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Operating experience at CEBAF

Description: CEBAF, the Continuous Electron Beam Accelerator Facility, is a 5-pass, recirculating, superconducting rf linac designed to provide exceptional beam quality at 4 GeV up to 200 {mu}A CW. It is made up of an injector, two 400-MeV linacs, and 9 recirculation arcs having a total beamline length of more than 4.5 km. On Nov. 5, 1995, CEBAF delivered a 4 GeV, 25-{mu}A CW electron beam to the first of 3 experimental halls and the experimental physics program was started 10 days later. Accelerator availability during the first month of the experimental run exceeded 75%. Beam properties measured in the experimental hall to date are a one sigma momentum spread of 5 x 10{sup -5} and an rms emittance of 0.2 nanometer-radians, better than design specification. CW beam has been provided from all 5 passes at 800 MeV intervals. Outstanding performance of the superconducting linacs suggests a machine energy upgrade to 6 GeV in the near term with eventual machine operation at 8-10 GeV. Results f rom commissioning and operations experience since the last conference are presented.
Date: July 1, 1996
Creator: Legg, R.
Partner: UNT Libraries Government Documents Department


Description: OAK-B135 The modulating voltages applied to the DIII-D gyrotrons are controlled by reference signals which are synthesized by arbitrary waveform generators.These generators allow ECH operators to pre-program reference waveforms consisting of ramps, flat tops, and various modulation shapes. This capability is independent of the DIII-D central timing and waveform facilities, which provides the ECH operators operational flexibility. The waveform generators include an amplitude modulation input, providing a means to control the pre-programmed waveform externally. This input is being used to allow the DIII-D plasma control system (PCS) to control gyrotron power in response top selected feedback signals. As the PCS control signal could potentially modulate the gyrotrons beyond operational limits or otherwise in a manner leading to recalcitrant rf generation, the control signal is conditioned so that its effect upon the ECH pre-programmed reference waveform is limited by conditions set by the ECH operators. The design of the circuitry which restricts the range over which the PCS control signal may modulate the reference waveform will be discussed. Test and DIII-D experimental results demonstrating the utility and effectiveness of gyrotron power modulated by the PCS will be presented.
Date: October 1, 2003
Partner: UNT Libraries Government Documents Department

The Use of a Solid State Analog Television Transmitter as a Superconducting Electron Gun Power Amplifier

Description: A solid state analog television transmitter designed for 200 MHz operation is being commissioned as a radio frequency power amplifier on the Wisconsin superconducting electron gun cavity. The amplifier consists of three separate radio frequency power combiner cabinets and one monitor and control cabinet. The transmitter employs rugged field effect transistors built into one kilowatt drawers that are individually hot swappable at maximum continuous power output. The total combined power of the transmitter system is 33 kW at 200 MHz, output through a standard coaxial transmission line. A low level radio frequency system is employed to digitally synthesize the 200 MHz signal and precisely control amplitude and phase.
Date: July 1, 2012
Creator: Kulpin, J. G.; Kleman, K. J. & Legg, R. A.
Partner: UNT Libraries Government Documents Department

Use of Near-Infrared Detector to Sense RF Antenna Heating

Description: The three antennas used for ion cyclotron heating (ICH) experiments on DIII-D have experienced localized heating of the Faraday shield rods during plasma operations which has resulted in some melting. This melting is of great concern not only because of the damage it does to the rf system's ability to deliver rf to the plasma, but because of its potential to contaminate the plasma during a shot and cast the experimental results from the shot into question. A real-time sensor to detect the temperature of the antennae during plasma operations is described. The sensor uses an avalanche photo diode (APD) with sensitivity from 0.4 to 1.0 {micro}m to monitor the temperature of the antennae. Calculations for the detector sensitivity based on Planck's law are compared with experimental results and detector data taken during plasma operations are presented.
Date: November 1, 1999
Creator: Legg, R.A.; Lee, R.L. & Baity, W.F.
Partner: UNT Libraries Government Documents Department

