Modelling of diagnostics for the ISIS ring

Ben Pine, Chris Warsop, Steve Payne

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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

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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 Monitor Working Principle Typical 10ms ISIS Profile

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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 potentialTransverse electrode potential

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Profile Monitor: Drift Field Errors (1)

Aperture scan under drift-field: detected ion position as a function of starting coordinate

(Initial x, Initial y)

(Detected x)

A new technique was developed to overcome limitations in particle creation and number in CST, and the results compared with the previous simulations

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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

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Profile Monitor: Space Charge Effects

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Simple model of ion trajectories and profile under effect of analytical space charge and ideal electrode field

Results plotted as electrode voltage is increased

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‘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)

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

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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