A third harmonic electron cyclotron emission diagnostic using ultrawide bandwidth ( = 40 GHz) heterodyne receivers centered on 120 GHz with 14 channels per radial view is describecj for localized, long wavelength (5 % X s 50 cm), fast time response ( = 1 ps) fluctuation studies in the PBX-M tokamak. The optically gray emission signal will have a y ie/ne + (3/0)Te/Te dependence on temperature and density fluctuations where y S 1 and 1 _ P: 3 depending on local optical depth. Electron temperature fluctuation sensitivity is estimated to be 0.2 % se &apos; Te/Te s 2.9 % depending on local optical depth and fluctuation frequency in the 0.1-1 MHz range. Spatial resolution of approximately 3 cm radially and 5 cm vertically are estimated for 2 keV plasmas with low suprathermal electron emission

Cohn, D.R.

Kesner, J.

Lane, B.

Luckhardt, S.

Machuzak, J.S.

Rhee, D.

Woskov, P.P.

P. P. Woskov

J. Kesner

B. Lane

S. Luckhardt

J. S. Machuzak

D. Y. Rhee

D. R. Cohn

CiteSeerX

Design of a 3rd Harmonic Electron Cyclotron Emission Diagnostic for Ballooning Mode Fluctuations in PBX-M

DSpace@MIT

PFC/JA-90-21Design of a 3rd Harmonic Electron CyclotronEmission Diagnostic for Ballooning ModeFluctuations in PBX-MP. P. Woskov, J. Kesner, B. Lane, S. Luckhardt,J. S. Machuzak, D. Y. Rhee, and D. R. CohnPlasma Fusion Center*Massachusetts Institute of TechnologyCambridge, Massachusetts 02139May 4,1990*Supported by U. S. Department of Energy Contract DE-AC02-78ET51013.Accepted by Review of Scientific Instruments.-1-AbstrwtA third harmonic electron cyclotron emission diagnostic using ultrawidebandwidth (= 40 GHz) heterodyne receivers centered on 120 GHz with 14 channelsper radial view is describecj for localized, long wavelength (5 % X s 50 cm), fasttime response (= 1 ps) fluctuation studies in the PBX-M tokamak. The opticallygray emission signal will have a y ie/ne + (3/0)Te/Te dependence on temperatureand density fluctuations where y S 1 and 1 _ P: 3 depending on local opticaldepth. Electron temperature fluctuation sensitivity is estimated to be 0.2% se' Te/Tes 2.9% depending on local optical depth and fluctuation frequency in the 0.1 -1MHz range. Spatial resolution of approximately 3 cm radially and 5 cm verticallyare estimated for 2 keV plasmas with low suprathermal electron emission.IntroductionLocalized temperature and density fluctuation diagnostics in a fluctuationwavelength range ( 0.1 < k < 1 cm-1 ) are likely to play a key role in advancing ourunderstanding of confinement in magnetic fusion devices. This fluctuationwavelength range is currently diagnosed with difficulty in tokamaks. However, itis an important range because density fluctuations due to turbulence and driftwaves are known to peak here [1]. In addition, ballooning modes which may bean important instability that could limit high beta and second stability tokamakoperation [2], fall in this range. Very little experimental data exists on ballooningmodes.-2-Conventional microwave to infrared scattering density fluctuationdiagnostics cannot provide spatially localized or wavenumber resolvedinformation in this wavelength range because the Bragg condition4nk= sin- (1)where k is the fluctuation wavenumber, X0 is the diagnostic wavelength and 0 isthe scattering angle, cannot be satisfied for diagnostic wavelengths that canaccess the plasma unless the scattering angle is made so small that thescattering volume is essentially chord averaged.Infrared forward scattering in the Raman-Nath regime [3] by eitherimaging the forward diffracted signal [4] or by phase contrast imaging [5] hasbeen shown to provide k resolution in this fluctuation range. Use of crossedbeams can make possible non-chord averaged spatially resolved measurements.However, this is a rather difficult diagnostic to implement, requiring throughputaccess across the entire plasma cross section being diagnosed and the opticalimaging of the signal on a many detector array.By comparison electron cyclotron emission (ECE) is a relativelysimple, passive (in contrast to active laser and particle beam techniques), andsingle view localized (in contrast to x-ray tomography) diagnostic of electrontemperature and its fluctuations when viewing an optically thick harmonic [6]. Itis sensitive to density fluctuations as well when viewing an optically thinharmonic. MHD modes can be observed by looking at either temperature ordensity fluctuations.