Furnstahl, R.J.

Pappas, D.S.

Wysocki, F.J.

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PFC/RR-82-14 DOE/ET-51013-49UC20FD-D NEUTRON ENERGY SPECTRA MEASUREMENTSIN ALCATOR CD. S. PAPPAS, F. J. WYSOCKI,t R. J. FURNSTAHLttPlasma Fusion CenterMassachusetts Institute of Technology -Cambridge, MA 02139August 1982tPresent Address: Princeton University, Department of Astrophysical SciencesttPresent Address: Stanford University, Department of PhysicsThis work was supported by the U.S. Department of Energy Contract No.DE-AC02-78ET51013. Reproduction, translation, publication, use and dis-posal, in whole or in part by or for the United States government is per-mitted.D-D NEUTRON ENERGY SPECTRA MEASUREMENTSIN ALCATOR CD. S. Pappas, F. J. Wysocki, R. J. FurnstahlPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, Massachusetts 02139 USAABSTRACTMeasurement of energy spectra of neutrons produced during high density(ne > 2 X 1014 cm- 3 ) deuterium discharges have been obtainedl using aproton-recoil (NE 213) spectrometer. A two foot section of light pipe(coupling the scintillator and photomultiplier) was used to extend thescintillator into a diagnostic viewing port to maximize the neutrondetection efficiency while not imposing excessive magnetic shieldingrequirements. A derivative unfolding technique was used to deduce theenergy spectra. The results showed a well defined peak at 2.5 MeV whichwas consistent with earlier neutron flux measurements on Alcator C thatindicated the neutrons were of thermonuclear origin 2.INTRODUCTIONRegimes of operation in present day fusion devices result in copiousproduction of fast neutrons. These fusion neutrons which emanate from thehot plasma core can not only give direct evidence of plasma having thermo-nuclear parameters, but also may allow determination of central ion temp-eratures a feature of prime importance in high density tokamaks such asthe Alcator devices3 .Under certain circumstances tokamak operation can lead to non-thermo-nuclear neutron emissions that can exceed expected thermonuclear rates 4,5,6,thus the origins of the neutron emissions must be determined.Measurement of neutron energy spectra can provide information concern-ing neutron origins as well as information on plasma properties. The re-sults described here were the first neutron spectra measurements fromAlcator C and were obtained shortly after the first introduction ofdeuterium into the device. Alcator C had commenced tokamak operation inhydrogen fill for the first six months, at which time photonuclear neutronrates were routinely reduced to less than 106 n/sec6 . Since the expect-ed thermonuclear yields were to be approximately 1010 n/sec during theinitial D-D operation, the spectrometer used for these experiments waschosen for its high efficiency rather than for maximum energy resolution.Specifically, the spectrometer consisted of a 5 cm X 5 cm NE 213 liquidorganic 7 ,8, 3 scintillator optically coupled to an RCA 8850 photomul-tiplier tube using a two-foot section of cast acrylic lucite plexiglassrod. The location of the spectrometer in relation to Alcator C is shown inFig. 1. Standard Ortec circuitry (Fig. 2) was used for n-y pulse shape- 2 -discrimination to allow pulse height analysis of neutron only pulsesduring the experiments.RESULTSShown in Fig. 3 is a nominal spectrum ('D(E)vs. E where P(E) = neutronflux at energy E in neutrons/cm2-sec, and E = neutron energy in MeV) for a1 keV D-D maxwellian plasma. The velocity distribution of the reactingdeuterons gives rise to a doppler broadening of the spectral peak. Theenergy spectra for maxwellian velocity distributions has been calculatedby Faust and Harris 9 . For a maxwellian deuterium plasma the width canbe written as FWHM = 83.25 (KT) 1/ 2 keV, with KT in keV. The