Mark Prokop

Frequency shift observer for an SNS superconducting RF cavity

In contrast to a normal conducting RF cavity, a superconducting RF cavity is very susceptible to shifts in its resonance frequency. The main sources of the shift are Lorentz force detuning and microphonics. In a spallation neutron source, to compensate for the frequency shift, a feedforward control is to be applied. In this paper, as an initiative step, a frequency shift observer is proposed which is simple enough to be implemented with a digital signal processor in real time. Simulation results of the proposed frequency shift observer show reliable performance and acceptable computation time for the real time implementation.

Lansce RF System Refurbishment

The Los Alamos Neutron Science Center (LANSCE) is in the planning phase of a refurbishment project that will sustain reliable facility operations well into the next decade. The linear accelerator was constructed in the late 1960s and commissioned as the Los Alamos Meson Physics Facility (LAMPF) in 1972. As the mission changed, LANSCE became a national user facility that provides pulsed protons and spallation neutrons for defense and civilian research and applications. The upgrade will replace all of the 201.25 MHz RF systems and a substantial fraction of the 805 MHz RF systems and high voltage systems. This paper will provide the design details of the new RF and high voltage systems.

Decoupling PI controller design for a normal conducting RF cavity using a recursive LEVENBERG-MARQUARDT algorithm

This paper addresses the system identification and the decoupling PI controller design for a normal conducting RF cavity. Based on the open-loop measurement data of an SNS DTL cavity, the open-loop systems bandwidths and loop time delays are estimated by using batched least square. With the identified system, a PI controller is designed in such a way that it suppresses the time varying klystron droop and decouples the In-phase and Quadrature of the cavity field. The Levenberg-Marquardt algorithm is applied for nonlinear least squares to obtain the optimal PI controller parameters. The tuned PI controller gains are downloaded to the low-level RF system by using channel access. The experiment of the closed-loop system is performed and the performance is investigated. The proposed tuning method is running automatically in real time interface between a host computer with controller hardware through ActiveX Channel Access.

FPGA implementation of a prototype PI Feedback control system for the LANSCE accelerator

The current LANSCE LLRF system is an analog PI Feedback control system which achieves amplitude and phase error of 1% and 1 degree respectively. The feedback system receives cavity amplitude and phase, crosstalk between the amplitude and phase paths is significant. We propose an In-phase (I) and Quadrature (Q) based feedback control system which easily decouples the crosstalk of the I and Q channels. A PI feedback controller is implemented with an Altera Stratix III FPGA. The control system is modeled with DSP Builder which automatically generates HDL. Altera SOPC Builder is used for the hardware integration of the DSP Builder model, memories, peripherals, and 32 bit NIOS II embedded processor. The NIOS II processor communicates with the host computer via Ethernet, uploads data, computes parameters, and downloads parameters. The network support of the design makes it possible to set and tune the control system parameters on-line and to conduct the calibration of the whole RF system easily. The proposed control system is successfully tested with a LANSCE sided-coupled linear accelerator at 720kw.

Gain scheduled neural network tuned pi feedback control system for the lansce accelerator

The current LANSCE LLRF system is an analog PI Feedback control system which achieves the amplitude and phase error within 1% and 1 degree. The feedback system receives the cavity amplitude and phase and the crosstalk between the amplitude and the phase paths is significant. We propose an In-phase (I) and Quadrature (Q) based feedback control system which easily decouples the crosstalk of I and Q channels. A gain scheduled PI feedback controller with optimally generated set point trajectory reduces the transient peak of the feedback controller and hence reduce the fatigue of the RF amplifier chain. An additional feature of the controller is the Neural network based self-tuning PI feedback, where the neural network tunes the feedback gains to minimize the errors in the least squares sense. The proposed control system is implemented with Altera Stratix II FPGA. The control system is modeled with DSP Builder and automatically generates HDL. Altera SOPC Builder is used for the hardware integration of the DSP Builder model, memories, peripherals, and 32 bit NIOS II embedded processor. NIOS II processor equipped with real time operating system communicates with the host computer via Ethernet, uploads data, computes parameters, and downloads parameters. The proposed control system is tested with the low power test-stand for the robustness of the algorithm.