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Title: Microwave generation for magnetic fusion energy applications

Abstract

This work strives to develop high average power FELs at voltages below I MV allowing for smaller and less costly power supplies. To achieve operation of an FEL with 100 GHZ [approx lt] f [approx lt] 150 GHz and with relatively modest voltage, we have been investigating the use of small period ([lambda][sub [omega]] [approximately] 1 cm) planar wiggler magnets together with sheet electron beams. The sheet beam geometry allows for an FEL interaction region in the form of a narrow slit with high wiggler field at the center plane where the electrons are concentrated. The total current and power may then be increased without making current density excessive by increasing the wide dimension of the sheet beam. Sheet beam FEL design parameters for both a Proof-of-Principle (PoP) FEL experiment, which is current in progress, and an ITER relevant FEL design are shown. A central issue in the sheet beam FEL concept is propagation of the beam through the interaction region without excessive interception by the walls. In section 2 below we describe a successful experimental demonstration of sheet beam propagation through a 56 period uniform wiggler. Cold testing and initial hot test operation of the (PoP) FEL amplifier aremore » also described. Finally, we present a theoretical investigation of the bandwidth of an FEL amplifier with a tapered wiggler operating in saturation is described.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Maryland Univ., College Park, MD (United States)
Sponsoring Org.:
USDOE; USDOE, Washington, DC (United States)
OSTI Identifier:
6544424
Report Number(s):
DOE/ER/52147-6
ON: DE93013864
DOE Contract Number:
FG05-87ER52147
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Fu; ECR HEATING; FREE ELECTRON LASERS; MICROWAVE AMPLIFIERS; PROGRESS REPORT; RESEARCH PROGRAMS; WIGGLER MAGNETS; AMPLIFIERS; DOCUMENT TYPES; ELECTRICAL EQUIPMENT; ELECTROMAGNETS; ELECTRONIC EQUIPMENT; EQUIPMENT; HEATING; HIGH-FREQUENCY HEATING; LASERS; MAGNETS; MICROWAVE EQUIPMENT; PLASMA HEATING; 700460* - Fusion Technology- Heating & Fueling Systems; Fuels- (1992)

Citation Formats

Antonsen, T.M. Jr., Destler, W.W., Granatstein, V.L., and Levush, B. Microwave generation for magnetic fusion energy applications. United States: N. p., 1993. Web. doi:10.2172/6544424.
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Antonsen, T.M. Jr., Destler, W.W., Granatstein, V.L., & Levush, B. Microwave generation for magnetic fusion energy applications. United States. doi:10.2172/6544424.
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Antonsen, T.M. Jr., Destler, W.W., Granatstein, V.L., and Levush, B. Mon . "Microwave generation for magnetic fusion energy applications". United States. doi:10.2172/6544424. https://www.osti.gov/servlets/purl/6544424.
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@article{osti_6544424,
title = {Microwave generation for magnetic fusion energy applications},
author = {Antonsen, T.M. Jr. and Destler, W.W. and Granatstein, V.L. and Levush, B.},
abstractNote = {This work strives to develop high average power FELs at voltages below I MV allowing for smaller and less costly power supplies. To achieve operation of an FEL with 100 GHZ [approx lt] f [approx lt] 150 GHz and with relatively modest voltage, we have been investigating the use of small period ([lambda][sub [omega]] [approximately] 1 cm) planar wiggler magnets together with sheet electron beams. The sheet beam geometry allows for an FEL interaction region in the form of a narrow slit with high wiggler field at the center plane where the electrons are concentrated. The total current and power may then be increased without making current density excessive by increasing the wide dimension of the sheet beam. Sheet beam FEL design parameters for both a Proof-of-Principle (PoP) FEL experiment, which is current in progress, and an ITER relevant FEL design are shown. A central issue in the sheet beam FEL concept is propagation of the beam through the interaction region without excessive interception by the walls. In section 2 below we describe a successful experimental demonstration of sheet beam propagation through a 56 period uniform wiggler. Cold testing and initial hot test operation of the (PoP) FEL amplifier are also described. Finally, we present a theoretical investigation of the bandwidth of an FEL amplifier with a tapered wiggler operating in saturation is described.},
doi = {10.2172/6544424},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Feb 01 00:00:00 EST 1993},
month = {Mon Feb 01 00:00:00 EST 1993}
}
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Technical Report:
View Technical Report (1.68 MB)
DOI:  10.2172/6544424
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  • A proof-of-principle short pulse ({approximately} 100 ns) experiment has successfully demonstrated operation of a sheet-beam FEL amplifier with output power of 250 kW at 86 Ghz and with 24 dB saturated gain and {approximately} 3% efficiency. Gain in the linear region was 30 dB. Measured performance parameters were in good agreement with predictions of a multi-mode, time dependence code. Also, a code has been developed to design depressed collectors which will enhance efficiency of ECRH sources (both FELs and gyrotrons). Extensive analytical and theoretical work in support of high power gyrotron development at Varian and MIT, and in support ofmore » ITER has been carried out. Specific studies are described. The effect of beam quality on the operation of the 145 GHz gyrotrons at MIT has been characterized using experimentally measured beam velocity distribution functions. The observed performance of these devices is consistent with a 10% RMS perpendicular velocity spread. An extensive study of mode competition in the 110 GHZ experiments at Varian and MIT has been carried out. Design criteria for the suppression of parasitic modes have been given for these experiments. The issues of mode competition and beam quality in the proposed 170 GHz megawatt gyrotrons for ITER have been investigated. Designs of cavities which eliminate unwanted modes have been made, and their sensitivity to beam quality studied. The constraints of lower power density and absence of mode competition coupled with the anticipated beam quality restrict efficiency. Efficiency can be improved by increasing the power density in the wall, improving beam quality, or perhaps by using a more advanced cavity. Studies of the causes of velocity spread in MIG guns have been initiated. Further, the effect of beam cavity misalignment on mode competition has been addressed.« less

