The 12C(3He,pγ)14N reaction cross section for γ-ray spectroscopy simulation of fusion plasmas

High resolution γ-ray spectroscopy measurements were performed in JET (3He)D plasmas with high energy ion populations driven by radio-frequency (RF) heating. One of the first reactions investigated was 12C(3He,pγ)14N, which was observed at low 3He concentrations. In order to interpret the measurements in this work, cross section data for the 12C(3He,pγ)14N reaction are evaluated. Available data for the population of excited states in 14N up to the eighth level are assessed in the range E(3He) = 0–5?MeV. Discrepancies and gaps in the database have been solved by means of interpolations and consistency analysis. The evaluated cross section data are used to predict the intensity ratio of characteristic 2.31 and 1.63 MeV γ-rays.     

Testing the neutron attenuator based on <Superscript>6</Superscript>LiH for γ-ray diagnostics of plasmas in the JET tokamak

A ⁶LiH attenuator of a neutron flux incident on a detector is used to reduce the γ-ray background induced by neutrons in the detector material. This attenuator has been tested during experiments with deuterium (DD) plasmas on the JET tokamak. A specimen of the neutron attenuator with dimensions of ?30 × 300 mm has been developed by the Russian Academy of Sciences’ Ioffe Physico-Technical Institute and inserted into a vertical collimator used for γ spectrometry of plasmas. To compare γ-ray spectra recorded with and without the ⁶LiH attenuator being mounted, identical discharges with heating of the DD plasma by a neutral particle beam have been selected. For γ rays with energies of <3 MeV, which are induced by neutrons in the detector material, the suppression factor is found to be ～100. A low attenuation (～2) observed at energies of >3 MeV can be attributed to the transparency of the ⁶LiH attenuator for γ rays. This portion of the spectrum is due to γ radiation of the plasma and γ rays induced by neutrons in the constructional materials of the tokamak. To estimate the efficiency of the ⁶LiH attenuators as a mandatory component of the ITER γ-ray diagnostic system, it is necessary that measurements be taken in deuterium-tritium (DT) discharges.     

Development of the large neutron imaging system for inertial confinement fusion experiments

Inertial confinement fusion (ICF) requires a high resolution (~10 μm) neutron imaging system to observe deuterium and tritium (DT) core implosion asymmetries. A new large (150 mm entrance diameter: scaled for Laser Me?gaJoule [P. A. Holstein, F. Chaland, C. Charpin, J. M. Dufour, H. Dumont, J. Giorla, L. Hallo, S. Laffite, G. Malinie, Y. Saillard, G. Schurtz, M. Vandenboomgaerde, and F. Wagon, Laser and Particle Beams 17, 403 (1999)]) neutron imaging detector has been developed for such ICF experiments. The detector has been fully characterized using a linear accelerator and a (60)Co γ-ray source. A penumbral aperture was used to observe DT-gas-filled target implosions performed on the OMEGA laser facility. [T. R. Boehly, D. L. Brown, R. S. Craxton, R. L. Keck, J. P. Knauer, J. H. Kelly, T. J. Kessler, S. A. Kumpan, S. J. Loucks, S. A. Letzring, F. J. Marshall, R. L. McCrory, S. F. B. Morse, W. Seka, J. M. Soures, and C. P. Verdon, Opt. Commun. 133, 495 (1997)] Neutron core images of 14 MeV with a resolution of 15 μm were obtained and are compared to x-ray images of comparable resolution.     

Development and characterization of sub-100 ps photomultiplier tubes

We describe the evaluation of a microchannel plate (MCP) photomultiplier tube (PMT), incorporating a 3?μm pore MCP and constant voltage anode and cathode gaps. The use of the small pore size results in PMTs with response functions of the order of 85 ps full-width-half-maximum, while the constant electric field across the anode and cathode gaps produces a uniform response function over the entire operating range of the device. The PMT was characterized on a number of facilities and employed on gas Cherenkov detectors fielded on various deuterium tritium fuel (DT) implosions on the Omega Laser Facility at the University of Rochester. The Cherenkov detectors are part of diagnostic development to measure Gamma ray reaction history for DT implosions on the National Ignition Facility.     

Fusion product diagnostics planned for Large Helical Device deuterium experiment

Deuterium experiment on the Large Helical Device (LHD) is now being planned at the National Institute for Fusion Science. The fusion product diagnostics systems currently considered for installation on LHD are described in this paper. The systems will include a time-resolved neutron yield monitor based on neutron gas counters, a time-integrated neutron yield monitor based on activation techniques, a multicollimator scintillation detector array for diagnosing spatial distribution of neutron emission rate, 2.5 MeV neutron spectrometer, 14 MeV neutron counter, and prompt γ-ray diagnostics.     

