Simulation of feedback BPM signals in an intense background environment

1. Simulation of feedback BPM signals in an intense background environment A Hartin, P Burrows, G Christian, C Clarke, B Constance, H Khah, C Perry, C Swinson (JAI, Oxford University, UK) R Arnold, S Molloy, S Smith, G White, M Woods (SLAC, Calfornia) A Kalinin (Daresbury, UK) Introduction The ILC beam-beam interaction produces a background environment that may affect the operation of feedback systems for beam alignment. One crucial element of the ILC feedback system is a stripline BPM placed near the IP in the extraction line. The operation of this feedback BPM in an intense background environment was tested at the T-488 experiment at SLAC End Station A. This experiment operated in two modes. Firstly, a “high spray mode" in which the primary beam was directed off axis into a graphite torus producing a large background spray flux. Secondly, a “low spray mode" in which an on-axis beam impinging on a thin radiator resulted in a strong central beam and a lower background spray flux. In the high spray mode, BPM voltage signals visibly distorted from the usual bipolar doublet, were recorded. Accurate simulations that reproduced these high spray mode signal shapes have been developed here. Experimental data from the low spray mode was combined with high spray mode data to predict the feedback BPM signal shapes expected at the ILC. Interpolation to the ILC The T-488 experiment also operating in the low spray mode passed the on-axis primary beam through a thin radiator. The lower flux of background spray resulted in a very low secondary emission signa. The low spray mode BPM difference signals in fact showed a 2% variation in amplitude, probably due to beam jitter The expected effect on the ILC feedback BPM position measurements due to secondary emission was estimated by using Geant to simulate the number of incident hits on striplines included in the ILC geant geometry and using ILC parameter set 14 (high luminosity with 1TeV centre of mass energy) and with the anti-DiD field. This configuration was known to produce the worst case scenario for background hits on the FONT BPM striplines. The ILC secondary emission to BPM signal ratio would be at least an order of magnitude smaller than that of the T-488 low spray mode and was therefore considered negligible. These tests give confidence that the FONT feedback system will operate successfully in the background environment expected at the ILC The BPM difference signals for the T-488 experiment with the beam passing through a thin radiator. The error on the peak readings of ~2% meant that no discernible effect due to background spray could be found The T-488 experimental module, containing the lowZ graphite torus, stripline BPM and connecting flanges 1st order nonlinear Pair Production Charge Bunch Bunch Fields 2nd order nonlinear Pair Production Background spray The T-488 module was modelled in Geant3. With an incident flux of 10000 electrons at different offsets on the x-axis, the x-profile of the scattered beam at the upstream end of the BPM strips was recorded. The x-profile of the beam was considered to consist of two current components; an azimuthally symmetric background spray Ispray and a remnant of the original beam Ib. The current induced on the striplines Islikewise has two components Incident hits and secondary emission from the BPM strips.Charges ejected from the back surface of the BPM strip quickly cross the 1mm gap between the strip and BPM wall. Charges approaching or leaving the front surface have a weighted contribution. Results of lowZ mode simulation The response of the oscilloscope that recorded the BPM stripline signals was simulated by use of a numerical second order, 1.2 GHz Butterworth low pass filter. Comparison with data occurred in the first instance with the beam on axis producing no background spray. Data and simulated bipolar doublets showed good agreement With the simulated beam off axis and producing significant background spray, all contributions to the stripline current were combined. Numerical calculations were performed using the Scilab program. Comparison of the distorted experimental and simulated BPM stripline signal showed good agreement, with the secondary emission signal visible as a reverse bipolar doublet superimposed over the usual bipolar doublet Background hits on the FONT feedback BPM striplines (S1-S4). The worst case scenario expected at the ILC would be at least an order of magnitude better than that measured in the low spray mode of the T-488 experiment. The x-profile of the beam after passage through the lowZ graphite torus and at the upstream end of the strips. Method of image charge The current induced on the BPM strip due to a beam current at (r,θ) is a function of r and θandof beampipe radius b and strip subtentding angle φ. An expression for Isb is obtained after solving the Laplace equation in 2 dimensions The contribution from the spray is calculated by considering an element of current confined to an area rdrdθ and integrating r and θ over the circular beampipe cross-section. The term dependent on cos(nθ) goes to zero and the contribution to the stripline current from the azimuthally symmetric spray is • Conclusion • Distorted stripline BPM voltagesignals were recorded at the ESA T-488 experimentwith the incident beam off-axis and impinging on a lowZ torus. The resultant residual beam and background spray was modelled using geant and separatedinto a number of components. • The effect on stripline currentwas calculated using the methodof image charges, and the time responsewas filtered through a low pass numerical filter. Matching experimental and simulated signals gave confidence in the method employed. • The simulations were applied to the T-488 low spray mode. No variation in BPM difference signal voltage amplitude beyond a 2% beam jitter was observed. • Predicted secondary emission signals at the ILC operating would be at least an order of magnitude smaller than those of the T-488 low spray mode. • It was concludedthat pair backgrounds incident on BPM striplines at the ILC would have a negligible effect. Cross-sectional slice of the BPM. An element of beam current passing down the beampipe at position (r,θ) induces a current on the stripline Further information tony.hartin@physics.ox.ac.uk http://www-pnp.physics.ox.ac.uk/~font +44(0)1865273381