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Author(s):
M. Osakabe
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Title:
Measurement of Neutron Energy on D-T Fusion Plasma Experiments
Date of publication:
Apr. 1995
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Key words:
TFTR tokamak, deuterium-tritium plasma, ICRF heating, neutron diagnostics, neutron energy spectrum, deuterium-tritium neutron, recoil proton telescope, plastic scintillator
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Abstract:
Recently, D-T burning plasma experiments have been performed on the Joint European Torus (JET) and the Tokamak Fusion Test Reactor (TFTR). These experiments are expected to bring valuable information for reactor relevant plasmas. In these experiment, the role of fusion products diagnostics, such as neutron diagnostics, becomes more important than ever before. The measurements of neutron flux as well as neutron energy will provide us the information on 1) fusion output, 2) high energy ions produced by the auxiliary heatings and 3) alpha-heating. These are the major issues not only on those experiments but also on the experiments of the next large tokamak facility such as International Thermonuclear Experimental Reactor (ITER). During past years, I Have developed a new type of neutron spectrometer, named COTETRA (counter telescope with thick radiator), to measure the neutron energy spectra around 14MeV. The COTETRA has a merit of compactness with good energy resolution and high detection efficiency, hence it is suitable for space resolved neutron energy spectrum measurement. It is based on the concept of a proton recoil telescope. In a conventional proton recoil telescope, protons are recoiled by incident neutrons in a thin polyethylene film (radiator). Some of protons enter into a detector (E-detector) placed in certain geometry, e.g. behind the radiator. The energy of the neutron is evaluated from the recoil proton energy being measured with E-detector, when the energy loss of the proton in the radiator is negligibly small to the proton energy. Therefore, the radiator is preferred to be as thin as possible to measure the neutron energy with good accuracy, although the usage of a thin radiator reduces the detection efficiency of the telescope. In COTETRA, a plastic scintillator (deltaE-detector) is used as a radiator. The energy of neutron is evaluated by the sum of the energy deposit of recoil proton in delta E- and that in E-detector. A thicker radiator can be used in COTETRA than a conventional telescope, since the energy deposit of proton in the radiator can be obtained in COTETRA. Therefore, COTETRA has better detection efficiency. A prototype of COTETRA was constructed to verify its operation principle. The calibration experiments were performed using a D-T neutron generator. The energy resolution of 5.3 pm 0.9% and the detection efficiency of 1.3x10^-4 counts/(n/cm^2)were achieved. A Monte Carlo code that simulates its detection process has been developed to evaluate the performance of COTETRA. The calculation agrees with the results of the calibration experiments within its margin of error. The calculation also suggests that the energy resolution up to 3% is achievable with the detection efficiency of ~10^-5 counts/(n/cm^2). In 1992, two types of COTETRA were developed for the application to TFTR D-T experiments. One uses a Si-diode (set-A) as an E-detector and the other uses NE102A plastic scintillator, insead (set- B). Both of them use NE102A as a deltaE-detector. Set-A was characterized by higher energy resolution, while set-B was by higher counting rate capability and detection efficiency. For the use under high neutron flux rate condition, both sets have smaller detection area and use faster electronics than the prototype. Calibration experiments for these sets were also performed. An energy resolution of 4.0% was obtained for set-A. Set-B is expected to work at a count rate of up to 10^4cps, which corresponds to a neutron flux rate of ~10^9 (n/cm^2)/s. The detection efficiency of set-B was ~ 6x10^-5counts/(n/cm^2). Both of them were installed at the multichannel neutron collimator of TFTR in 1993. At this location, COTETRA's are viewing plasmas perpendicularly to the magnetic field. A data acquisition system was newly developed for each set. Two functions are newly applied to the system. One function provides the timing information of neutron events during shots. Another function provides the information of accidental coincidence between deltaE- and E-detector. The accidental coincidence causes serious problem when the neutron flux is extremely high. On the calibration experiment, the effect of accidental coincidence was evaluated by placing a tantalum plate between deltaE- and E-detector. It was not possible to apply this method on TFTR. Therefore, the evaluation was made by the timing-shift technique. Using this technique along with normal coincidence technique, two types of coincidence spectra, called foreground and background spectra, were obtained. The foreground spectrum was obtained by accumulating the coincidence events of the three PMT's (two attached to deltaE-detector and one to E-detector). This spectrum contains events due to accidental coincidences between deltaE- and E-detector as well as those due to true coincidences, in which protons recoiled at deltaE-detector is detected by E-detector. The background spectrum was obtained by coincidence events between the timing signal of deltaE-detector and the 10-nsec shifted timing signal of E-detector. Then, It contains only events due to accidental coincidence. It was verified that these two new functions were working successfully on TFTR. The Monte Carlo code NESFP was developed to calculate the energy spectra of neutrons emerged from D-T fusion Plasmas. With this code , the neutron energy spectrum and the fusion reactivity of a plasma for any velocity distributions of deuterium and tritium ion species can be calculated. The validity of the code was checked by comparing its results with those in other publications in the case where deuterium and tritium ions have Maxwellian velocity distribution. The D-T fusion plasma experiments have been performed since November 1993 on TFTR. Tritium was introduced to the torus by gas puffing and/or by neutral beam infection (NBI). As auxiliary heatings, the NBI heating and ion cyclotron range of frequency (ICRF) heating were provided. The D-T neutron energy spectra were obtained for 'NBI and ICRF' heated plasmas and for NBI heated plasmas. These were the first D-T neutron spectra obtained from D-T plasmas in the world. The full width at half maximum (FWHM) of the peak for the ICRF heated plasma is wider than that for NBI heated plasma. The calculated energy spectra suggested this broadening was due to the existence of the high energy tritium ion tail of 100 ~ 400keV. The calculation also suggested that, for D-T plasmas, it is necessary to measure the energy spectrum in evaluating the existence of high energy ions besides the total neutron yield, which might decrease at higher ion temperature. Space resolved measurements of D-T neutron energy spectra will be necessary for ICRF heated D-T plasma experiments on the next large tokamak, such as ITER, to evaluate the mechanism of ICRF heating in the plasma. The COTETRA will be a suitable diagnostic for this purpose because of its good energy resolution and compactness.
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