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

2 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

3 Detector Electrode Detector Beam Ions Electric field Electrode Profile Monitor: Principle of Operation  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 ISIS Profile MonitorWorking PrincipleTypical 10ms ISIS Profile

4  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? Profile Monitor: Focus of Work Longitudinal electrode potential Transverse electrode potential

5 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

6 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

7 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

8 ‘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 2.12x10 13 ppp) Profile Monitor: HV Testing  Experiment increased electrode voltage from KV  Fitted curve changes by less than 1cm between KV  New HV Towers for monitors will provide 60 KV  Profile model will be compared directly with experimental results

9 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