Tomasz Plawski

A digital self excited loop for accelerating cavity field control

We have developed a digital process that emulates an analog oscillator and ultimately a self excited loop (SEL) for field control. The SEL, in its analog form, has been used for many years for accelerating cavity field control. In essence the SEL uses the cavity as a resonant circuit - much like a resonant (tank) circuit is used to build an oscillator. An oscillating resonant circuit can be forced to oscillate at different, but close, frequencies to resonance by applying a phase shift in the feedback path. This allows the circuit to be phased-locked to a master reference, which is crucial for multiple cavity accelerators. For phase and amplitude control the SEL must be forced to the master reference frequency, and feedback provided for in both dimensions. The novelty of this design is in the way digital signal processing (DSP) is structured to emulate an analog system. While the digital signal processing elements are not new, to our knowledge this is the first time that the digital SEL concept has been designed and demonstrated. This paper reports on the progress of the design and implementation of the digital SEL for field control of superconducting accelerating cavities.

Overview of SNS Cryomodule Performance

Thomas Jefferson National Accelerating Facility (Jefferson Lab) has completed production of 24 Superconducting Radio Frequency (SRF) cryomodules for the Spallation Neutron Source (SNS) superconducting linac. This includes one medium-β (0.61) prototype, eleven medium-β and twelve high-β (0.81) production cryomodules. Nine medium-β cryomodules as well as two high-β cryomodules have undergone complete operational performance testing in the Cryomodule Test Facility at Jefferson Lab. The set of tests includes measurements of maximum gradient, unloaded Q (Q0), microphonics, and response to Lorentz forces. The Qext’s of the various couplers are measured and the behavior of the higher order mode couplers is examined. The mechanical and piezo tuners are also characterized. The results of these performance tests will be discussed in this paper.

CEBAF new digital LLRF system extended functionality

The new digital LLRF system for the CEBAF 12GeV accelerator will perform a variety of tasks, beyond field control [1]. In this paper we present the superconducting cavity resonance control system designed to minimize RF power during gradient ramp and to minimize RF power during steady state operation. Based on the calculated detuning angle, which represents the difference between reference and cavity resonance frequency, the cavity length will be adjusted with a mechanical tuner. The tuner has two mechanical driving devices, a stepper motor and a piezo-tuner, to yield a combination of coarse and fine control. Although LLRF piezo processing speed can achieve 10 kHz bandwidth, only 10 Hz speed is needed for 12 GeV upgrade. There will be a number of additional functions within the LLRF system; heater controls to maintain cryomodules heat load balance, ceramic window temperature monitoring, waveguide vacuum interlocks, ARC detector interlock and quench detection. The additional functions will be divided between the digital board, incorporating an Altera FPGA and an embedded EPICS IOC. This paper will also address hardware evolution and test results performed with different SC cavities.

System study using injection phase locked magnetron as an alternative source for superconducting radio frequency accelerator

As a drop-in replacement of Continuous Electron Beam Accelerator Facility (CEBAF) 5kW CW klystron system, a 1497MHz, high efficiency magnetron using injection phase lock [1] and slow amplitude variation using magnetic field trimming and anode voltage modulation has been studied systematically using MatLab/Simulink simulations. The magnetron model is based the characteristics of experiment and manufacture chart on a 2.45GHz cooker type CW magnetron. To achieve high performance of a superconducting radio frequency (SRF) acceleration cavity with an electron beam loading, the magnetrons low level radio frequency (LLRF) control has been studied in two lock loops. In the frequency lock loop, the characterized anode V-I curve, output power (the tube electronic efficiency) and frequency dependence to the anode current (pushing by Vaughan model) and the Rieke diagram (frequency pulling by the reactive load) are simulated. The magnetic field B and anode voltage V in Hartree condition are satisfied and the effect of filament heater power to the frequency lock is also included. In the phase lock loop, the Adler equation governing injection phase stability is included in this study. The control of the magnet trim-coil power-supply and of the anode voltage modulation-switching power-supply has been also simulated to achieve the amplitude modulation. The result of linear responses to the amplitude and phase of SRF cavity will be presented in this paper. The requirement of LLRF control will be given by this result.

Simulation and experimental studies of a 2.45GHz magnetron source for an SRF cavity with field amplitude and phase controls

As a drop-in replacement for the CEBAF 8kW CW klystron system, A 1497 MHz, CW type high- efficiency magnetron using injection phase lock with amplitude variation is attractive. Amplitude control using magnetic field trimming and anode voltage modulation has been studied using MatLab/Simulink simulations [1]. Along with a magnetron we are planning to use an FPGA based digital LLRF system, which allows to apply various types of control algorithms in order to achieve the required accelerating field stability. Since the 1497 MHz magnetron has not yet been built, the proof of principle measurements of a typical 2450MHz commercial magnetron are made to characterize the anode I–V curve, output power (the tube electronic efficiency), frequency dependence on the anode current (frequency pushing) and the Rieke diagram (frequency pulling by the reactive load). In the Simulink simulation, extension of the Adler equation governing injection phase stability by Chens model is included. The results of the linear response to the amplitude and phase control of an SRF cavity, and the specification of the new LLRF control chassis for both 2450 and 1497MHz systems will be presented in this paper.

New RF control system for the 12 GeV energy upgrade of the CEBAF accelerator at Jefferson Lab

The CEBAF Accelerator at Jefferson Lab presently consists of 50 MeV injector, two anti-parallel superconducting linacs and two arcs for transporting beam between the linacs. By the mid-1990s, the accelerator was providing electrons up to 5.75 GeV. The 12 GeV Upgrade is a major accelerator project aimed at doubling the energy by adding 10 (five per linac) new high gradient cryomodules, each providing 100 MV of field. The new cryomodule will consist of eight 7-cell superconducting cavities operating at an average accelerating gradient of 19.5 mv/m and with an external Q = 3.2 x 107 The high gradient, very high quality factor and large Lorentz detuning coefficient (KL up to 4) pose significant challenges beyond what the present analog low level RF (LLRF) control systems can handle reliably; therefore, a new digital LLRF control system has been developed. The main highlights of the new RF Control system include: a RF frontend with low temperature drift and good linearity, a large FPGA platform for digital signal processing, an EPICS Input Output Computer (IOC) using a PC-104 and a digital Self Exciting Loop (SEL) based algorithm. This paper provides an overview of the 6 GeV and 12 GeV Upgrade CEBAF machines, a summary of sources of cavity field variation that drive LLRF system performance, and discusses recent developments and progress in Jefferson Labs new LLRF system design.