1. 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. 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

4. 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

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. 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
‘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)

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