Location and correction of 60 hz in the CEBAF injector

Description: CEBAF produces a continuous electron beam with an emittance of 2-3 nm-rad. Transverse low frequency magnetic oscillations act to dilute this emittance. These fields are typically associated with AC line conductors. The CEBAF injector is approximately 40 m long. To locate the source(s) of the beam motion, measured offsets were back propagated along the beamline using the DIMAD model. Field measurements were then made at the calculated field source positions and correlated with the measured effects. Corrections and final beam measurements were made to verify the corrections. 2 refs., 4 tabs.
Date: August 1, 1996
Creator: Legg, R.; Douglas, D.; Krafft, G.A. & Saulter, Q.
Partner: UNT Libraries Government Documents Department

Precision intercomparison of beam current monitors at CEBAF

Description: The CEBAF accelerator delivers a CW electron beam at fundamental 1497 MHz, with average beam current up to 200 {mu}A. Accurate, stable nonintercepting beam current monitors are required for: setup/control, monitoring of beam current and beam losses for machine protection and personnel safety, and providing beam current information to experimental users. Fundamental frequency stainless steel RF cavities have been chosen for these beam current monitors. This paper reports on precision intercomparison between two such RF cavities, an Unser monitor, and two Faraday cups, all located in the injector area. At the low beam energy in the injector, it is straightforward to verify the high efficiency of the Faraday cups, and the Unser monitor included a wire through it to permit an absolute calibration. The cavity intensity monitors have proven capable of stable, high precision monitoring of the beam current.
Date: December 31, 1995
Creator: Kazimi, R.; Dunham, B.; Krafft, G.A.; Legg, r.; Liang, C.; Sinclair, C. et al.
Partner: UNT Libraries Government Documents Department

Automated path length and M{sub 56} measurements at Jefferson Lab

Description: Accurate measurement of path length and path length changes versus momentum (M{sub 56}) are critical for maintaining minimum beam energy spread in the CEBAF (Continuous Electron Beam Accelerator Facility) accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The relative path length for each circuit of the beam (1,256m) must be equal within 1.5 degrees of 1497 MHz RF phase. A relative path length measurement is made by measuring the relative phases of RF signals from a cavity that is separately excited for each pass of a 4.2 {micro}s pulsed beam. This method distinguishes the path length to less than 0.5 path length error. The development of a VME based automated measurement system for path length and M{sub 56} has contributed to faster machine setup time and has the potential for use as a feedback parameter for automated control.
Date: August 1, 1997
Creator: Hardy, D.; Tang, J.; Legg, R.; Tiefenback, M.; Crofford, M. & Krafft, G.A.
Partner: UNT Libraries Government Documents Department

Interactive troubleshooting guide for accelerators

Description: Modern accelerators face the challenge of supporting increased machine complexity and higher levels of utilization while relying on downsized maintenance and support groups. To improve system availability, reduce reliance on system experts and provide a systematic approach to problem solving, an interactive troubleshooting guide has been implemented for the CEBAF injector at Jefferson lab. This guide, which uses a tree structure with appropriate hypertext links, traces problems from a set of symptoms, through a series of diagnostic tests, to a specific corrective action. This guide is used in conjunction with an active parameter monitoring system that is part of the EPICS control system tool kit. The monitoring system generates operator alarms when injector RF or magnet parameters fall outside pre-set windows. Operators receive immediate feedback when injector components vary from their standard values and the troubleshooting guide provides a systematic means to identify and ultimately correct the problem.
Date: December 1, 1997
Creator: Legg, R.; Dunham, B.; Higgins, S.; Kazimi, R.; Kehne, D. & Oren, T.
Partner: UNT Libraries Government Documents Department

An emittance measurement system for a wide range of bunch charges

Description: As a part of the emittance measurements planned for the FEL injector at the Thomas Jefferson National Accelerator Facility (Jefferson Lab), the authors have developed an emittance measurement system that covers the wide dynamic range of bunch charges necessary to fully characterize the high-DC-voltage photocathode gun. The measurements are carried out with a variant of the classical two-slit method using a slit to sample the beam in conjunction with a wire scanner to measure the transmitted beam profile. The use of commercial, ultra-low noise picoammeters makes it possible to cover the wide range of desired bunch charges, with the actual measurements made over the range of 0.25 pC to 125 pC. The entire system, including its integration into the EPICS control system, is discussed.
Date: June 1, 1997
Creator: Dunham, B.; Engwall, D.; Hofler, A.; Keesee, M. & Legg, R.
Partner: UNT Libraries Government Documents Department

