Phase and amplitude stability of a pulsed RF system on the example of the CLIC drive beam LINAC

The CLIC drive beam accelerator consists of the Drive Beam Injector (DBI) and two Drive Beam Linacs (DBLs). The drive beam injector is composed of a thermionic electron source, 3 Sub Harmonic Bunchers (SHBs), a pre-buncher, and several acceleration structures. In the electron source the DC electron beam is produced from a thermionic cathode. The following buncher cavities group ("bunch") the electrons to be accelerated by RF later on. Each electron bunch has an energy of 140 keV, a length of 3 mm, and a charge qb = 8.4 nC. Afterwards the electrons are accelerated in the 1 GHz accelerating structures up to 50MeV. The pulsed Radio Frequency (RF) power for this acceleration is provided by 1 GHz, 20MW modulator-klystron units, one per acceleration structure. A klystron is an RF amplifier based on a linear-beam vacuum tube. The high voltage modulator supplies the acceleration voltage to this tube. A DC electron beam gets modulated with an input signal, the modulation enhances in a drift space, and finally the power gets coupled out into a waveguide. Due to the fact that the main beam of CLIC is powered by the drive beam, the quality and stability of the main beam depends on the current, phase, and energy stability of the drive beam. There are very stringent requirements on the beam stability of the main beam which are then translated into requirements of the amplitude and phase stability of the klystron units of the drive beam. The drive beam current stability is predefined by the electron gun, while the drive beam phase and energy quality is defined by the klystron output phase and amplitude. Since the klystron is an amplifier the output stability depends heavily on the input signal and the supply voltages. For example, the 100 kV supply voltage of the klystron needs a 10-5 stability. The specification for the RF drive signal to the klystron is an amplitude stability of 0.1% and a phase stability of 0.05 degrees pulse to pulse. The subject of this thesis is the generation and pre-amplification of the pulsed RF klystron input signal of the CLIC drive beam accelerator, with the above specifications.