-3-Analytical BasisPBX-M is ideally suited for ECE diagnostics because of its high aspect ratio(> 5). When the aspect ratio is equal to 5 as shown in Figure 1, the secondharmonic, as defined by 2c where OCc is the electron cyclotron resonancefrequency, is not overlapped across the entire plasma cross-section. In additionthe third harmonic, 3 0 c, is not overlapped for most of the plasma cross-sectionfrom the outside to over three-fourths of the distance toward the inside wall.Therefore the 1/R dependent emission frequencies for these harmonics can beuniquely related to radial location.PBX-M has as a goal to operate at fairly high densities approaching 1014cm-3 . To avoid cutoffs and refraction the PBX-M ECE fluctuation diagnostic mustmake use of the third harmonic. For a nominal central field of 1.5 Tesla, thiswould correspond to a frequency range of 105 to 140 GHz.The harmonic resonances plotted in Figure 1 assume only the presence ofthe vacuum toroidal field. With a plasma there can be additional poloidal andtoroidal field components that can add and subtract to the total field andconsequently affect the diagnostic's localization. In high beta experiments onPBX-M, these components will be much larger than in typical tokamakexperiments. However even an extreme case, of a poloidal field 20% of thevacuum toroidal field, would only increase the total field by 2% because the fieldsadd vectorially.The ECE intensity from a thermal plasma is given byIece = Ibb (1 - e-e) (2)whereO)2TeIbb = (3)8WC2-4-is the emission level of a blackbody at a temperature Te andC Ope R 22t.1 kTe )-1 (4)=2 cC (t-l)! 2mec2is the optical depth in a tokamak of harmonic t where R is the tokamak majorradial component. The form of the optical depth given here is that of Engelmannand Curatolo [71 which is valid for nonrelativistic electron temperaturesTe 5 8 keV.For PBX-M plasmas the optical depth for third harmonic emission will be T35 1, a region referred to as optically gray. In this case it can be shown that thefluctuating component of the emission will have a y ffe/ne + (3/) Te/Te dependencewhere y 5 1 and 1 s . s 3 are determined by the local optical depth. Thefluctuation signal will be strongly dependent on electron temperature with aweaker radial dependence on electron density. This should not pose a problem fora gross identification of ballooning modes, but for a detailed interpretation of theradial structure, electron and temperature profile data from other diagnosticsmust be used.Spatial resolution for a horizontally viewing diagnostic geometry will bedetermined in the radial direction by relativistic Doppler broadening given byAr = R 27t (5)For typical PBX-M parameters ( R = 1.6 m and T = 2 keV ) this corresponds to aradial resolution of 2.8 cm. In the vertical and toroidal dimensions the spatialresolution is diffraction limited which for a Gaussian beam is given byAh = 1.27 F# (6)-5-where F# =L/d is the plasma viewing optics f-number with L the focal length andd the clear aperture. At 100 GHz and with an F# of 14 this would correspond to aresolution of approximately 5 cm.Experimental DesignThe ECE diagnostic system will be composed of a single sideband ultrawidebandwidth (6 40 GHz) heterodyne receiver in the 100 to 140 GHz range, 14intermediate frequency (IF) channels using state of the art multiplexer filters, aquasi-optical antenna, and an overmoded circular TEii transmission line with aTE11 to HE11 mode converter at the antennna. The bandwidth of this fixedfrequency