavity, is used to bias (offset) the RF scanned beam vertically downward below the centerline. With this dc biasing technique, the RF scanned beam is returned to the centerline once during each RF cycle so that a given (adjustable) fraction of the incident beam is transmitted through a small diameter chopping collimator and injected into the linac only during the period when both the energy spread and the rate of change of the transverse momentum introduced by the chopper cavity are tending to zero. The phase width (bunch length) of the transmitted portion of the chopped beam is a function of the ratio of collimator to beam diameter (D/d), the maximum RF deflection of the beam (χM) and the dc bias deflection (B). The 17 GHz chopped beam transmission for both the unbiased (symmetrically scanned beam) and biased operating conditions are shown in Figures 1 and 2, respectively. With biased operation (B=χM), a χM value equal to 3 beam diameters is required to produce chopped bunch lengths of 100° for subsequent compression to 15° by the prebuncher cavity. The chopper cavity was designed to achieve this beam deflection with a drift distance of 35 cm using an RF peak input power of only 1.4 kW to give a peak transverse magnetic field of 6060 0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 D/d = 1.50 D/d = 1.25 D/d = 1.00 TEST DATA (D/d=1.4) Gaussian (d/2 = 2σ) BIAS = 0 BEAM DIAMETER = d CHOPPER COLLIMATOR DIA = D PEAK RF BEAM DEFLECTION = χ M