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Last Name: Hartman
Full Name: S. Hartman
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25 papers
| title: | Status of UCLA IR FEL |
| format: | preprint |
| year: | 1996 |
| 21 authors: | | | | | | | | | | | | | | | | | | | | | |
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| title: | Surface Plasmon Enhanced Photoemission |
| format: | preprint |
| year: | March 18, 1993 |
| 1 author: | |
| abstract: | Photoemission studies have been carried out recently at the UCLA photo injector which cannot be explained by a straight forward application of the standard Fowler-Dubridge (1,2) theory for photoemission. This theory is used to calculate the contributions to the extracted current from field emission, thermal emission and the bulk photoemission process. The Fowler-Dubridge theory predicts a photoyield of 5x10(-6), which is two orders of magnitude less than that observed experimentally at 70 degrees incidence. However, if one assumes optical coupling to surface polasmons via surface roughness. Then one can calculate photoyields of 10(-4) as observed experimentally. Also, if the vector nature of the photoemission process is accounted for then one can calculated photoyields which agree with experimental data as a function of incident illumination angle, polarization angle, and laser wavelength. A few ideas are then set forth on photocathode preparation and materials so as to increase photoyield. |
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| title: | Space Charge Depleted Emission from a Damaged Photocathode |
| format: | preprint |
| year: | February 4, 1993 |
| 2 authors: | | |
| abstract: | There has been a long-standing question about the amount of charge extracted from the UCLA photo-injector as a function of incident laser energy (quantum efficiency). One expects the charge measured at the exit of the rf gun to be a linear function of applied laser energy to the photocathode. However, this is not the case fo the UCLA experiment thus far. However, the quantum efficiency data which measures the total extracted charge out of the rf gun as a function of laser energy, is very non linear. From zero nC to approximately 0.5nC the charge extracted follows a linar trend. From 0.5nC to 3nC the extracted charge falls off unexpectedly. Because of the highly dynamical behaviour of the electron beam near the cathode, a series of PARMELA simulations have been carried out to model the evolution of the beam in this region. These simulations give excellent agreement with the experimental data. Also, a simple model is put forth to explain the physical process which limits the emission of charge. |
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| title: | Space Charge Depleted Emission and Electron Beam Pulse Lengthening from a Damaged Photocathode |
| format: | preprint |
| year: | February 4, 1993 |
| 2 authors: | | |
| abstract: | There have been a long-standing questions about the amount of charge extracted from the UCLA photo-injector as a function of incident laser energy (quantum efficiency), and the observed pulse lengthening in time. One expects the charge measured at the exit of the rf gun to be a linear function of applied laser energy to the photocathode. However, this is not the case for the UCLA experiment thus far. The quantum efficiencey data, which measures the total extracted charge out of the rf gun as a function of laser energy, is very non linear. From zero nC to approximately 0.5 nC the charge extracted follows a linear trend. From 0.5 nC to 3 nC the extracted charge falls off unexpectedly. In addition, streak camera measurements indicate that the beam's temporal length expands considerably from that of the laser. Because of the highly dynamical behavior of the electron beam near the cathode, a series of PARMELA simulations have been carried out to model the evolution of the beam in this region, both in time structure and in maximum allowed charge. These simulations give excellent agreement with the experimental data. Also, a simple model is put forth to explain the physical process which limits the emission of charge. |
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| title: | Cold Test Measurements of the UCLA Photo-Injector |
| format: | preprint |
| year: | April 30, 1993 |
| 2 authors: | | |
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| title: | Space Charge Depleted Emission II: Further Exploration of the Model, and Comparison to Experiment |
| format: | preprint |
| year: | April 1, 1993 |
| 2 authors: | | |
| abstract: | In a previous note (CAA-TEC-NOTE-48), we presented theory and simulation of space charge suppression of the emitted beam charge in from gun. The simple theory, based on calculation of the longitudiinal electric field of the beam and its image charge at teh cathode, explained both the data and the simulation well. The saturation phenomenon of the emitted beam chartge was found to depend only on the applied electric field and the effective emission area of the photoelectrons. The purpose of this note is to further explore the applicability of this theoretical model, and to show that the 70 degree quantum efficiency data quantitatively support the model. |
