Sung-Il Kwon

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.

SNS Superconducting RF cavity modeling-iterative learning control

The Spallation Neutron Source (SNS) Superconducting RF (SRF) linear accelerator is operated with a pulsed beam. For the SRF control system to track the repetitive electromagnetic field reference trajectory, both feedback and feedforward controllers have been proposed. The feedback controller is utilized to guarantee the closed loop system stability and the feedforward controller is used to improve the tracking performance for the repetitive reference trajectory and to suppress repetitive disturbances. As the iteration number increases, the feedforward controller decreases the tracking error. Numerical simulations demonstrate that inclusion of the feedforward controller significantly improves the control system performance over its performance with just the feedback controller.

Uncertain system modeling of SNS RF control system

This paper addresses the modeling problem of the linear accelerator RF system for SNS. The cascade of the klystron and the cavity is modeled as a nominal system. In the real world, high voltage power supply ripple, Lorentz force detuning, microphonics, cavity RF parameter perturbations, distortions in RF components, and loop time delay imperfection exist inevitably, which must be analyzed. The analysis is based on the accurate modeling of the disturbances and uncertainties. In this paper, modern control theory is applied for modeling the disturbances, uncertainties, and for analyzing the closed loop system robust performance.

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.

CONTROL SYSTEM ANALYSIS FOR THE PERTURBED LINEAR ACCELERATOR RF SYSTEM

This paper addresses the modeling problem of the linear accelerator RF system in SNS. Klystrons are modeled as linear parameter varying systems. The effect of the high voltage power supply ripple on the klystron output voltage and the output phase is modeled as an additive disturbance. The cavity is modeled as a linear system and the beam current is modeled as the exogenous disturbance. The output uncertainty of the low level RF system which results from the uncertainties in the RF components and cabling is modeled as multiplicative uncertainty. Also, the feedback loop uncertainty and digital signal processing signal conditioning subsystem uncertainties are lumped together and are modeled as multiplicative uncertainty. Finally, the time delays in the loop are modeled as a lumped time delay. For the perturbed open loop system, the closed loop system performance, and stability are analyzed with the PI feedback controller.

System identification of the linac RF system using a wavelet method and its applications in the SNS LLRF control system

For a pulsed LINAC such as the SNS, an adaptive feed-forward algorithm plays an important role in reducing the repetitive disturbance caused by the pulsed operation conditions. In most modern feed-forward control algorithms, accurate real time system identification is required to make the algorithm more effective. In this paper, an efficient wavelet method is applied to the system identification in which the Haar function is used as the base wavelet. The advantage of this method is that the Fourier transform of the Haar function in the time domain is a sinc function in the frequency domain. Thus we can directly obtain the system transfer function in the frequency domain from the coefficients of the time domain system response.

SNS LLRF control system model design

This paper addresses part of the design of the LLRF control system for the Spallation Neutron Source. Based on the Matlab/Simulink model of the Klystron and Cavity, considered as a two input two output (TITO) system, we design a PID controller which achieves the tracking of the set point reference. The PID controller design method modifies a relay-feedback-based PID auto-tuner for a single input single output (SISO) system. The original modeling was developed for the Low Energy Demonstration Accelerator and has been modified for the SNS. The advantage of this method is that the only system information required for tuning the PID controller gains is the oscillation gain (critical gain) and the oscillation frequency (critical frequency) from the relay-feedback control of the open loop system. From the oscillation gain (critical gain), we obtain the proportional gain, and from the oscillation frequency (critical frequency), we obtain the integration time and the derivative time by applying some algebraic rules.

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.

Digital LLRF control system design and implementation for APT superconducting cavities

With a rapid development of the digital signal processor in recent years, a full digital control system for the superconducting section of APT is designed and implemented by using TIs C6x processor. A digital control system is first modeled and simulated using Matlab/Simulink. Simulation results confirmed the feasibility and flexibility of the digital control system for LLRF feedback system. Due to flexibility of DSP, different control algorithms are implemented and compared in the system. Particular attention is paid to the inherent pipeline delay problem associated with a digital control system and its effect and limitation on the overall system performance.

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.