Kurup A

Letchford AP

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ELECTROMAGNETIC DESIGN OF A RADIO FREQUENCYQUADRUPOLE FOR THE FRONT END TEST STAND AT RALA. Kurup, Imperial College London, UKA. Letchford, CCLRC/RAL/ISIS, Chilton, Didcot, Oxon, UKAbstractThe goal of the RAL front end test stand is to demon-strate cleanly chopped bunches of a 60mA H− ion beamat 3MeV. The acceleration of the H− ions from 65keV to3MeV will be done using a radio frequency quadrupole(RFQ) operating at a resonant frequency of 324MHz. Thetwo types of RFQ considered were a 4-vane and a 4-rod.The 4-vane has a higher Q-value but the post-productionadjustment is limited. The 4-rod design is easier to man-ufacture but requires complicated cooling at 324MHz.The results of electromagnetic simulations using CST Mi-crowave Studio are presented for the 4-vane type and 4-rodtype RFQ.INTRODUCTIONHigh power proton drivers (∼ 4 MW) that employ chargeexchange injection require high efficiency H− ion front-ends. RFQs provide simultaneous acceleration and fo-cussing for low energy ions, which results in a high trans-mission efficiency. The RAL front end test stand is de-signed to demonstrate the technology needed for the frontend of a high power proton driver. The RFQ is required toaccelerate a 60mA H− ion beam, with input emittance of0.25 π mm mrad, from 65 keV to 3 MeV.Particle tracking simulations were performed, usingcode developed by A. Letchford [3], that gave a transmis-sion efficiency of 95%. From these studies the radius ofthe rods and the distance between rods were determined.These parameters were also used for the vane tip radiusand distance between vane tips and were fixed in all thesimulations.The frequency choice of 324 MHz favours the 4-vane de-sign from the point of the Q-value but post-manufacturingadjustment difficulties and the influence of dipole modesmay lead to the 4-rod design being favoured. The aim ofthese simulations was to create a design that maximised theQ-value without creating non-uniform heating or electricfield distributions, which would affect the beam transport.Cold-models of both the 4-rod and 4-vane designs wouldthen be made to compare with the simulations.ELECTROMAGNETIC SIMULATIONMETHODCST Microwave Studio was used to calculate the eigen-modes of the RFQ and the Q-value of each mode. The sim-ulations were done by taking the engineering model (drawnusing Autodesk Inventor) and removing the features thatwould not significantly affect the electromagnetic simula-tions. The inside volume of the model was then importedinto Microwave Studio. Using Inventor to draw the modeleliminates the need to maintain two separate versions intwo different environments and ensures that the model usedfor the simulations is realistic and can be built. Hidingthe engineering features allows the meshing to be doneon a simplified shape therefore reducing the complexity ofthe calculation and simplifying the application of bound-ary conditions. Once the final design has been obtained theengineering features can then easily be reapplied.Microwave Studio uses the Finite Integration Methodand for these simulations, the automatic hexahedral mesherwas used with the Perfect Boundary Approximation(PBA) [1] option. Using the PBA option allows a coarsemesh to describe a finer geometry. The Jacobi-Davidsonmethod eigenmode solver was then used to determine theresonant frequencies.SIMULATIONS OF A 4-VANE TYPE RFQThe simulation of the 4-vane RFQ included tuning holes,vacuum ports, coupling ports and vane cutaways. Fig. 1shows the cross-section of the CAD model and the in-side volume imported into Microwave Studio. The diam-eter marked on Fig. 1 was varied, whilst keeping the dis-tance between vanes fixed, until a resonant frequency of324 MHz was obtained.Figure 1: Cross section of the 4-vane Model (left) and theinside volume imported into Microwave Studio. The di-ameter, d, was varied to obtain a resonant frequency of324 MHz.Fig. 2 and Fig. 3 show the variation of the frequency andQ-value, respectively, with the number of mesh lines perwavelength used in the simulation.Proceedings of EPAC 2006, Edinburgh, Scotland MOPCH11604 Hadron AcceleratorsA15 High Intensity Proton Machines315Figure 2: Plot showing the variation of the resonant fre-quency of the first three modes with the number of linesper wavelength used to mesh the geometry. Mode 1 is thequadrupole mode and Mode 2 and 3 are the adjacent dipolemodes.Figure 3: Plot showing the variation of the Q-value of thefirst three modes with the number of lines per wavelengthused to mesh the geometry.TUNING SIMULATIONS OF THE 4-VANETYPE RFQA simple plug tuning mechanism was simulated to inves-tigate the total tuning range possible using a plug