Qingzi Xing

Mode Analysis of High-Power Microwave Generation in the Inward-Emitting Coaxial Vircator Based on Computer Simulation

A mode analysis is carried out with the MAFIA TS3 module to study the high-power microwave generation in the inward-emitting coaxial vircator with two opposed nonaxisymmetric emitting surfaces. The central angle thetas of the emittors, and the axial distance D between the cathode and anode reflector, are verified to have a large influence on the mode component. For a fixed diode voltage of 250 kV, the coaxial diode current is 13.7 kA when the beam injection is axisymmetric (thetas = 180deg). In this situation, the simulation gives the highest efficiency of 8.3% for D = 5.0 cm and 10.9% for D = 10.0 cm , comparing with the nonaxisymmetric beam injection. For the case of two opposed nonaxisymmetric emitting surfaces with central angle thetas, there exists a range for thetas to acquire a rather pure output of the TE11 mode (80% of the total power), which is 80deg < thetas < 160deg. However, the efficiency decreases with smaller thetas. The polarized direction of the main TE11 mode is consistent with the directional layout of the two opposed emitters. FFT transformation shows that the dominant frequency for thetas < 80deg can be estimated by the equations derived from the 2-D theoretical analysis of the symmetric coaxial vircator.

Design of the Low Energy Beam Transport Line for Xi‘an Proton Application Facility

Xi`an Proton Application Facility (XiPAF) is a new proton project which is being constructed for singleevent-effect experiments. It provides proton beam with the maximum energy of 230MeV. The accelerator facility of XiPAF mainly contains a 7MeV Hlinac injector [1] and a proton synchrotron accelerator. The 7MeV Hlinac injector is composed of an ECR ion source, a Low Energy Beam Transport line (LEBT), a Radio Frequency Quadrupole accelerator (RFQ) and a Drift Tube Linac (DTL). The 50keV 10mA Hbeam (pulse width of 1ms) extracted from the ion source is expected to be symmetric transversely with the Twiss parameters Į=0 and ȕ=0.065 mm/mrad. With an adjustable aperture and an electric chopper in the 1.7m-long LEBT, the beam pulse width of 40ȝs and peak current of 6mA can be obtained. The Hbeam is matched into the downstream RFQ accelerator with Į=1.051 and ȕ=0.0494 mm/mrad. This paper presents the detailed design process of the LEBT and the beam dynamics simulation result with the TRACEWIN code. LEBT STRUCTURE In general, Low Energy Transport line (LEBT) is used to match the Hbeam between the Ion source and RFQ accelerator. For the linac injector of Xi`an Proton Application Facility (XiPAF), the beam which is expected to be symmetric transversely is extracted from an ECR ion source. A two-solenoid structure is capable of matching the beam in usual. The RFQ accelerator is expected to accelerate the beam with the pulse width of 40ȝs and input peak current of 6mA, while the Ion Source is designed to produce the beam with the pulse width of 1 ms and peak current of 10 mA. Thus a square aperture and a chopper are inserted into the 1.7m long LEBT. Figure 1 shows the layout of the LEBT. Those elements are related to the beam dynamics. The beam diagnostics devices like faraday cup, ACCT and emittance scanner are included in the design. With the operation experience of the Compact Pulsed Hadron Source (CPHS) at Tsinghua University, it is difficult to manipulate the beam for the field of the solenoids is overlapped with the field of the steering magnets [2]. The rotating of the beam caused by the solenoids couples with the offset of the beam caused by the steering magnets. Therefore, at XiPAF`s linac injector, those two kinds of magnets are set at different position. The beam is focused and steered separately. It is much easier to match the beam in LEBT. The aperture is set after the steering magnets. The off-axis of the beam is avoided at the position of the aperture.