Gamma bang time analysis at OMEGA

Absolute bang time measurements with the gas Cherenkov detector (GCD) and gamma reaction history (GRH) diagnostic have been performed to high precision at the OMEGA laser facility at the University of Rochester with bang time values for the two diagnostics agreeing to within 5 ps on average. X-ray timing measurements of laser-target coupling were used to calibrate a facility-generated laser timing fiducial with rms spreads in the measured coupling times of 9 ps for both GCD and GRH. Increased fusion yields at the National Ignition Facility (NIF) will allow for improved measurement precision with the GRH easily exceeding NIF system design requirements.     

Absolute calibration method for laser megajoule neutron yield measurement by activation diagnostics

The laser megajoule (LMJ) and the National Ignition Facility (NIF) plan to demonstrate thermonuclear ignition using inertial confinement fusion (ICF). The neutron yield is one of the most important parameters to characterize ICF experiment performance. For decades, the activation diagnostic was chosen as a reference at ICF facilities and is now planned to be the first nuclear diagnostic on LMJ, measuring both 2.45 MeV and 14.1 MeV neutron yields. Challenges for the activation diagnostic development are absolute calibration, accuracy, range requirement, and harsh environment. At this time, copper and zirconium material are identified for 14.1 MeV neutron yield measurement and indium material for 2.45 MeV neutrons. A series of calibrations were performed at Commissariat a? l'Energie Atomique (CEA) on a Van de Graff facility to determine activation diagnostics efficiencies and to compare them with results from calculations. The CEA copper activation diagnostic was tested on the OMEGA facility during DT implosion. Experiments showed that CEA and Laboratory for Laser Energetics (LLE) diagnostics agree to better than 1% on the neutron yield measurement, with an independent calibration for each system. Also, experimental sensitivities are in good agreement with simulations and allow us to scale activation diagnostics for the LMJ measurement range.     

A technique for selecting γ rays with energies above 50 GeV from the background of charged particles in the GAMMA-400 space-based γ-ray telescope

The task of selecting neutral γ rays from the background of charged particle fluxes, which arises in investigation of high-energy (>50 GeV) cosmic rays, is complicated by the presence of the backsplash effect. The backsplash is composed of a great number of low-energy (~1 MeV) particles produced in an electromagnetic shower being developed in the calorimeter of the γ-ray telescope. A technique of charged particle rejection using an anticoincidence system has been developed. A method for discriminating events of charged particle detection from γ-ray detection events accompanied by the backsplash phenomenon is proposed. This method is based on the difference of the signals in time and makes it possible to maintain a high detection efficiency even for high-energy γ rays.     

Neutron spectroscopy results of JET high-performance plasmas and extrapolations to DT performance

In a fusion reactor with high energy gain, the fusion power will be mainly thermonuclear (THN). Measurements of the THN neutron rate are a good performance indicator of a fusion plasma, requiring neutron emission spectroscopy (NES) measurements to distinguish thermal and nonthermal contributions. We report here on recent NES results from JET high-performance plasmas with high fractions (about 65%) of THN emission. The analysis is made with a framework for analyzing NES data, taking into account THN reactions and beam-target reactions. The results are used to extrapolate to the equivalent DT rates. Finally, we discuss the applicability of using NES in the deuterium phase of ITER, both for the extrapolations to ITER’s future DT performance as well as for the measurements of confined energetic ions.     

Active spectroscopic measurements using the ITER diagnostic system

Active (beam-based) spectroscopic measurements are intended to provide a number of crucial parameters for the ITER device being built in Cadarache, France. These measurements include the determination of impurity ion temperatures, absolute densities, and velocity profiles, as well as the determination of the plasma current density profile. Because ITER will be the first experiment to study long timescale (～1?h) fusion burn plasmas, of particular interest is the ability to study the profile of the thermalized helium ash resulting from the slowing down and confinement of the fusion alphas. These measurements will utilize both the 1 MeV heating neutral beams and a dedicated 100 keV hydrogen diagnostic neutral beam. A number of separate instruments are being designed and built by several of the ITER partners to meet the different spectroscopic measurement needs and to provide the maximum physics information. In this paper, we describe the planned measurements, the intended diagnostic ensemble, and we will discuss specific physics and engineering challenges for these measurements in ITER.     