designedto fit within a KF40 flange. Fig. 4 shows the model with allfours tuning plugs pushed all the way in, giving the maxi-mum effect on the resonant frequency.Figure 4: 4-vane model with four tuning plugs pushed allthe way in.Fig. 5 and Fig. 6 show the variation of the resonant fre-quency and Q-value, respectively, with the distance that thetuning plugs have been moved in.Figure 5: Plot showing the variation of the resonant fre-quency, of the first three modes, with the position of thetuning plugs.Figure 6: Plot showing the variation of the Q-value, of thefirst three modes, with the position of the tuning plugs.CUTAWAY OPTIMISATIONA delay in the schedule for the manufacture of the 4-vanecold-model allowed some optimisation of the vane cutawayregion to be done. Fig. 7 shows the surface current distribu-tions for the old and new models. While both show uniformwall losses over most of the inner surface, the original cut-away design shows more localized heating near the vanetip, which would require cooling. The new cutaway designhas surface currents concentrated away from the vane tipswhere machining of cooling channels is much easier.SIMULATIONS OF A 4-ROD TYPE RFQA design similar to the ISIS RFQ [2] was first used forthe simulations. Fig. 8 shows the parameters (and their finalvalues) that were varied to obtain a resonant frequency of324 MHz. This compact design allows for many coolingchannels, since the distance between the stems is relativelysmall. However, this could affect the ability to efficientlycouple in rf power.Fig. 9 shows the surface current distribution for thismodel. This shows that this 4-rod design has localizedMOPCH116 Proceedings of EPAC 2006, Edinburgh, Scotland316 04 Hadron AcceleratorsA15 High Intensity Proton MachinesFigure 7: Surface current distributions showing the old cut-away shape (top) and the new cutaway shape.Figure 8: 4-rod Model.Table 1: Results for the first three eigenmodes of the 4-rodsimulation.Mode Frequency (MHz) Q-value1 (quadrupole) 323.7 43232 (dipole) 526.8 37513 (dipole) 536.4 4013heating near the join between the rod and the stem, whichis a critical region. Some preliminary thermal studies havebeen done to investigate cooling of the stems [4].This model was then modified to include cooling blocksand filleting of the edges. Simulations with various 4-roddesigns are currently ongoing.Figure 9: Surface current distribution for the simple 4-roddesign.CONCLUSIONSThe simulations of the 4-vane RFQ produced a modelwith a resonant frequency of (324.0 ± 1.2) MHz and a Q-value of (8782±133). A conservative estimate of the errorswas made by taking the RMS of the results from Fig. 2 andFig. 3. The adjacent dipole mode is separated by 7.1 MHzand therefore should not overlap with the quadrupole mode.Using four simple tuning plugs showed that the expectedtuning range is 12 MHz. Thus, to tune the RFQ by 1.2 MHzrequires moving the tuning plugs by 6mm, which corre-sponds to a change in the Q-value of 243. To allow tuningin both directions, the model has to be built for a lowerresonant frequency and operated with the tuning plugs par-tially in. However, this will mean operating at a lower thanideal Q-value.A cold-model of the 4-vane RFQ is already in construc-tion [4] and the 4-rod cold-model will follow shortly after.The 4-vane cold-model is based on the optimised cutawaysimulations. However, since the material had already beenpurchased, it was not possible to increase the model sizeto obtain a resonant frequency of 324 MHz. Therefore,the cold-model is expected to have a resonant frequency of(334.4± 1.2) MHz.Initial simulations with the 4-rod design showed thatthere is localized heating around the critical region betweenthe rod and the stem. Further simulations are being done toreduce the localized heating and to investigate the efficacyof different cooling options.REFERENCES[1] B. Krietenstein, R. Schuhmann, P. Thoma, T. Weiland, “ThePerfect Boundary Approximation Technique Facing the BigChallenge of High Precision Field Computation”, LINAC’98,Chicago (1998), p. 860.[2] H. Vormann, A. Schempp, U. Bessler, C. W. Planner, A.Letchford,“A New High Duty Factor RFQ Injector for ISIS”,EPAC’98, Stockholm (1998), p. 782.[3] A. Letchford, “A Comparision Of 4-Rod and 4-Vane RFQ-Fields”, EPAC’98, Stockholm (1998), p. 1204.[4] P. Savage, Y. A. Cheng, A. Letchford, J. K. Pozimski, “Me-chanical Design and RF Measurement on RFQ for Front-endTest Stand (FETS) at RAL”, these proceedings.Proceedings of EPAC 2006, Edinburgh, Scotland MOPCH11604 Hadron AcceleratorsA15 High Intensity Proton Machines317

Electromagnetic Design of a Radio Frequency Quadrupole for the Front End Test Stand at RAL

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