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FETS RFQ Beam Dynamics Studies

FETS RFQ Beam Dynamics Studies. Simon Jolly 8 th June 2010. FETS Layout. Ion Source. Beam Diagnostics. Laserwire Tank. LEBT. RFQ. MEBT/ Chopper. RFQ. RFQ Geometry: A Reminder. RFQ provides transverse focussing along with longitudinal bunching/acceleration in a single element.

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FETS RFQ Beam Dynamics Studies

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  1. FETS RFQ Beam Dynamics Studies Simon Jolly 8th June 2010

  2. FETS Layout Ion Source Beam Diagnostics Laserwire Tank LEBT RFQ MEBT/ Chopper Simon Jolly, Imperial College

  3. RFQ Simon Jolly, Imperial College

  4. RFQ Geometry: A Reminder • RFQ provides transverse focussing along with longitudinal bunching/acceleration in a single element. • Transverse focussing comes from opposing voltages on 4 vanes/rods: • B-fields from RF power induce currents on vane surfaces. • Currents give voltages that provide quadrupole field. Simon Jolly, Imperial College

  5. RFQ Geometry: A Reminder (2) • Longitudinal bunching and acceleration comes from modulation of vanes: • Longitudinal modulations cause field lines to curve. • Stretching modulation period gives acceleration. Positive vane Field lines Positive bunch Negative Vane tips Lines of Force Positive vane Simon Jolly, Imperial College

  6. RFQ Integrated Design • RFQSIM (written by A. Letchford for ISIS RFQ) generates r, a, m and L parameters (and accompanying field map) that describe RFQ vane modulation for every cell. • Integrated design takes these parameters and creates: • Full CAD model of RFQ vane tips (Autodesk Inventor). • Electrostatic field map from CAD model (CST/Comsol). • Compare beam dynamics simulations using both types of field map. Simon Jolly, Imperial College

  7. CST vs. RFQSIM: Quiver Plot Simon Jolly, Imperial College

  8. CST vs. RFQSIM: Potential Simon Jolly, Imperial College

  9. CST vs. RFQSIM: Potential (Zoom) Simon Jolly, Imperial College

  10. Conclusions From Jan ’10 Update • Finally seeing match between RFQSIM optimised field map and CST field map from CAD modelling. • Each step has required a lot of effort for small progress, but that progress has proved extremely valuable. • Advantages of this process already starting to become apparent: for example, very easy to modify CAD model based on thermal/stress simulations (Scott Lawrie) and measure effect on beam dynamics. • Next steps: • Output CAD model to Comsol, repeat process from CST to produce more easily adaptable field map (tighter integration with Inventor and Matlab). • Compare CST coarse, fine, Comsol and RFQSIM field maps point-by-point to determine whether discrepancies are a result of poor field mapping or more accurate modelling of vane tips. Simon Jolly, Imperial College

  11. 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” (MOPEC076). • Needed to compare beam dynamics simulations for all 3 field mapping methods. • Simulations in GPT to compare CST and Comsol with RFQSIM field. • Vary current between 0-120mA and measure transmission and final energy spread. Simon Jolly, Imperial College

  12. Results: RFQSIM Transmission Energy Spread (60mA) Simon Jolly, Imperial College

  13. Results: CST Transmission Energy Spread (60mA) Simon Jolly, Imperial College

  14. Results: Comsol Transmission Energy Spread (60mA) Simon Jolly, Imperial College

  15. RFQSIM vs. CAD Model Maps • RFQSIM shows 92% transmission for 60 mA input current. • CST and Comsol give very similar results to each other for both transmission and energy, but significantly poorer transmission than RFQSIM. • 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… Simon Jolly, Imperial College

  16. Increased Field Strength • 10% increase in field strength recovers transmission: we’re back in business! • Is such an increase feasible in reality? We have available RF power, but what about heating? • Does it compare to known RFQ’s? • Not yet sure of the origin of this difference: might be mesh-based, might be real. Simon Jolly, Imperial College

  17. Conclusions • 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. • Need to produce very high mesh models: • Compare single RFQ cell with deep modulations between RFQSIM and Comsol: how do Ez-fields compare? • Mesh RFQ in very short sections (200 mm) in Comsol and see if transmission improves at 60mA. • Next steps: • Resolve mesh-based issues in Comsol (required mesh density, tangential boundaries). • Modify RFQSIM parameters based on CAD results: deeper vane modulations? • Incorporate thermal and mechanical stresses from Scott’s models. Simon Jolly, Imperial College

  18. rod axis r0 (mm) ma r0 (mm) a L/2 beam axis L RFQ Design Parameters • RFQ parameterised by 3 (+1) parameters: • a and m parameters define modulation depth. • r0 defines the mean vane distance from the beam axis and is derived from a and m. • r gives the radius of curvature (vane) or mean radius (rod). • L defines the length of each cell (half sinusoidal period). • For RFQSIM, these values generated for idealised RFQ field. Simon Jolly, Imperial College

  19. RFQ Parameters (MOPEC076, IPAC’10) Simon Jolly, Imperial College

  20. 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: • xmax = ymax = 2.2mm. • x’max = y’max = 90mrad. • erms = erms = 0.25 p mm mrad. Simon Jolly, Imperial College

  21. 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). Simon Jolly, Imperial College

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