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No abstract

This paper presents the results of the adaptive feedforward system in use on the Ground Test Accelerator (GTA). The adaptive feedfonvard system was fihown to correct repetitive, high-frequtmq errors in the amplitude and phase of the RF field of the pulsed accelerator, The adaptive feedforward system was designed as an augmentation to the RF field feedback control system and was able to extend the closed-loop bandwidth and disturfx nce rejection by a factor of ten. Within a second implementation, the adaptive feedforward hardware was implemented in place of the feedback control system and was shmvn to negate both beam transients and phase droop in the klystron ampl~ler.

A technique for measuring both resistive and reactive particle-beam-loading effects on an rf accelerating cavity is described. A high-power, six-port reflectometer is used to measure the complex cavity impedance, both with and without beam, as a function of cavity resonant frequency. Resistive and reactive beam loading appears as a decrease in the cavity quality factor Q and as a change in the cavity resonant frequency, respectively. An equivalent circuit model for the drive system, cavity, and beam is used to quantify these effects as a function of cavity voltage and phase, beam current, and cavity detuning. Measurements on a heavily beam-loaded two-cell drift-tube linac are compared with predictions from the equivalent circuit model and good agreement is found. The measurements are performed while maintaining constant cavity-field amplitude and phase. Thus, this technique is a nonperturbative measurement.

Due to the time-varying nature of the radio-frequency (RF) accelerator, RF field amplitude and phase parameters must be precisely controlled in order to confine and accelerate the charged particle beam. Typically, a feedback control system regulates the RFfield, rejects noiseanddisturbances, and maintains operational stability over changes in the electrical structure of the accelerator. This paper describes a multivariable control system that compensates the electrical structure of the acceleratorby using gain-shaping in the frequency domain. The amplitude and phase quantities have been resolved into in-phase and quadrature (I&Q) variables. These orthogonal variables have simple mathematical relationships, and can be analyzed using linear transfer function matrices. The transfer matrix theory has been applied to the design of the multivariable control system that regulates the RF field in-phase and quadrature components. Frequencydomain controllers compensate these two signals to provide desired frequency response characteristics. A control predistorter performs an inverse coupling function, so that the I&Q components are effectively decoupled by the accelerator. Furthermore, computer interface circuitry allows the adaptive optimization of the mathematical transfer functions of the compensators.