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| title: | Initial measurements of the UCLA RF photoinjector |
| format: | conference procceeding |
| conference: | Conference on High Intensity Electron Sources |
| year: | 1994 |
| 14 authors: | | | | | | | | | | | | | | |
| abstract: | The 1.5 cell standing wave RF photoinjector has been operated for the past several months using a copper cathode. The photoinjector drive laser produces sub 2 ps pulses of UV ( lambda =256 nm) light with up to 200 mu J/pulse which generates up to 3 nC of charge, The emittance of the photoinjector was measured as a function of charge, RF launching phase, and peak accelerating field. Also, the quantum efficiency and pulse lengths of the laser beam and the electron beam were measured. (15 References). |
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| title: | Initial measurements of the UCLA RF photoinjector |
| format: | conference procceeding |
| conference: | 15th International Free Electron Laser Conference |
| year: | 1994 |
| 2 authors: | | |
| abstract: | An RF photocathode gun which, along with a compact linac, forms the injection system for a planned 10 mm free-electron laser amplifier experiment, has been commissioned in the Particle Beam Physics Laboratory at UCLA. This high-gradient gun, based on the Brookhaven design, has emitted several picosecond, [right angle bracket]100 A electron beams of up to 4 MeV in energy. These beams have been characterized by a variety of diagnostics. The quantum efficiency of the copper cathode used has been measured at normal incidence, and at 70 degrees incidence, where the polarization dependence was also examined. Limits on laser intensity due to surface damage, and to longitudinal space charge suppression of photoemission have been explored. The energy and energy spread of the beam were characterized using a dipole spectrometer, while the time structure was examined using a picosecond resolution streak camera. Both energy spread and pulse length were found to be adversely affected by longitundinal space charge forces. The emittance of the beam was measured using the pepper pot Technique, and its dependence on space charge and rf phase were found. The impact of these results on imporving the design and operation of high brightness photoinjectors is discussed, in particular with respect to SASE FEL amplifiers such as the UCLA 10mm FEL, and the proposed SLAC X-ray FEL. |
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| title: | Experimental demonstration of plasma lens focusing |
| format: | conference procceeding |
| conference: | 1993 Particle Accelerator Conference |
| year: | 1993 |
| 7 authors: | | | | | | | |
| abstract: | The magnetic self-focusing of a relativistic electron beam propagating through a plasma is demonstrated. The plasma which is produced by an RF discharge in a glass tube with no externally applied magnetic field focuses a 3.5 MeV, 25 ps (FWHM) long electron beam from an initial size of 2.5 mm (FWHM) to about 0.5 mm (FWHM) at a focal length of 18 cm. (7 References). |
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| title: | The plane wave transformer linac development at UCLA |
| format: | conference procceeding |
| conference: | 1993 Particle Accelerator Conference |
| year: | 1993 |
| 3 authors: | | | |
| abstract: | A plane wave transformer linac (PWT), offering advantages of high efficiency, compactness, fabrication simplicity and cost, is being developed at UCLA. The PWT prototype at UCLA is an 8-cell, pi -mode, S-band standing-wave linac. To fully understand its physical properties, numerical modeling of the PWT prototype has been carried out by using the 3-D code MAFIA. A microwave test-stand with a network analyzer has also been set up to test these properties. In this paper. We present the important physical features, such as mode structures, dispersion curves, wake field, from the computation and/or the experiment. The measurements show good agreement with the numerical computation. (7 References). |
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| title: | BPM Revisit: Summary of the Previous Test Results |
| format: | preprint |
| year: | 1993 |
| 5 authors: | | | | | |
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| title: | Quantum efficiency measurements of a copper photocathode in an RF electron gun |
| format: | conference procceeding |
| conference: | 1993 Particle Accelerator Conference |
| year: | 1993 |
| 8 authors: | | | | | | | | |