  • ; ; ; ...
    This report details progress over the past year in the research program Free Electron Lasers with Short Period Wigglers.'' The work is performed jointly by the laboratory for Plasma Research and the Electrical Engineering Department of the University of Maryland and is funded by the US Department of Energy Office of Fusion Energy. The goal of the work is the development of an electron cyclotron resonance heating (ECRH) scheme for magnetic fusion plasmas such as the Compact Ignition Tokamak (CIT). Our approach is the development of a free electron laser using a sheet electron beam and a short period wigglermore » magnet. The specific requirements for the heating method include 10 to 30 MW of average power with pulse durations of several seconds to CW at a frequency near 300 GHz ({approximately}600 GHz) in the case of second harmonic (ECRH). Compatible with the experimental nature of the program, radiation frequency flexibility of 30% total bandwidth and 5% rapid dynamic ({approx lt}10 ms) bandwidth is desirable. As the source will eventually be applied to a reactor, priority is placed upon high system efficiency and reliability. Use of established technologies is encouraged where possible.« less

  • ; ; ; ...
    This progress report encompasses work on two separate projects, both related to developing sources for electron cyclotron resonance heating of magnetic fusion plasmas. The report is therefore divided into two parts as follows: Free electron laser with small period wigglers; and theory and modeling of high frequency, high power gryotron operation. Task A is experimental and eventually aims at developing continuously tunable, cw sources for ECRH with power per unit as high as 5 megawatts. Task B provides gryotron theory and modeling in support of the gryotron development programs at MIT and Varian.

  • ; ; ; ...
    This progress report encompasses work on three separate projects, all related to developing sources for electron cyclotron resonance heating of magnetic fusion plasmas. The report is therefore divided into three parts as follows: free electron lasers with small period wigglers; theory and modeling of high frequency, high power gyrotron operation; and depressed collectors for energy recovery in gyrotrons. Task A is experimental and eventually aims at developing CW sources for ECRH at frequencies as high as 600 GHz. Task B provides gyrotron theory and modeling in support of the gyrotron development programs at MIT and Varian. Task C was phasedmore » out in the current contract year but recent publications are included in this report in order to create a permanent record for the use of future researchers with interest in the topic of increasing gyrotron efficiency through the use of depressed collectors.« less

  • ; ; ; ...
    One of the main issues in the development of high power gyrotrons is the present discrepancy between the theoretically predicted efficiency and that observed in the experiments. Recent 140 GHz experiments by MIT employed three cavities; two of the cavities have different interaction lengths and the third cavity is a complex two-section cavity. In all cases, the maximum experimental efficiency is well below the theoretically predicted one. A better theoretical understanding of the causes of these discrepancies is essential to the scaling of gyrotrons to higher power and higher frequency. We continued our investigation with the objective of determining whethermore » mode competition and velocity spread in the electron beam could in some way be influencing the current result from the MIT 140 GHz gyrotrons experiments and determining to what extent these effects would influence operation of a 1 MW, 280 GHz, TE{sub 42,7} gyrotron as conceived at MIT. In our investigations, we used two models, the fixed axial field profile'' model and the self-consistent determination of the axial field profile'' model. With the fixed axial field profile'' model we studied the effect of thermal spread on the efficiency of gyrotrons operation, mode competition between unequally spaced modes, and we simulated the start up of the MIT 140 GHz gyrotrons. Our main conclusions from all these studies are discussed in this paper.« less