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Title |
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| MOPPH005 |
Three-Dimensional Theory for a Smith-Purcell Free-Electron Laser with Grating Sidewalls
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17 |
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- H. L. Andrews, C. A. Brau, J. D. Jarvis
Vanderbilt University, Nashville, TN
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We present an analytic theory for the operation of a Smith-Purcell free-electron laser in three dimensions that includes side walls bounding the grating. We allow the width of the electron beam and the width of the grating to vary independently, and require the walls be high compared with the wavelength of the evanescent wave. For both amplifier and oscillator operation, when the electron beam width is narrow compared to the grating, we find the system has a higher growth rate than that predicted by the two-dimensional theory*. Additionally, we find evidence of gain guiding in the system in this configuration. When the grating and electron beam widths are the same, we recover the results from the two-dimensional theory as expected.
* H. L. Andrews and C. A. Brau, Phys. Rev. ST Accel. Beams 7, 070701 (2004)
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| MOPPH047 |
Experimental Observation of the Evanescent Wave in a Smith-Purcell Free-Electron Laser
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83 |
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- H. L. Andrews, C. A. Brau, J. D. Jarvis
Vanderbilt University, Nashville, TN
- R. Durant, C. F. Guertin, T. H. Lowell, M. R. Mross, A. O'Donnell
Vermont Photonics, Bellows Falls, VT
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We present the first experimental observations of the evanescent wave in a Smith-Purcell free-electron laser (FEL). This wave, predicted by both theory and simulations*, has a wavelength longer than the Smith-Purcell radiation, can travel anti-parallel to the electron beam, and for sufficiently high current, will provide feedback to bunch the electron beam. This feedback is the basis of oscillator operation of the Smith-Purcell FEL. The wavelengths observed agree with theoretical predictions.
* Andrews and Brau, PRSTAB 7, 070701 (2004); Donohue and Gardelle, PRSTAB 8, 060702 (2005); Li, Yang, Imasaki and Park, PRSTAB 9, 040701 (2006); Kumar and Kim, PRE 73, 026501 (2006)
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| TUAAU02 |
Superlinear Current Dependence in a Grating-Based Tunable THz Source
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192 |
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- H. L. Andrews, C. A. Brau, J. D. Jarvis
Vanderbilt University, Nashville, TN
- R. Durant, C. F. Guertin, T. H. Lowell, M. R. Mross, A. O'Donnell
Vermont Photonics, Bellows Falls, VT
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Recent experiments have demonstrated a strongly superlinear dependence of Smith-Purcell radiation on electron-beam current, similar to previous observations*. This results in an increase of output power of up to 1000 times that expected from a linear current dependence, which makes the device a useful source of THz radiation. This behavior strongly suggests superradiant effects caused by bunching of the electron beam on length scales on the order of the optical wavelength**. However, the observed spectrum of emitted radiation remains unchanged over the entire current range. For this to be consistent with a superradiant mechanism, the bunching frequency must be smaller than the spectrometer resolution, which is on the order of 10 GHz. The magnitude of such bunching would increase with increasing current to account for the large power increase. The modulation might be caused by virtual-cathode oscillations or other electron-beam instabilities. To test this mechanism, we can look for peaks in the output radiation spectrum with a higher-resolution spectrometer or measure the GHz modulation on the electron beam directly.
* Urata, Goldstein, Kimmitt, Naumov, Platt and Walsh, PRL 80, 516 (1998)
** Andrews, Boulware, Brau and Jarvis, PRSTAB 8, 110702 (2005)
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Slides
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| TUPPH014 |
Transverse Emittance of Diamond Field-Emitter Arrays
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262 |
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- J. D. Jarvis, H. L. Andrews, C. A. Brau, B. K. Choi, J. L. Davidson, W. P. Kang, S. Raina, Y. M. Wong
Vanderbilt University, Nashville, Tennessee
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We present progress in transverse-emittance measurements of ungated diamond field-emitter arrays. Fine-pitch arrays are conditioned to provide uniform emission prior to emittance testing. The testing will be performed using a pepperpot technique with microlithographed silicon aperture arrays and a ZnO phosphor screen. Results will be compared to simulations performed in POISSON/GPT.