| abstract: | A 4.5 MeV photocathode RF gun has been commissioned at UCLA. A photo-injector drive laser produces sub 2 ps pulses of UV ( lambda =266 nm) light with up to 200 mu J/pulse, and illuminates a copper cathode. The photoelectrons are accelerated to an energy of 3.5 MeV within the gun. The electron beam charge is measured as a function of laser energy using an integrating current transformer (ICT). We present measurements of quantum efficiency as a function of laser polarization for injection angles of 2 degrees and 70 degrees with respect to the cathode normal. At 70 degrees incidence a 50% enhancement in quantum efficiency (>10(-4)) is observed for p-polarized light over s-polarized light. (7 References). |
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| title: | Streak camera measurements of electron bunch length from a copper photocathode in an RF gun |
| format: | conference procceeding |
| conference: | 1993 Particle Accelerator Conference |
| year: | 1993 |
| 8 authors: | | | | | | | | |
| abstract: | Short laser pulses (sub 2 ps) of UV ( lambda =266 nm) light with 200 mu J/pulse are used to produce electrons from a copper cathode in an RF gun. The electron bunch length is measured by streaking the Cerenkov radiation ( lambda =530 nm) from a thin (250 mu m) fused silica etalon. Streaks for both 0 degrees and 70 degrees laser incidence angles with respect to the cathode normal are presented with a temporal resolution of 3.6 ps. The shortest electron bunch length measured was 9 ps. (3 References). |
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| title: | RF Focusing Effects and Multi-bunch Beam Breakup in Superconducting Linear Colliders |
| format: | conference procceeding |
| conference: | 1993 Particle Accelerator Conference |
| year: | 1993 |
| 3 authors: | | | |
| abstract: | A high gradient standing wave linear accelerator provides axisymmetric transverse focusing due to the presence of strong alternating gradient transverse electromagnetic fields arising from the backward rf wave. This effect is second order in both the field amplitude and in gamma(-1), so it is of importance only for high gradient, relatively low energy beams. The purpose of the present analysis is to examine the effect of this focusing on multi-bunch beam breakup in a superconducting linear collider, which has both a high accelerating gradient and long bunch train. As an interesting test case, we discuss the beam breakup problem in the TESLA test bed at DESY. |
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| title: | MAFIA Studies of the Plane Wave Tranformer Linac |
| format: | preprint |
| year: | Januray 6, 1992 |
| 1 author: | |
| abstract: | In designing a high power rf-accelerator one needs to examine any filed disturbances created by asymmetries in teh structure. In the following we present a study of the disturbance of the modes using the three-dimensional frequency domain solver MAFIA[1]. We pay particular attention to perturbations in the accelerating mode and also to higher order modes which can be excited by the electron beam itself. These perturbations in the accelerating mode and off axis kicks due to the higher order modes give rise to emittance growth. Some ideas of how to redesign the support structure in order to iminimze accelerating mode perturbation effects and also of how to damp out unwanted higher order modes are discussed. |
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| title: | The UCLA IR FEL project |
| format: | conference procceeding |
| conference: | 14th International Free Electron Laser Conference |
| year: | 1993 |
| 21 authors: | | | | | | | | | | | | | | | | | | | | | |
| abstract: | A 10.6 mu m free electron laser (FEL) operating in the high gain regime is under construction at UCLA. FEL physics significant to future short wavelength operation is emphasized including optical guiding, superradiance, saturation and self-amplified spontaneous emission (SASE). A 5 MeV RF photocathode gun illuminated by a UV laser supplies a high brightness electron beam which is injected into a plane wave transformer (PWT) linac for acceleration to 20 MeV. Recent measurements of the gun emittance as well as quantum efficiency are presented. The undulator is of a modified hybrid design producing approximately 7.5 kG peak field on axis with 5 mm gap spacing and 1.5 cm pole period. Simulation results which include three-dimensional effects are furnished. The present status and future plans of the project are summarized. (16 References). |
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| title: | Photoelectron beams from the UCLA RF gun |
| format: | conference procceeding |
| conference: | 3rd Advanced Accelerator Concepts Workshop |
| year: | 1993 |
| 12 authors: | | | | | | | | | | | | |
| abstract: | A high brightness, low emittance photocathode rf gun is starting operation at UCLA as an injector to a 20 MeV linac. This linac will initially be used to drive FELs, plasma wakefield accelerators, and to test plasma lenses. The gun is a 1 1/2 cell pi-mode standing wave structure running at 2.856 GHz, and has a copper photocathode. In the initial commissioning of the gun, photoelectron beams of up to 2.5 nC at 4.5 MeV have been produced. We report on the current status of the system, experimental data taken with 50 ps UV laser pulses, and plans for the future. |