local oscillator (LO) system is sufficient to view the entire unoverlappedthird harmonic emission in PBX-M at a field of 1.5 Tesla.The quasi-optical antenna and transmission line system areillustrated in Figure 2. The corrugated horn will produce a Gaussianantenna beam which is focused by the teflon lens into the plasma through awedged quartz window. The HE11 corrugated horn mode will be convertedto a circular TE11 mode for transmission in a 12.7 mm diameter copperguide with a transition to WR-8 guide at the receiver. Miter bends will beused if necessary. A viewing dump [8] constructed of a suitable absorbingvacuum material such as macor, boron nitride, or silicon carbide will beused on the inner vacuum vessel wall. Transmission line losses areestimated to be -1.5 dB for a 10 m long line from the horn to receiver. Anadditional -1.2 dB loss is expected due to reflections at the lens and windowsurfaces.The receiver, shown in Figure 3, has a much larger bandwidth thanpreviously used in this type of system [9]. It will have a 100 GHz high pass filter-6-following the scalar feedhorn to prevent viewing ECE emission in the receiver'slower sideband. The RF signal will be downconverted in frequency from102-140 GHz to 2-40 GHz with a mixer having a 6.5 dB conversion loss and 100 GHzGunn diode LO. The mixer will have a -40 GHz instantaneous IF bandwidthwhich is possible with planar circuit technology and Schottky barrier beam leaddiodes (101. The resulting ultrawide IF bandwidth will be amplified by abroadband amplifier from 2-40 GHz which is just now becoming availablecommercially [11,12]. Monolithic microwave integrated circuit (MMIC) andmicrowave integrated circuit (MIC) technologies have improved to the point ofproviding workable low noise (-12 dB noise figure) and moderate gain (-10 dB)ultrawide bandwidth amplifiers. The resulting system noise temperatureassuming a 4 dB front end loss is estimated to be TS = 50,000 K or 4.3 eV at theplasma.The amplified signal will be split into 14 individual channnels from 2 to 40GHz with a combination of a diplexer and two 7 channel multiplexers [131. An off-the-shelf diplexer will split the IF bandwidth into two channels of 2-18 GHz and18-40 GHz. Additional amplification will be provided by commercially availableamplifiers for each of these frequency ranges [12] following the diplexer. Anotheroff-the-shelf multiplexer can easily be modified to provide 7 channels ofapproximately 2.3 GHz bandwidth which will range from 2 to 18 GHz. A 7channel multiplexer from 18-40 GHz with approximately 3 GHz bandwidths canalso be fabricated with suspended substrate stripline technology [13].The 18-40 GHz multiplexer is technically harder to design, andcorrespondingly higher in cost (factor of approximately 3) than lower frequencymultiplexers; however, an alternative receiver design based on two 0.1-18 GHz IF-7-receivers to cover a - 40 GHz bandwidth would cost approximately the same.Relative phase measurements between channels across the entire emissionspectrum for fluctuation wavenumber determination would also be more reliablyaccomplished with a receiver using only one mixer/LO combination. The output2.3 and 3 GHz multiplexer channel bands would be detected with square lawdetector -diodes and amplified with 30 dB of voltage gain before being digitized.The fluctuation level sensitivity of this instrument is given by the well-known result of radiometer theory which was experimentally verified for tokamakECE [14]rece 1Tlece=where Af is the channel bandwidth and t is the integration time. Maximizing thechannel bandwidth to correspond to the -2 GHz Doppler broadening at 2 keVminimizes the observable fluctuations without sacrificing spatial resolution. Theintegration time is limited by the flucutation frequencies of interest, which in thiscase for Ballooning modes are expected to fall in the 0.1 to 1 MHz range. Thecorresponding fluctuation level sensitivity therefore falls