resolutionof the high efficiency spectrometer used in this work was 20% or about500 keV at 2.45 MeV. The efficiency of the detector however was approxi-mately 104 times the efficiencies of the usual (He3, Li sandwich, etc.)high resolution spectrometers. Thus for the neutron rates expectedduring the early D-D operation, verification of the fusion origin of theneutron emissions was possible from data averaged over approximately tenshots.To unfold the measured proton recoil spectra a derivative unfoldingcode10 called FLYSPEC was used. Basically the flux P is given by-E 1 dP(E),(E) = - - -VTnH onp(E) dEwhere C(E) = neutron flux at energy E (n/cm 2-sec)dP- = first derivative of the proton recoil distributiondE- 3 -V = detector volume (cm3)T = counting time (sec)nH = density of target protons (H atoms/cm3)np = n-p elastic scattering cross section (cm2)The unfolded measured neutron energy spectrum from a high density Al-cator C D2 discharge is shown in Figure 4. The spectrum indicates a large2.5 MeV peak neutron emission from the D(D,n)He3 fusion reaction in accor-dance with expectations due to the low runaway electron levels (i.e., con-tributions due to photonuclear processes and the electro disintegration ofdeuterons were expected to be quite low). The ion temperature determinedboth by global neutron emission and charge exchange were approximately800 eV. The agreement between neutron flux deduced and charge exchangededuced ion temperatures together with the corresponding measured 2.45MeV peak in the neutron spectra is a clear confirmation that the majorityof the neutron emissions for the typical Alcator C high density dis-charges are of thermonuclear origin.- 4 -ACKNOWLEDGMENTThe authors would like to thank Ron Parker and the entire Alcatorteam for their encouragement and support during the course of this work.- 5 -REFERENCES1. Pappas, D.S., Furnstahl, R.J., Bull. Am. Phys. Soc. 25 (1980) 952.2. Pappas, D.S., Bull. Am. Phys. Soc. 24, (1979) 997.3. Pappas, D.S., Parker, R.R., M.I.T. Plasma Fusion Center Report PFC/RR-78-S.4. TFR Group, Phys. Lett. 60 A (1977) 219.5. Strachan, J., Meservey, E., Stodiek, W., Naumann, R., Girshick, F.,Nuclear Fusion 17 (1977) 140.6. Pappas, D.S., Furnstahl, R.J., Kochanski, G.P., M.I.T. Plasma FusionCenter Report PFC/RR-81-22.7. Burrus, W.R., Verbinski, V.V., Nucl. Instr. and Methods 67 (1969) 181.8. Morgan, G.L., England, A.C., Nucl. Instr. and Methods 129 (1975) 1.9. Faust, W.R., Harris, E.G., Nuclear Fusion 1 (1960) 62.10. Slaughter, D.R., Lawrence Livermore Laboratory Report, UCRL-77849,(1976).- 6 -FIGURE CAPTIONSFigure 1.Figure 2.Figure 3.Figure 4.- 7 -The Alcator C device and relative positions of the spectrometerand neutron flux diagnostics.Block diagram of the spectrometer instrumentation.Calculated neutron energy spectrum for a 1 keV D-D (maxwellian)plasma.Measured neutron energy spectrum from Alcator C high densitydeuterium discharge. The peak at 2.5 MeV together with theagreement between charge exchange and neutron flux deducedion temperatures indicates that the neutrons are of thermo-nuclear origin.LU0Ui*IILlLU IOD DOI.'-102U 1L rJOIuQQjw-j'IZ ______LLJD 0~LLJLLCfLU0z-4CD0-4/WZt 0wLCDPM LinearBase OutputORT55A-FractionOutputInputEC2B-FractionOutputStart StpInput InputORTEC SCA467 OutputTPHCOutputGateORTEC427AOutputSignalInputTN-1 7 101 MCAFigure 2InoutA ORTEC46oBipolar UnOutput buipilartputFIGURE 3(I)co:wr>FIGURE 4zD-NOMINAL8642u864SPECTRUM EXPECTED FROMT;= IkeV D-D PLASMA1.0 2.0 3.0NEUTRON ENERGY (MeV)4$'L -sO/- -MEASURED 02SPECTRUM Ti=780eV-II I2 3NEUTFRON ENtERGY (McV)I I -7I I i I I2I

D-D neutron energy spectra measurements in Alcator C

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D-D neutron energy spectra measurements in Alcator C

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