Fast longitudinal diagnostics for the HERA proton ring

Abstract

After the luminosity upgrade of the electron proton collider HERA, the proton bunch length will become relevant for the achievable luminosity. New, strong focussing, superconducting magnets inside the detectors H1 and ZEUS lead to smaller cross sections at the interaction regions and with that, to higher luminosity. Due to the strong focussing, the beta function and the bunch length have a comparable magnitude. This enhances the effective cross section, ‘hour-glass-effect’ [1]. A reduction of the bunch length would result in smaller effective cross sections and so further increase the luminosity. Even at HERA energies of 920 GeV, protons do not lose a significant amount of energy, due to synchrotron radiation, because of their large rest mass. Hence, there is no natural damping of unwanted beam oscillations, as is the case for electrons or positrons. Coherent oscillations of the proton beam lead to an increase of the beam dimensions. Shorter bunch lengths at high energy are achievable by suppressing the coherent oscillations at injection and during acceleration. For an investigation of the origin of coherent oscillations, powerful longitudinal diagnostic tools are indispensable. Therefore, a fast longitudinal diagnostic system was developed, which permits real time measurements of bunch phase and length for all 180 bunches during one turn. Beam loading transients and the accelerating voltages can be examined with a fast cavity field diagnostic. The vacuum system of the HERA proton ring was carefully designed to keep the impedance, which could drive beam oscillations, as low as possible. Therefore, measures are needed to suppress the oscillations. The fast longitudinal diagnostic system provides the signals, needed for a RF feedforward and a coupled-bunch feedback. The new diagnostic system permits novel observations and experiments at HERA. One can observe coupled-bunch oscillations during acceleration, which are correlated with an increase in the longitudinal emittance. The impact of bunch oscillations on the beam loading transients can be demonstrated. Measurements of the longitudinal beam transfer function and measurements of decoherence times, together with beam echoes, yields important beam dynamical parameters. Since a further reduction of the all over impedance is not easily achievable, the suppression of coupled-bunch oscillations must be done by active systems. The results obtained in this thesis, provide the necessary information for the design of a coupled-bunch feedback system, whose task is the preservation of the longitudinal emittance.