RF manipulations in SIS 18 and SIS 100 O. Chorniy

Introduction Scheme of the SIS18 RF cycle RF capture Fast bunch compression Bunch merging Tomography and diagnostics tools

RF cycle in SIS18 Full cycle pre-compression step presently RFC amplitude RF frequency BCC amplitude BK V V(h=4) V(h=1) V(h=4) V(h=2) V(h=1) alternative pre-compression step time In order to avoid the coasting beam phase, where beam is less controled injection capture acceleration compression de-bunching re-capture merging

RF capture. General requirements Transition from coasting beam to bunched beam has to be done without increase of longitudinal rms emittance The dilution factor D is a function of: 1.Initial voltage 2.Type of the capture 3.Initial emittance Find the condition for increase RF voltage -> growing RF bucket area - longitudinal rms emittance, or area in phase space covered by particles

RF capture in single RF bucket Linear ramp linear RF capture in 20 ms 0 If bucket height created by is higher than 2 rms momentum spreads of coasting beam (red bucket), then emittance will be increased.

RF capture in single RF bucket is the RF amplitude after first quarter of synchrotron oscillation Empirical condition for emittance conservation found from the simulation We need number of synchrotron oscillations performed during the time along RF ramp For linear RF ramp

Threshold parameters for linear RF ramp Simulation results for dilution factor and threshold capture time (red) Condition for

RF capture with space charge Without space chargeWith space charge ? Space charge improves RF capture allowing faster capture times. The mechanism is not clear yet.

RF capture with other ramp RF ramp Single RF bucket Dual RF bucket

RF capture. Measurements and simulation For more exact (quantitative) analysis in future the emittances and dilution factors will be compared. For this aim the tomography reconstruction will be used. All analysis above is based on the simulation results. The simulation results of RF capture in a single RF bucket are in agreement with measurement.

Fast bunch compression. Injection energy. Bunch compressor in SIS18 Space charge ? E=11.4 MeV/u Compression MeasurementTheory

Compression with space charge From paper “Effect of space sharge on bunch compression near transition”, G. Franchetti, I. Hofmann and G. Rumolo Comparison of analytic results with simulation results Space charge cannot be the reason for the reduced compression in measurements

Fast bunch compression. Top energy. pre-compression and compression steps V(h=4) V(h=1) Vbk(h=1) time de-bunching re-capture RF amplitude 4 kV Bunch compressor amplitude 38 kV Ions E=300 MeV/u The compression coefficient cannot be obtained Distorted pre-compressed bunch profile due to: 1.Missmatch of beam energy with respect to RF frequency 2. Too fast RF re-capture

Fast bunch compression. Top energy. Comparison of measurements with simulation results Simulation (black line): The capture time is as in the measurements RF frequency is shifted to one value of rms momentum spread of coasting beam Simulation (red line): Long capture time No missmatch Different conditions in simulation: Thus, the simulations show that the compressed bunch length can be improved (reduced).

Fast bunch compression. Top energy. The peak current and the fraction of particles that remains inside the compressed bunch is also important Simulation results The minimum bunch length can be achieved by compression of purely coasting beam (84 ns). At the same time only half of all particles remains inside bunch area. If we use for example 2 kV pre-compression amplitude then all particles inside bunch area but the bunch length is 2 times higher. Next slides is devoted to improvement of compression of purely coasting beam

Compression of the coasting beam In order to reduce the compressed bunch length the RF cavity also can participate in compression process. The “natural” rise time of RF cavity has non-zero value. Bunch compressor and RF cavity compressing the coasting beam V(h=4) V(h=1) Vbk(h=1) time If rise then the reference parameters: The effect of RF cavity finite rise time can be compensated by the proper time shift between starts of RF cavity and bunch compressor shift Simulation results

V(h=4) V(h=2) Vbk(h=1) Higher harmonic of RF in order to improve the performance RF cavity signal is in phase with bunch compressor signal at synchronous point, similar to shortening mode RF cavity signal phase is shifted with 90 degree with respect to phase of bunch compressor signal at synchronous point, similar to lengthening mode shift shortening mode lengthening mode Compression of the coasting beam Simulations results

Bunch merging. For the compression with pre-bunched beam the merging scheme can be used in order to avoid “RF frequency-beam energy” misalignment V(h=4) V(h=2) Bunch merging scheme used in simulations time Contour plot from simulations

Bunch merging. Influence of bunch length Dilution factor diagram for different merging times and bunch lengths Dilution factor here is defined as means that sum of rms emittances af all bunches before and after merging is constant ? Another merging scheme (ramp) to improve the performance

Longitudinal diagnostic Longitudinal signal from Phase Detector Longitudinal signal from Fast Current Transformer ! Improved longitudinal density resolution.

Tomography Using longitudinal signal from Phase Detector Using longitudinal signal from Fast Current Transformer ! Improved phase space density resolution. More exact value of the rms emittance

Outlook The analysis of beam loading influence for all mentioned RF manipulations has to be done Space charge effect for the bunch merging process has to be studied Development and integration of tomography scheme in the SIS diagnostic system