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Description
Power variators (PV) or variable attenuators are required to control the RF power transfer to accelerator cavities and to realize dual energy LINACs operated at Megawatt peak power. Dual energy LINACs are used in medical radiation therapy (dual photon energy and imaging mode), as well as in industrial X-ray inspection systems for enhanced material detection in cargo scanning and non-destructive testing (NDT). PVs may consist of a passive waveguide network - including 3-dB-couplers, power splitters/ combiners, loads/ absorbers and tunable phase shifters as actuators for attenuation control - connected between the Magnetron or Klystron tube and the LINAC. Most solutions are based on stepper- or servo-motorized mechanical waveguide phase shifters. With a response time in the range of some hundred milliseconds or seconds those devices are not suitable to perform pulse to-pulse power variation at an RF pulse repetition rate of several hundred Hz.
This contribution introduces a fast ferrite phase shifter (Fast Ferrite Tuner, FFT), where the phase shift is delivered by the continuously tunable permeability of a biased microwave ferrite. A reflection-type ferrite waveguide phase shifter was designed for Magnetron-driven S-Band LINACs operated at 2998 MHz, 4 MW peak power, 4 kW average power with typical RF pulse length between 4 and 10 µs. The ferrites are in a shorted WR284 waveguide-section, water cooled and pressurized with SF6 at a pressure of 2 bar gauge. The ferrite bias is provided by permanent magnets and coils embedded in a closed magnet system formed of laminated iron yokes. A continuous tuning of permeability and phase is achieved by applying a tuning coil current of ± 10 A. A special narrow line width YIG ferrite material is chosen, and it is operated above ferromagnetic resonance, to obtain a high tunability of the effective permeability at very low loss.
Test results are presented for the FFT phase shifter device, achieving a differential phase shift of more than 110° at a power loss of only < 0.05 dB (0° state) and < 0.15 dB (90° state). The response time for a full phase-range switch is about 1 ms, using a four-quadrant power supply at ± 10 A, ± 48 V.
A reflection-type power variator unit is introduced, consisting of a compact arrangement of a magic-T power splitter, a load and FFT phase shifters. The unit provides continuous or switched-mode power variation in the range of 0 to 4 dB by using a single FFT or from 0 to >20 dB by using two FFT phase shifters. The low response time of the FFT allows for pulse-to-pulse power variation up to 1 kHz pulse repetition frequency, as demonstrated in an S-band dual energy LINACs with 3-dB power variation.
It is shown, how a reflection-type FFT power variator can be embedded in a 4-port circulator environment, to combine a variable attenuator with an isolator function. Inserted in the RF chain of a LINAC system, the subsystem provides reliable isolation between tube and PV as well as between PV and LINAC. Using such an isolated power variator in a dual energy LINAC permits to operate a pulsed Magnetron at a constant nominal peak power without high-voltage modulation. This way the lifetime of the Magnetron is improved, and the tube is effectively prevented from damage. A closed-loop power control via the FFT driver electronics enables precise and continuous adjustment of the photon energy as well as high energy stability, as required in high-resolution systems.
