A Proposed RF System for the Fusion Materials Irradiation Test Facility
Abstract: Preliminary rf system design for the accelerator portion of the Fusion Materials Irradiation Test (FMIT) Facility is in progress. The 35-MeV, 100-mA, cw deuteron beam will require 6.3 MW rf power at 80 MHz. Initial testing indicates the EIMAC 8973 tetrode is the most suitable final amplifier tube for each of a series of 15 amplifier chains operating at 0.5-MW output. To satisfy the beam dynamics requirements for particle acceleration and to minimize beam spill, each amplifier output must be controlled to ±1° in phase and the field amplitude in the tanks must be held within a 1% tolerance. These tolerances put stringent demands on the rf phase and amplitude control system.
Summary (1 min read)
Low Power rf
- The low-power rf stage will supply an rf signal of the proper frequency, phase, and amplitude (O-100 W) to drive the PA such that the PA output is also at the required frequency, phase, and amplitude (O-500 kW) for optimum accelerator performance.
- The accelerating field is the vector sum of the rf fields produced by the energy delivered to each tank by several rf power amplifiers.
- Feedback control loops around each PA and around each accelerator tank insure that the field in the tank remains at the correct phase and amplitude with or without beam,.
- The phase and amplitude controlling takes place at the 1-W level, The rf drives for all three tanks are essentially the same so only one amplifier chain will be described in detail.
- Figure 2 contains the block diagram for one complete rf amplifier chain including feedback control loops,.
FREQUENCY CONTROL 8. PHASE FIEF: LINE
- The tank resonant frequency is critically temperature dependent.
- In the search mode the rf source is switched to a voltage controlled oscillator (VCO), which generates 1-ms pulses with a pulse repetition frequency of 120.
- The central control system (CC'S) generates an amplitude set-point signal proportional to the desired field level in the accelerator tank.
- The tank amplitude set-point signal is subtracted from the tank feedback signal to generate a control signal proportional to their difference .
- The amplitude control circuit of each PA receives four control signals: 1. Amplitude set point, proportional to the average power output of each PA.
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