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Effect of RFQ Modulations on Frequency and Field Flatness
Recap: Tuning the End Region to Achieve Correct Frequency
Final Design
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
Original Matcher
No Matcher
Optimised Matcher
New matcher design achieves considerably flatter field and ensures (323.5 ± 0.5) MHz along the entire RFQ
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, r0
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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’00A. 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 thelarger capacitance.”
Evidence For Modulation-Induced Field Flatness
Errors
“Complete RF design of the HINS RFQ with CST and HFSS”, MOP042, LINAC’08G. Romanov & A. Lunin, Fermilab
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)
E-field Plots for Cell 222
Transverse (cell centre)
Longitudinal (x=0 plane)
Comparing HINS & FETS RFQs
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
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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