The Upgrade of the DIII-D GHz ECH System to 6 MW

Description: ECH power has proven capabilities to both heat and drive current in energetic plasmas. Recent developments in high power sources have made the use of these capabilities in energetic plasmas feasible. For the second phase of ECH power on DIII-D, there will be three 1 MW sources added to the existing 3 MW for a total generated power of 6 MW. The upgrade is based on the use of single disc CVD (chemical vapor deposition) diamond windows on 1 MW gyrotrons developed by CPI. AU gyrotrons are connected to the tokamak by low-loss-windowless evacuated transmission lines using circular corrugated waveguide for propagation in the HE{sub 11} mode. Each waveguide system incorporates a two-mirror launcher which can steer the rf beam poloidally from the center to the outer edge of the plasma and toroidally for either co- or counter-current drive. The total system overview and integration with existing systems will be discussed along with the new aspects of the upgrade from building modifications to the new launchers. Much of the upgrade is comprised of existing designs, which will need only slight modifications, while some components have required new designs because of longer pulse lengths.
Date: November 1, 1999
Creator: Cary, W.P.; Callis, R.W.; Lohr, J.M.; Ponce, D. & Legg, R.A.
Partner: UNT Libraries Government Documents Department

Experimental results from a DC photocathode electron gun for an IR FEL

Description: A 350 keV DC photocathode gun capable of delivering the high-brightness CW electron beam necessary for Jefferson Lab`s infrared free-electron laser is described. The gun is to be used with a superconducting radiofrequency linac operating at 1.497 GHz and is mode-locked to the 40th subharmonic of the fundamental using a Nd:YLF drive laser. The gun provides 20--25 ps bunches at up to 135 pC/bunch. Experimental measurements of transverse and longitudinal beam properties are presented. Transverse emittance is measured using a slit-wire scanner emittance meter, and energy spread is measured using the slit and a spectrometer magnet. Longitudinal emittance is measured using a combination of sampling aperture, kicker cavity, slit and spectrometer. Measurements for bunch charges of 135 pC are described and compared with simulations.
Date: October 1, 1997
Creator: Kehne, D.; Engwall, D.; Legg, R. & Shinn, M.
Partner: UNT Libraries Government Documents Department


Description: The DIII-D National Fusion Facility at General Atomics is completing the upgrade of its electron cyclotron heating (ECH) capability from the previous 3 MW at 110 GHz to 6 MW of generated microwave power. An 8.4 MW modulator/regulator (M/R) power system has been designed and constructed. Surplus hardware that was acquired from the Lawrence Livermore National Laboratory (LLNL) Mirror Fusion Test Facility (MFTF program) was used as part of the design foundation. The power system, with a nominal output of -80 kV and 80 A, can supply a pair of gyrotrons with up to 10 second long pulses that may or may not be modulated. The modulator/regulator was designed about the BBC CKQ200-4 tetrode, which was the key component acquired from the LLNL program. In order to meet the performance goals of the program, substantial design modifications were needed to be made on the grid driver amplifier and the closed-loop feedback regulator circuits. Also, a newly designed crowbar switch system, featuring a high speed, thyratron-like triggered gas switch, was implemented. The modulator/regulator performance to date has been demonstrated as having <0.06% peak-to-peak ripple and square wave modulation of 50% amplitude at 2 kHz. The key features of the design of the power system and its performance will be presented in this paper.
Date: November 1, 2000
Creator: PRONKO, S.G.E.; DELAWARE, S.W.; HARRIS, T.E.; HOYT, D.; KELLMAN, D.H.; LEGG, R.A. et al.
Partner: UNT Libraries Government Documents Department


Description: A powerful microwave system operating at the second harmonic of the electron cyclotron frequency has been commissioned on the DIII-D tokamak. The primary mission of the microwave system is to permit current profile control leading to the improved performance of advanced tokamak operation in quasi-steady state. Initial performance tests and experiments on current drive both near and away from the tokamak axis and on transport have been performed.
Date: September 1, 2000
Partner: UNT Libraries Government Documents Department