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| TUPPH016 |
Fabrication of Diamond Field-Emitter-Array Cathodes for Free-Electron Lasers
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269 |
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- J. D. Jarvis, H. L. Andrews, C. A. Brau, B. K. Choi, J. L. Davidson, W. P. Kang, S. Raina, Y. M. Wong
Vanderbilt University, Nashville, Tennessee
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Field-emitter arrays (FEAs) have several advantages as cathodes for free-electron lasers (FELs): they are rugged, require no laser driver, and generate little heat. We have developed two methods to fabricate diamond FEAs for FEL applications. In the first method, pyramids are formed on a Si substrate and sharpened by microlithography and then coated with CVD nanodiamond. The advantages of this approach are a rigid, planar Si substrate, and microelectronic type fabrication. Typically, tip radii on the order of hundreds of nanometers are formed on 20-μm pyramids. In the second method, all-diamond pyramids are formed by a mold-transfer process in which they become sharpened from an oxide layer in the mold process. The diamond array is then brazed to a Mo substrate and the Si mold removed. The advantage of this process is that the tips are sharper, with tip radii smaller than 10 nm formed on 10-μm pyramids. The fabrication techniques and the performance of these cathodes will be discussed and compared.
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| TUPPH028 |
Uniformity Conditioning of Diamond Field-Emitter Arrays
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302 |
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- J. D. Jarvis, H. L. Andrews, C. A. Brau, B. K. Choi, J. L. Davidson, W. P. Kang, S. Raina, Y. M. Wong
Vanderbilt University, Nashville, Tennessee
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We present results in the conditioning of diamond field-emitter arrays towards uniform emission. Post-fabrication conditioning procedures consisting of thermal annealing, gas exposure, and high field/emission operation have been examined. A high degree of emission uniformity was successfully achieved by thermal-assisted field evaporation of the diamond nanotips. This uniform emission was stable up to currents of 15 microamps per tip, at which point the phosphor anode began to degrade from the high input power density.
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| TUPPH034 |
Toward Energy-Spread Measurements of Diamond Field-Emitter Arrays
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317 |
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- J. D. Jarvis, H. L. Andrews, C. A. Brau, B. K. Choi, J. L. Davidson, W. P. Kang, S. Raina, C. L. Stewart, Y. M. Wong
Vanderbilt University, Nashville, Tennessee
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A high-resolution retardation energy analyzer is being developed to examine the energy spread of electron beams from diamond field-emitter arrays. Analyzer design was guided by previous work at UMER*, and simulations performed in SIMION. The anaylzer incorporates a cylindrical focusing electrode which, when properly tuned, gives millivolt resolution for multi-kilovolt beams. The analyzer is integrated into a low-energy cathode teststand, which allows arbitrary adjustment of the anode-cathode spacing and planarity during operation.
* Y. Cui, Y. Zou, A. Valfells, M. Reiser, M. Walter, I. Haber, R. A. Kishek, S. Bernal, and P. G. O’Shea, Rev. Sci. Instrum. 75, 8 (2004).
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| THAAU04 |
Development of Diamond Field-Emitter Arrays for Free-Electron Lasers
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477 |
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- J. D. Jarvis, H. L. Andrews, C. A. Brau, B. K. Choi, J. L. Davidson, W. P. Kang, S. Raina, Y. M. Wong
Vanderbilt University, Nashville, Tennessee
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We report recent advances in the development of diamond field-emitter arrays as a promising electron source for free-electron lasers. Both sparse and close-packed arrays have been produced using an inverse-mold transfer process. High-pitch arrays have been used in the development of conditioning techniques, which drive the emitters toward uniformity in a self-limiting fashion. Properties of these cathodes including I-V response, emitted energy spread, transverse emittance, temporal stability, and operational lifetime are being examined in two DC teststands. Highly uniform, stable emission current of 15 μA (DC) per tip has been achieved. The resulting high-input-power density destroyed the phosphor anode locally; therefore, higher currents could not be attempted. In an RF gun, pulsed picosecond operation will allow much higher peak currents, and back bombardment from sublimated anode material will not be present. The maximum DC-current densities observed scale to approximately 300 Amps per square centimeter for fine-pitch arrays, demonstrating great promise for use in free-electron lasers.
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