Curt Hovater

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.

JLAB high efficiency klystron baseline design for 12 GeV upgrade

A computer design of a 13.5 kW, 1497 MHz, CW type, 55% efficiency, 0.8 /spl mu/P beam perveance, /spl sim/40 dB gain, 5-cavity klystron has been developed for the JLAB 12 GeV Upgrade project. The design uses TRICOMP codes to simulate the gun, modulating anode section, solenoid focus channel and beam dump. The klystron tube was designed by JPNDISK (1D) code initially and then optimized by MASK (2D) code for the baseline parameters. All of these codes have been bench marked by JLAB 5 kW operational klystrons. The details of scaling laws, design parameters and the simulation results are presented.

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.

A DSP-based beam current monitoring system for machine protection using adaptive filtering

The CEBAF accelerator at Jefferson Lab is currently using an analog beam current monitoring (BCM) system for its machine protection system (MPS), which has a loss accuracy of 2 /spl mu/A. Recent burn-through simulations predict catastrophic beam line component failures below 1 /spl mu/A of loss, resulting in a blind spot for the MPS. The revised MPS requirements target an ultimate beam loss accuracy of 250 nA. A new beam current monitoring system has been developed which utilizes modern digital receiver technology and digital signal processing concepts. The receiver employs a direct-digital down converter integrated circuit, mated with a Jefferson Lab digital signal processor VME card. Adaptive filtering is used to take advantage of current-dependent burn through rates. Benefits of such a system include elimination of DC offsets, generic algorithm development, extensive filter options, and interfaces to UNIX-based control systems.

Two Applications of Direct Digital Down Converters in Beam Diagnostics

The technologies of direct digital down converters, digital frequency synthesis, and digital signal processing are being used in many commercial applications. Because of this commercialization, the component costs are being reduced to the point where they are economically viable for large scale accelerator applications. This paper will discuss two applications of these technologies to beam diagnostics. In the first application the combination of direct digital frequency synthesis and direct digital down converters are coupled with digital signal processor technology in order to maintain the stable gain environment required for a multi-electrode beam position monitoring system. This is done by injecting a CW reference signal into the electronics as part of the front-end circuitry. In the second application direct digital down converters are used to provide a novel approach to the measurement of beam intensity using cavity current monitors. In this system a pair of reference signals are injected into the cavity through an auxiliary port. The beam current is then calculated as the ratio of the beam signal divided by the average of the magnitude of the two reference signals.

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.

A low-cost, NIST-traceable, high performance dielectric resonator master oscillator [for CEBAF]

The current CEBAF Master Oscillator (MO) uses a quartz-based 10 MHz reference to synthesize 70 MHz and 499 MHz, which are then distributed to each of the klystron galleries on site. Due to the specialised nature of CEBAFs MO requirements, it has been determined that an in-house design and fabrication would provide a cost-effective alternative to purchasing or modifying vendor equipment. A Global Positioning System (GPS) disciplined, Direct Digital Synthesis (DDS) based MO is proposed which incorporates low-cost consumer RF components, designed for cellular communications. A 499 MHz Dielectric Resonant Oscillator (DRO) Voltage Controlled Oscillator (VCO) is phase-locked to a GPS-disciplined 10 MHz reference, and micro-tuned via a DDS, in an effort to achieve the lowest phase noise possible.

An injection mode locked Ti-sapphire laser for synchronous photoinjection

The CEBAF 4 GeV accelerator has recently begun delivering spin-polarized electrons for nuclear physics experiments. Spin-polarized electrons are emitted from a GaAs photocathode that is illuminated with pulsed laser light from a diode laser system synchronized to the injector chopping frequency (499 MHz). The present diode laser system is compact, reliable and relatively maintenance-free; however, output power is limited to less than 500 mW. In an effort to obtain higher average power and thereby prolong the effective operating lifetime of the source, we have constructed an injection mode locked Ti-sapphire laser with picosecond pulsewidths and gigahertz repetition rates.