FETS RFQ Beam Dynamics Simulations for RFQSIM, CST and Comsol Field Maps Simon Jolly 2 nd June 2010

RFQ Field Map Comparisons We now have 3 consistent methods of producing field maps for the RFQ: –RFQSIM (from coefficients). –CST (optimised field from August 2009). –Comsol (just finished in time for IPAC’10). IPAC’10 RFQ beam dynamics paper: “Integrated Design Method And Beam Dynamics Simulations For The Fets Radiofrequency Quadrupole” Needed to compare beam dynamics simulations for all 3 field mapping methods. Simulations in GPT to compare CST and Comsol with 2 types of RFQSIM field (approximated and full). Vary current between 0-120mA and measure transmission and final energy spread. 02/06/10Simon Jolly, Imperial College2

Input Conditions Current variation from 0-120mA; bunch length: 1RF period; SCtree3D space charge simulates bunched beam. Using same input distribution as for previous publications: –x max = y max = 2.2mm. –x’ max = y’ max = 90mrad. –  rms =  rms = 0.25  mm mrad. 02/06/10Simon Jolly, Imperial College3

Field Map Differences Differences between RFQSIM approximated and full fields at 5% level: –Smooth variation of coefficients between cells. –Full Bessel functions rather than truncated series. CST uses maximum mesh density (4,700 points) with 6 RFQ sections (matching section, 2x500mm, 3x1m). Comsol uses same vane model but not yet using tangential boundaries. All field maps use 0.5mm point spacing (RFQSIM field maps match CST and Comsol, but different from previous simulations). 02/06/10Simon Jolly, Imperial College4

Results: RFQSIM /06/10Simon Jolly, Imperial College5 TransmissionEnergy Spread (60mA)

Results: RFQSIM 2010 (Simple) 02/06/10Simon Jolly, Imperial College6 TransmissionEnergy Spread (60mA)

Results: RFQSIM 2010 (Full) 02/06/10Simon Jolly, Imperial College7 TransmissionEnergy Spread (60mA)

Results: CST 02/06/10Simon Jolly, Imperial College8 TransmissionEnergy Spread (60mA)

Results: CST 02/06/10Simon Jolly, Imperial College9 TransmissionEnergy Spread (60mA)

Preliminary Conclusions RFQSIM simple field gives very similar results to old simulations: this is good! Differences probably due to GPT interpolation since point spacing is different. RFQSIM simple and full fields also give very similar transmission results: –92% transmission at 60 mA for both. –Full field expansion gives slightly higher final energy. CST and Comsol also give very similar results, both for transmission and energy. But… 02/06/10Simon Jolly, Imperial College10

Poor CAD Model Transmission CST and Comsol give significantly poorer transmission than RFQSIM for higher beam currents. Why the difference? Poor meshing or real RFQ properties? Try increasing the field strength by up to 30% to see if we can recover transmision… 02/06/10Simon Jolly, Imperial College11

Increased Field Strength 02/06/10Simon Jolly, Imperial College12 10% increase in field strength recovers transmission: we’re back in business! Is such an increase feasible in reality? Does it compare to known RFQ’s? Not yet sure of the origin of this difference: might be mesh-based, might be real.

Conclusions Lots of results that make sense (a turn-up for the books!): –Simple and Full RFQSIM field maps show virtually no difference: slightly better RF capture and acceleration from Full field gives higher final energy. –CST and Comsol give very similar results: looks like we’re producing the same map through the same method. Clear differences between CAD-based methods and RFQSIM: –Field strength nominally correct, since no extra transverse losses. –Longitudinal fields give problems: poor RF capture and acceleration. –But we can recover transmission by increasing the field strength: maybe field is closer to reality? –Longitudinal vane curvature certainly more subtle than transverse: need better mesh longitudinally. Perhaps try a single Comsol simulation with very high mesh density (200mm RFQ sections) and see if transmission improves at 60mA. Should we start optimising on “realistic” beam? 02/06/10Simon Jolly, Imperial College13