A NOVEL DEVICE FOR NON-INTERSECTING BUNCH SHAPE MEASUREMENT AT THE HIGH CURRENT GSI-LINAC

For bunch structure determination in the range of 0.1 to 5 ns a novel device has been realized at the GSI heavy ion LINAC. It uses the time spectrum of secondary electrons created by residual gas interaction. Those electrons are accelerated by an electric field of 420 V/mm toward an electro-static energy analyzer. This is used to restrict the effective source region. The time spectrum is transformed into a spatial separation by an rf-deflector driven by the main acceleration frequency (36 or 108 MHz). Single electron detection is performed by a multi-channel plate equipped with a phosphor screen and observed by a digital CCD camera. The achievable time resolution is 50 ps, corresponding to 2 0 of the 108 MHz acceleration frequency. DETECTOR TECHNOLOGIES The determination of the longitudinal density distribution of a bunched beam is an important issue because it is required for an optimal matching between different LINAC-modules as well as for the comparison with numerical calculations also taking space charge effects into account. The bunch structure cannot be determined by capacitive pick-ups due to the non-relativistic beam velocities ( β< 20% at the GSI-LINAC) causing a faster propagation of the electric field of the bunches. At most LINACs the bunch structure is determined by secondary electrons emitted from a wire crossing the beam [1, 2]. The wire is biased with about −10 kV to pull the secondary electrons toward a slit outside the beam path. An rf-deflector follows, where the electrons are modulated in transverse direction by an electric rf-field. The deflection angle depends on their relative phases, i.e. the device transforms the time information into a spatial difference. For the high current beam operation at GSI with heavy ions and currents up to 20 mA [3], the beam power is sufficient to melt intersecting materials. The described principle is adapted to a non-intersecting device by performing the time spectroscopy of secondary electrons created by atomic collisions between beam ions and residual gas molecules. The electrons are accelerated by a homogeneous electrical field formed by electrodes outside of the beam pass, as usually used for residual gas profile monitors. To restrict the source region for the secondary electrons, an aperture system and an electro-static energy analyzer is used. The time-to-spatial transformation is performed with the standard type rf-deflector developed at INR (Moscow) [2].