Modelling of diagnostics for the ISIS ring Ben Pine, Chris Warsop, Steve Payne

Motivation It is important to understand and predict high intensity behaviour on ISIS In order to compare theory and simulation with experimental results, we must have confidence in the accuracy of our diagnostic instruments A detailed analysis of instruments’ physical operation compliments the diagnostic hardware upgrades This talk describes work in progress studying the ISIS Residual Gas Profile Monitors with CST Studio Suite and other tools Modern computing power and software used to look for new insights into these well known devices

Profile Monitor: Principle of Operation ISIS Profile Monitor Working Principle Typical 10ms ISIS Profile Detector Electrode Detector Beam Ions Electric field Electrode Residual gas atoms left in the beam pipe are ionised as the beam comes past The ions are swept with an electric field and the resulting current plotted against transverse position The gas ions will also be affected by the electric field of the beam itself Radial electric field of beam may distort the detected profile

Profile Monitor: Focus of Work Two major effects limiting resolution: Non-linear drift field Space charge of the beam Effect of space charge can be minimised by increasing the electrode voltage When will the drift field dominate over space charge? What are the errors associated with the drift field alone? Longitudinal electrode potential Transverse electrode potential

Profile Monitor: Drift Field Errors (1) Aperture scan under drift-field: detected ion position as a function of starting coordinate A new technique was developed to overcome limitations in particle creation and number in CST, and the results compared with the previous simulations

Profile Monitor: Drift Field Errors (2) Mathematica tracking program and CST field used to calculate profile error and position offset due to drift field WORK IN PROGRESS Initial Results

Profile Monitor: Space Charge Effects Simple model of ion trajectories and profile under effect of analytical space charge and ideal electrode field Results plotted as electrode voltage is increased

Profile Monitor: HV Testing Experiment increased electrode voltage from 10-30 KV Fitted curve changes by less than 1cm between 60-100 KV New HV Towers for monitors will provide 60 KV Profile model will be compared directly with experimental results ‘Investigations into the relationship between the Drift Field Voltage and the measure Beam Width in the ISIS Ring Beam Profile Monitor System‘, [S. J. Payne, C. M. Warsop, A. H. Kershaw, D. M. Wright] (Measurements taken at 0ms in the ISIS cycle, with 2.12x1013 ppp)

Conclusion Understanding diagnostics is an essential part of machine development What is being learnt about the diagnostics will help optimisation of the synchrotron and prepare for the detailed measurements required to study high intensity behaviour This is work in progress - results will be compared with experiment as new hardware becomes operational Aim to model any 2D beam distribution, with arbitrary beam intensity and electrode voltage, then predict the detected profile Split-electrode beam position monitors have also been modelled; Envelope monitors will hopefully follow by the summer