A coherent lidar system has been constructed for the measurement of alpha particles in a burning plasma. The lidar system consists of a pulsed CO{sub 2} laser transmitter and a heterodyne receiver. The receiver local oscillator is a cw, sequence-band CO{sub 2} laser operating with a 63.23 GHz offset from the transmitter

Bennett, C. A.

Hutchinson, D. P.

Richards, R. K.

Simpson, M. L.

UNT Digital Library

Application of Coherent Lidar to Ion Measurements in Plasma Diagnostics Donald P. Hutchinson Oak Ridge Xationai Laboratory Oak Ridge. TN 3783 1 (423)574-4730 E-mail: hutchinsondp@ornl.gOv P. 0. BOX 2008. MS-6004 Roger K. Richards Oak Ridge Xational Laboratory Oak Ridge. TN 3783 1 E-mail: richardsrk@oml.gov P. 0. BOX 2008. MS-6004 (423)576-2582 Charles A. Bennett Oak Ridge National Laboratory Oak Ridge. TX 3783 1 E-mail: bennen@ornl.gov P. 0. BOX 2008. MS-6004 (423)576-2582 Marc L. Simpson Oak Ridge National Laboratory Oak Ridge. TN 3783 1 (423)574-4171 E-mail: mls@ornl.gov P. 0. BOX 2008. MS-6004 Submitted: 9" Conference OR Coherent Laser Radar. Linkoping Sweden. June 23-27. 1997. ABSTRACT A coherent lidar system has been constructed for the measurement of alpha particles in a burning piasma The iidar system consists oi  ;f pulsed CO, laser transmitter and a heterodyne receiyer. The receiver local osciilator is a cw. sequence-band CO, laser operating with a 63 .3  GHz offset from the transmitter. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- tJ or respomibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof. Portions of this dowment may be illegible in electronic image products. Images are produced from the best available original document. Application of Coherent Lidar to Ion Measurements in Plasma Diagnostics Donaid P. Hutchinson, Roger K. Richards. Charles A. Bennett, Marc L. Simpson Oak Ridge National Laboratory! Oak Ridge, TX 3783 I (423)574-4730 P. 0. BOX 2008. MS-6004 Submitted: 9" Conference on Coherent Laser Radar, Linkoping, Sweden, June 23-27,1997. ABSTRACT . A coherent lidar system has been constructed for the measurement of alpha panicies in a burning plasma The lidar system consists of a pulsed CO, laser transminer and a heterodyne receiver. The receiver local oscillator is a cw, sequence-band CO, laser operating with a 63.23 GHz offset from the transmitter. SUMMARY A coherent lidar system has been constructed for the measurement of fusion-produced alpha particles in a burning plasma. The diagnostic techmque used for the measurement is called collective Thomson scattering (CTS). This technique works by scattering a high-power CO, laser beam from clouds of electrons surrounding ions in a high temperature plasma. In a high temperanxre plasma all of the atoms of the filling gas. deuterium and tritium in the case of a fusion reactor, are completely ionized and will remain so if the temperature is maintained. The ions attract the fast moving electrons but recombination into atoms is prohibited by the high energy of the particles. If a probing wavelen-gh is chosen that is comparable in size to or larger than this cloud of electrons hovering around each ion. then the spectrum of the scartered laser light. Doppler-shifted by the motion of the cloud. will be characterized by the ion velocity distribution. The criterion used to establish the proper conditions for such ion measurements in a plasma is known as the Salpeter parameter. a. given by' IbO a =  where a = the Salpeter parameter -!vfanaged by Lockheed Martin Energy Research Corp. For the U.S. Depamnent of Energy under contract DE-AC05-960R2216.C -I. ... e suhmtzed manuscript :?as 5een a-chorea by a cont rac tor  of  :.*.e C . 3 -  Gsverr-Tenr: .aaer cont rac t  No.  2E-.AC?5- ?50R22;66. Accorcinaiy, :he C . 3 .  Jsverrxent r s t a i n s  a nonexcl-sive. rDyal:y-free l icense  t o  yzblish a r  zeproauce tke  pubiisned fsrm of :.*.is c=ntr:Suaon. ~r silow others  ds so. _ L A  ;-- 5. 5. GGverrment purposes.' * .  