in the range f ece/Iee =0.9 - 2.9% = y ie/ne + (3/p)Te,Te for the 2.3 GHz channels. This would also be thesensitivity to electron temperature fluctuations if the plasma is optically thick (y =0, 0 = 3). However if the plasma is optically thin (y = P =1) and fie/ne = Te,Te thenthe observable fluctuation threshold would decrease to Te/Te = 0.2 - 0.7%.It is planned that two of these ultrawide bandwidth receivers be built. TheECE diagnostic will then simultaneously acquire temperature and densityfluctuations at 14 volumes along each of two horizontal chords; one equatorial-8-chord will provide information to the radial mode structure, while an off-equatorial chord will provide signals dependent on a combination of the radialand poloidal structure of the mode. The second chord could also be displacedtoroidally for sampling toroidal structure. Such a simultaneous kr, k0 , and kmeasurement capability has not been previously attempted by ECE fluctuationdiagnostics.The data acquisition electronics would include 100 kilosamples persecond digitizers for all 28 channels to economize on cost, and 2 or more fasttransient digitizers of at least 20 megasamples per second for fastfluctuation studies in select channels.References[1] P. C. Liewer, Measurements of microturbulence in tokamaksand comparisons with theories of turbulence, Nuclear Fusion,vol. 25, pp. 543-621, 1985.[2] M. J. Gerver, J. Kesner, and J. J. Ramos, Access to the secondstability region in a high-shear, low-aspect-ratio tokamak,Phys. Fluids, vol. 31, pp. 2674-2682, 1988 and referencestherein.[3] M. Born and E. Wolf, Principles of Optics, Pergamon, Oxford,1964.(4] D. E. Evans, E. J. Doyle, D. Frigione, M. vonHellermann, andA. Murdoch, Measurement of long wavelength turbulence in atokamak by extreme far forward scattering, Plasma Physics,vol. 25, pp. 617-640,1983.[5] H. Weisen, The phase contrast method as an imagingdiagnostic for plasma density fluctuations, Rev. Sci. Instrum.,vol. 59, pp. 1544-1549,1988.[6] D. A. Boyd, Plasma diagnostics using electron cyclotronemission, University of Maryland, Intern. Report PL 81-023,1980.-9-[7] F. Engelmann and M. Curatolo, Cyclotron radiation from ararefied inhomogeneous magnetoplasma, Nuclear Fusion ,vol. 13, pp. 497-507, 1973.[8] K. Kato and I. H. Hutchinson, Design and performance ofcompact vacuum compatible submillimeter viewing dumps,Rev. Sci. Instrum., vol. 57, pp. 1242-1247, 1986.[9] H. J. Hartfuss and M. Tutter, Fast multichannel heterodyneradiometer for electron cyclotron emission measurement onstellarator W VII-A, Rev. Sci. Instruments, vol. 59, pp. 1703-1705,1985.[10] Millitech Corp., South Deerfield Park, Massachusetts.[11] MITEQ, Hauppauge, N.Y.[12] Celeritek, 617 River Oaks Parkway, San Jose, California[131 Filtronic, Royal London Industrial Estate, Acorn Park,Charlestown, Shipley BD17 7SW., W. Yorkshire, England.[14] A. Cavallo and R. Cano, Measurement of apparent turbulenttemperature fluctuations on TFR tokamak, Plasma Physics,vol. 23, pp. 61-65,1981.-10-Figure CaptionsFig. 1 Frequencies of the first five electron cyclotron harmonics as afunction of major tokamak radius. The shaded areas indicatefrequency overlapped regions.Fig. 2 Illustration of the receiver front end components for thePBX-M ECE diagnostic.Fig. 3 Block diagram of a 14 channel ultrawide heterodyne receiverusing commercially available components for electroncyclotron emission fluctuation diagnostics.-11-U3., .~ .0U7 r ILO)E-0>4uz0W.(n N0D-4LOCDUl$0U,0IIqCNco/m) Aouanbazra-12-C-4___ ___ ___ ___ _003e N30MaQaP"CqzuC9*-13-CC0 cc0-0 coowom 020l0~ X4uU.s00 hICLNO ze0L9) LL

Design of a 3rd harmonic electron cyclotron emission diagnostic for ballooning mode fluctuations in PBX-M

http://dspace.mit.edu/bitstream/1721.1/95083/1/90ja021_full.pdf

Design of a 3rd harmonic electron cyclotron emission diagnostic for ballooning mode fluctuations in PBX-M

Abstract

Similar works

Full text

Available Versions

CiteSeerX

DSpace@MIT