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| title: | Status of RF photoinjector and PWT linac at UCLA |
| format: | conference procceeding |
| conference: | 1992 Linear Accelerator Conference |
| year: | 1992 |
| 12 authors: | | | | | | | | | | | | |
| abstract: | The authors report the present status of RF photoinjector and plane wave transformer (PWT) linac for the production of 20 MeV/c electron beam. The photoinjector is a 1/sup 1///sub 2/ cell pi -mode standing wave structure operating at 2.856 GHz with photoelectrons generated on a copper cathode by 4 ps long laser pulse at 206 nm. Measurements of the beam of photoelectrons are underway at various experimental parameters. The PWT has been tested at low power to investigate its mode structure. An overview of the system, latest data, and future directions are presented. (5 References). |
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| title: | PARMELA simulation of the U.C.L.A. compact linac |
| format: | preprint |
| year: | February 11, 1991 |
| 1 author: | |
| abstract: | The compact laser-driven R.F. linac for the SATURNUS project has been modeled using the particle dynamics simulation code PARMELA....etc. |
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| title: | Solenoid Field Measurements |
| format: | tech note |
| year: | April 1, 1991 |
| 1 author: | |
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| title: | Photocathode driven linac at UCLA for FEL and plasma wakefield acceleration experiments |
| format: | conference procceeding |
| conference: | 1991 Particle Accelerator Conference |
| year: | 1991 |
| 17 authors: | | | | | | | | | | | | | | | | | |
| abstract: | The UCLA compact 20-MeV/c electron linear accelerators is designed to produce a single electron bunch with a peak current of 200 A, an RMS energy spread of 0.2% or less, and a short 1.2-ps RMS pulse duration. The linac is also designed to minimize emittance growth down the beamline so as to obtain emittances on the order of 8 pi mm-mrad in the experimental region. The linac will feed two beamlines, the first will run straight into the undulator for FEL experiments while the second will be used for diagnostics, longitudinal bunch compression and other electron beam experiments. A description is given of the considerations that went into the design of the accelerating structures and the transport to the experimental areas. (8 References). |
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| title: | PARMELA Simulation of the SATURNUS Compact 20 MeV linac |
| format: | preprint |
| year: | 11-11-91 |
| 2 authors: | | |
| abstract: | The R.F. photoinjector and Plane WAve Transformer have been modeled in the past 1. The results of these simulations have shown that ther eis an optimum parameter space to operate in. Optimum meaning parameters which give a picosecond, low emittance and low energy spread beam. |
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| title: | 1.5 Cell R.F. Gun Simulation with coil and Space Charge U.C.L.A. IRFEL Note H.3 |
| format: | preprint |
| year: | January 9, 1990 |
| 1 author: | |
| abstract: | Simulations were done to explore the effect of placing a coil along the beamline to minimize the divergent lense effect of the exit of the last cell of the gun....etc. |
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| title: | The UCLA IR-FEL |
| format: | conference procceeding |
| conference: | 1990 IEEE International Conference on Plasma Science |
| year: | 1990 |
| 11 authors: | | | | | | | | | | | |
| abstract: | A high-brightness RF gun operating at 2.856 GHz is being used as the injector for an IR-FEL (free-electron laser) experiment. The RF gun consists of a disk loaded, 1.5 cell cavity and a photocathode in the end wall of the cavity. A strong electric field within the half cell near the photocathode quickly accelerates the electrons, thus preventing emittance blow-up. The photocathode is to be driven by a Nd:YAG laser system producing a 10-ps pulse. The power for the RF gun will be supplied by a 25-MW klystron. The RF power is coupled to the cavity via an S-band waveguide attached to the sidewall of the gun. The TE_10 mode excited in the waveguide strongly couples to the pi-mode with the RF gun while the zero mode is suppressed. (0 References). |
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| title: | 3.5 Cell R.F. Gun Simulation with Spacecharge and Focusing Coil U.C.L.A. IRFEL Note H.1 |
| format: | preprint |
| year: | November 9, 1989 |
| 1 author: | |
| abstract: | Using the particle simulation code PARMELA, I calculated the emittance values for a three and one half cell gun. Since the beam suffers a R.F. defocusing at the exit of the last cell a solenoidal focusing element was added to cancel the divergence effect. |
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