L,= incilr~nt laser waveiengh h,= Debye length 8= scanering angle. / BLACKBODY SOCRCE / ,-\ '> I.VJECTX0.V LASER P HgCdTe .MIXER L O U L  OSCILLATOR HOP CO, CELL b---------- 2 BEAM DUMP ~~ ~~ F i g u r e  1 Schematic diagram of scaLtering system. In order to measure the ion spectra in a plasma cz must be geater than one. For conditions typical in a fusion plasma this means that for a 10 micron C02 laser. the scattering angle 9 must be less than 1 -degree. A schematic dia-pm of the system is shown in Figure I. The lidar system consists of an injection-locked. unstable resonator pulsed CO, laser transmitter and a heterodyne receiver. The transmitter is locked to the IOP20 line of CO, by the injection of 250 mW of power from a waveguide h e r  operating on the 1OP20 transition of carbon dioxide. The transmitter cavity is a positive branch confocal resonator with a rnagnificarion of 2 i d  a cavity length of 3-meters. The 50 mm diameter rear mirror of the cavity has a concave radius of curvature of 12 meters and the convex scrapper output mirror has a radius of curvature of 6 meters. The ourput mirror is glued to an .Ut-coated NaCI flat which acts a pressure window on the laser cavity enclosure. The injection seed beam enters the pulsed cavity through a 2-mm hoie covered by a ZnSe brewster window in the back curved mirror of the cavity. This iaser produces a 1-microsec pulse with an energy of approximately 1 joules. The back-propagated local oscillator (BPLO) beam intersects the transmitter beam at an angle of 0.S6"in the center of the plasma. The scattered Iight from the piasma is directed through a triple- pass hot CO, cell which acts as a notch filter to reduce strav light from the transmitter. The local oscillator for the receiver is a cw. sequence band CO1 -mting-tuned. waveguide faser opentinp with a 63.23 GHz offset from [he pulsed laser frequency. The waveguide local oscillator laser has an intra-cavity cell containing approxirnateiy lo0 torr of CO, heated to 600 deg-C to force operation on the iOP15 sequence band line shifted +9.68 GHz above the 10P18 transition. The detector is a HgCdTe photovoltaic diode with a bandwidth of 1.5 GHz and a quantum efficiency of 35%. Approximately 10 mW of local oscillator power is focused on the photodetector with a 25-mm dia. ZnSe lens. The scattered signal from the plasma is combined with the Iocal osciiIator Ii@t on a ZnSe beamsplitter. The beamsplitter is coated to provide approximately 2% reflectivity for the local oscillator beam and 98% transmission for the sipai beam. The output of the HgCdTe photovoltaic CENTRAL ELECTRON DENSITY (10~~/cm3 Figure  2 Scattering Data from ATF 3roof-of-principle t e s t  compared xi th theory .  mixer is fed into two 1-GHz wideband amplifiers and then an RF detector. X video amplifier (BW=2 ,MI323 folIows the RF detector to set the postdetection bandwidth oithe receiver. The high speed data are logged by a 32-iMHZ CLMAC digitizer for viewing the puke shape and noise level. Ion scattering in the ATF experiment is not feasible because of the low ion temperature. To properly simulate CTS for alpha particle measurements. a proof-of-principle test has been periormed which relies on scatterin,o from plasma electrons at large shifts from the pulsed laser frequency near the electron plasma resonance’. These resonances are only weakiy dependent on electron temperature and have 3 scaling of n:’ ’. Figure 1. contains a summary of data from scattering experiments that were performed on the ATF device as the plasma density was varied from 0 to about 7 s loi9 m’. The solid and dotted curves in the figure represent theoretical calculations of the scattered spectrum as a function of plasma density. I . J. Sheffield. "Plasma Scattering oi Electromagnetic Radiation." Academic Press, Sew York. 1975. 2 . R. K. Richards, D. P. Hutchinson. C. A. Bennett. H. T. Hunter. and C. H. Ma, "-Measurement of CO, Laser Small-Xngie Thomson Scanering on a Magnetically Confined Plasma" Applied Physics Letters. 62, P. 28-30. January 1993. 

Application of coherent lidar to ion measurements in plasma diagnostics

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Application of coherent lidar to ion measurements in plasma diagnostics

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