Research On Bent-vane Type Radio Frequency Quadrupole Accelerator

Radio Frequency Quadrupole(RFQ)is widely used as the injector for high energy accelerator due to its outstanding capabilities in strong focusing,bunching,and accel-erating simultaneously for low velocity beams.Mainly,there are two types of RFQ structures,the four-vane type and the four-rod type.The four-rod type RFQ structure is usually employed in the low frequency cases,and is mostly used in the low duty factor operations,which is limited by the complicated cooling systems.The four-vane type RFQ structure is mainly used in high frequency cases and has simple but efficient cool-ing systems,which is usually operated at high duty factor and continuous wave(CW)modes.Four-vane type RFQs with low frequencies were adopted by some projects to get effective cooling for CW operations,but with quite large section diameters,thus making the manufacture process more difficult and increasing the cost.Four vane with windows RFQ has been proved to be effective to reduce the transverse dimensions in low frequency field.But the cooling system is complicated,and the field flatness is distorted near the electrode tips between adjacent vanes,which will introduce adverse effects on the beam emittance and transmission efficiency.To get a small section size at low frequency and a simple but efficient cooling sys-tem at the same time,the bent-vane type RFQ(BV-RFQ)has been proposed to operate at CW mode with intense beam current and high transmission efficiency.For the BV-RFQ structure,additional capacitances could be generated among the bent vanes,which are expected to reduce the section size.And similar to the four-vane structure,the BV-RFQ is rotationally symmetric,thus a simple cooling system could be efficient for CW operation.In order to demonstrate the principle of the BV-RFQ structure,we constructed a self-consistent equivalent circuit model.The full geometry of the cross section for a single vane could be defined by 13 independent parameters,and optimized parameters could be attained with a parameter sweeping method.A proof-of-principle aluminum cavity was built and tested to demonstrate this new concept.The beam dynamics design of the BV-RFQ prototype cavity is performed by RFQ-Gen based on‘Four sections theory'design method.This prototype cavity is proposed to accelerate 5 mA He~+(q/A=1/4)beam from 4 keV/u to 42.5 keV/u with 96.9%transmis-sion efficiency at the 81.25 MHz resonant frequency based on 530.07 mm vane length.The inter-vane voltage is selected to be 70 kV.The parametric resonance is avoided and the transverse and longitudinal emittance is effectively controlled.Tolerance analyses have also been done for this prototype cavity.The electromagnetic studies of the BV-RFQ prototype cavity have been completed with CST microwave studio.The full cavity model was designed based on the cross section profile and parameter values of the optimized BV-RFQ.Two plunger tuners are employed in each quadrant thus resulted to a total tuner number of eight for this whole prototype cavity.These tuners are inserted 25 mm into the cavity as the baseline,the tuning range is between-276.97—483.12 kHz,tuning sensitivity is 12.67 kHz/mm.The nearest dipole mode is 10.609 MHz higher than the operation mode.The quality factor of the cavity is 12003,shunt impedance is 269.8 k?m,the cross section diameter is 567.34 mm.Stability study of the BV-RFQ prototype cavity includes the multiphysics analyses and multipacting analyses.Multiphysics analyses are done with ANSYS Workbench,stable CW operation could been guaranteed by a simple cooling system,temperature rise,deformation and stress are all controlled under safe values.Frequency sensitivities due to the vane and wall cooling water temperature shift are calculated.The tolerance studies are also done.Multipacting analyses have been done with CST Particle Studio,which could be refered during the RF conditioning and operation.Finally,the manufacture and test of the BV-RFQ prototype aluminum cavity are introduced.The manufacture tolerance could be fully satisfied,and the cold test show great results.