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Published byAnderson Burkinshaw Modified over 2 years ago

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Effect of RFQ Modulations on Frequency and Field Flatness

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Recap: Tuning the End Region to Achieve Correct Frequency

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Final Design

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CST and ANSYS results for final geometry, 4m RFQ with flush tuners. Taking into account meshing accuracy for these large models (see slides 3 & 4), both agree with the frequency being (323.5 ± 0.5) MHz

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Original Matcher No Matcher Optimised Matcher New matcher design achieves considerably flatter field and ensures (323.5 ± 0.5) MHz along the entire RFQ

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Now: What About Modulations? Transverse position of copper in each cell varies ∴ Capacitance varies throughout the cell But surely, on average, capacitance stays constant if average bore radius is constant? Average bore radius, r 0

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Evidence For Modulation- Induced Field Flatness Errors “RF Test of a 324-MHz, 3-MeV, H - RFQ stabilised with PISLs”, TUD02, LINAC’00 A. Ueno & Y. Kondo, KEK “…tuners were set at the design positions (99.5mm from the beam axis) …The longitudinal field non- uniformity is speculated to be produced by the modulation effect. …The larger modulation is speculated to have the larger capacitance.”

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Evidence For Modulation- Induced Field Flatness Errors “Complete RF design of the HINS RFQ with CST and HFSS”, MOP042, LINAC’08 G. Romanov & A. Lunin, Fermilab

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My Simulation Strategy 1.Import Simon’s full 4m vane-tips geometry. 2.Import vane geometry with tips removed. 3.Create cavity vacuum by taking inverse of copper. 4.Macro to automate slicing & mesh of one cell. 5.Eigenmode solution of each individual cell. Calculate for each RFQ cell: Resonant frequency Q Voltage at cell centre (position of quadrupole transverse & maximum longitudinal field)

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My Simulation Strategy Transverse Mesh Longitudinal Mesh

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E-field Plots for Cell 222 Transverse (cell centre) Longitudinal (x=0 plane)

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Comparing HINS & FETS RFQs

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Solution? ✕ Complete redesign of beam dynamics and modulation parameters to get correct capacitance and frequency for each cell ✕ Vary inductance by altering the quadrant radius to suit each modulation Alter quadrant radius of each 1m section so each section is roughly flat to start with, easing tuning procedure

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Leave quadrant radius at 44.1mm Increase quadrant radius to ~44.2mm Change quadrant radius half way along this section? Next step: Try this solution to see if frequency and voltage do flatten out

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