The 110 GHz Gyrotron System on DIII-D: Gyrotron Tests and Physics Results

Description: The DIII-D tokamak has installed a system with three gyrotrons at the 1 MW level operating at 110 GHz. Physics experiments on electron cyclotron current drive, heating, and transport have been performed. Good efficiency has been achieved both for on-axis and off-axis current drive with relevance for control of the current density profile leading to advanced regimes of tokamak operation, although there is a difference between off-axis ECCD efficiency inside and outside the magnetic axis. Heating efficiency is excellent and electron temperatures up to 10 keV have been achieved. The gyrotron system is versatile, with poloidal scan and control of the polarization of the injected rf beam. Phase correcting mirrors form a Gaussian beam and focus it into the waveguide. Both perpendicular and oblique launch into the tokamak have been used. Three different gyrotron designs are installed and therefore unique problems specific to each have been encountered, including parasitic oscillations, mode hops during modulation and polarization control problems. Two of the gyrotrons suffered damage during operations, one due to filament failure and one due to a vacuum leak. The repairs and subsequent testing will be described. The transmission system uses evacuated, windowless waveguide and the three gyrotrons have output windows of three different materials. One gyrotron uses a diamond window and generates a Gaussian beam directly. The development of the system and specific tests and results from each of the gyrotrons will be presented. The DIII-D project has committed to an upgrade of the system, which will add three gyrotrons in the 1 MW class, all using diamond output windows, to permit operation at up to ten seconds per pulse at one megawatt output for each gyrotron.
Date: December 1, 1999
Creator: Lohr, J.; Calahan, P.; Callis, R.W.; Chu, T.S.; deGrassie, J.S.; Gorelov, I. et al.
Partner: UNT Libraries Government Documents Department

Real-Time Transverse Emittance and Phase-Space Monitor

Description: A real-time multislit [1] transverse-emittance monitor has been developed for diagnosing the space-charge-dominated beam in the 10MeV injection line of the FEL at Thomas Jefferson National Accelerator Facility (formerly CEBAF). It gives emittance, Twiss parameters, and phase-space contours (without any symmetry assumptions) at the update rate of 1Hz. It reduces measurement noise in real-time, and incorporates a special algorithm for constructing the phase-space matrix, which yields more accurate results by sweeping the beam across the slits. In this paper we will discuss issues relevant to the software design and implementation. Experimental results obtained from a 250keV photocathode gun will also be presented and compared with other methods and with PARMELA simulations.
Date: January 1, 1998
Creator: Jordan, K.; Song, J.; Piot, P.; Legg, R.; Kehne, D.; Li, R. et al.
Partner: UNT Libraries Government Documents Department

Design Alternatives for a Free Electron Laser Facility

Description: The University of Wisconsin-Madison is continuing design efforts for a vacuum ultraviolet/X-ray Free Electron Laser facility. The design incorporates seeding the FEL to provide fully coherent photon output at energies up to {approx}1 keV. The focus of the present work is to minimize the cost of the facility while preserving its performance. To achieve this we are exploring variations in the electron beam driver for the FEL, in undulator design, and in the seeding mechanism. Design optimizations and trade-offs between the various technologies and how they affect the FEL scientific program will be presented.
Date: July 1, 2012
Creator: Jacobs, K; Bosch, R A; Eisert, D; Fisher, M V; Green, M A; Keil, R G et al.
Partner: UNT Libraries Government Documents Department

First Lasing of the Jefferson Lab IR Demo FEL

Description: As reported previously [1], Jefferson Lab is building a free-electron laser capable of generating a continuous wave kilowatt laser beam. The driver-accelerator consists of a superconducting, energy-recovery accelerator. The initial stage of the program was to produce over 100 W of average power with no recirculation. In order to provide maximum gain the initial wavelength was chosen to be 5 mu-m and the initial beam energy was chosen to be 38.5 MeV. On June 17, 1998, the laser produced 155 Watts cw power at the laser output with a 98% reflective output coupler. On July 28th, 311 Watts cw power was obtained using a 90% reflective output coupler. A summary of the commissioning activities to date as well as some novel lasing results will be summarized in this paper. Present work is concentrated on optimizing lasing at 5 mu-m, obtaining lasing at 3 mu-m, and commissioning the recirculation transport in preparation for kilowatt lasing this fall.
Date: May 1, 1999
Creator: Benson, Stephen; Biallas, George; Bohn, Court; Douglas, David; Dylla, H.F.; Evans, R. et al.
Partner: UNT Libraries Government Documents Department