Development of a 10 MW hollow beam klystron for the ILC

Present efforts to achieve a high efficiency 1.3 GHz RF source for the ILC are centered on the development of multiple beam klystrons (MBK) and more recently, on a sheet beam klystron (SBK). We have initiated the development of a hollow beam klystron (HBK) to fulfill the stringent requirements imposed by the ILC. That is, the minimum efficiency should be 65% at an RF output power of 10 MW and the cathode current density should be less than 5 A/cm². The pulse width must be 1.5 milliseconds at a pulse rate frequency of 15 Hz. We have simulated several 7 cavity interaction circuits using our fully relativistic 2frac12D klystron large signal code, KLSC. To date, we have achieved the following: beam voltage: 120 kV, beam current: 140 A, beam perveance: 3.37 micropervs, efficiency: 63%, RF output power: 10.6 MW, saturated gain: 47.4 dB, maximum gradient: 137kV/cm, beam current density: 30 A/cm², beam area convergence: 6:1. These parameters were obtained by varying the diameter of the drift tube and the inner and outer diameter of the electron beam, the cavity gap-gap spacing and the cavity gap length. We will present graphical date showing that the electron bunching is almost solely axial modulation with minimal radial modulation, unlike conventional solid beam klystrons. We will also present performance data for other circuits where the efficiency is lower but with collector depression, can be increased above 65%.