Measurement of the ratio of hydrogen to deuterium at the KSTAR 2009 experimental campaign

The control of the ratio of hydrogen to the deuterium is one of the very important issues for ion cyclotron range of frequency (ICRF) minority heating as well as the plasma wall interaction in the tokamak. The ratio of hydrogen to deuterium during the tokamak shot was deduced from the emission spectroscopy measurements during the KSTAR 2009 experimental campaign. Graphite tiles were used for the plasma facing components (PFCs) at KSTAR and its surface area exposed to the plasma was about 11?m(2). The data showed that it remained as high as around 50% during the campaign period because graphite tiles were exposed to the air for about two months and the hydrogen contents at the tiles are not fully pumped out due to the lack of baking on the PFC in the 2009 campaign. The validation of the spectroscopy method was checked by using the Zeeman effects and the ratio of hydrogen to the deuterium is compared with results from the residual gas analysis. During the tokamak shot, the ratio is low below 10% initially and saturated after around 1 s. When there is a hydrogen injection to the vessel via ion cyclotron wall conditioning and the boronization process where the carbone is used, the ratio of the hydrogen to the deuterium is increased by up to 100% and it recovers to around 50% after one day of operation. However it does not decrease below 50% at the end of the experimental campaign. It was found that the full baking on the PFC (with a high temperature and sufficient vacuum pumping) is required for the ratio control which guarantees the efficient ICRF heating at the KSTAR 2010 experimental campaign.     

稀燃天然气掺氢发动机循环变动的试验研究

对火花点火天然气发动机而言,稀燃是提高发动机燃油经济性和降低发动机排放的一种有效途径,但稀燃导致的循环变动是拓宽稀燃极限的一个主要限制因素,在天然气中掺混氢气作为燃料,可解决稀燃工况下循环变动过大的问题。为了研究掺氢对发动机循环变动影响,选取纯天然气和掺氢体积比为20%的天然气掺氢燃料,在6缸进气道喷射增压稀燃天然气发动机上进行不同点火提前角和空燃比工况下的试验研究。通过对缸内压力特性参数、燃烧特性参数以及排放数据的分析,结果表明:掺氢可有效降低发动机最高压力循环变动和平均指示压力循环变动,在稀燃情况下效果更为明显;掺氢可降低火焰发展期和燃烧持续期及其循环变动;在稀燃工况下,掺氢对控制发动机NOx及未燃HC排放有利。     

A method for measuring the detector response function for monochromatic electrons based on Compton scattering

A method for calibrating the energy scale of a scintillation detector using γ rays has been proposed and implemented. The technique is based on Compton scattering in the scintillation detector, followed by photoelectric absorption of a scattered γ-ray photon in a Ge detector. The novelty of the method consists in placing the γ-ray source and the scintillation and Ge detectors tightly to each other. The method is efficient for detectors with a low-Z material for which the ratio of the cross sections for Compton scattering and photoeffect is great in value and the attenuation length of the γ-ray flux is comparable to the detector dimensions. The described technique can be used to precisely investigate the dependence of the light yield in a scintillator on the electron energy.     

Calibration of the neutron detectors for the cluster fusion experiment on the Texas Petawatt Laser

Three types of neutron detectors (plastic scintillation detectors, indium activation detectors, and CR-39 track detectors) were calibrated for the measurement of 2.45 MeV DD fusion neutron yields from the deuterium cluster fusion experiment on the Texas Petawatt Laser. A Cf-252 neutron source and 2.45 MeV fusion neutrons generated from laser-cluster interaction were used as neutron sources. The scintillation detectors were calibrated such that they can detect up to 10(8) DD fusion neutrons per shot in current mode under high electromagnetic pulse environments. Indium activation detectors successfully measured neutron yields as low as 10(4) per shot and up to 10(11) neutrons. The use of a Cf-252 neutron source allowed cross calibration of CR-39 and indium activation detectors at high neutron yields (～10(11)). The CR-39 detectors provided consistent measurements of the total neutron yield of Cf-252 when a modified detection efficiency of 4.6×10(-4) was used. The combined use of all three detectors allowed for a detection range of 10(4) to 10(11) neutrons per shot.     

Development of an x-ray imaging system for the Laser Megajoule (LMJ)

This imaging system aims at recording images of the core size and shape of an imploding deuterium-tritium (DT) microballoon on LMJ inertial confinement fusion (ICF) experiments. Image acquisition is difficult due to the harsh surrounding created by the fusion reaction, which affects system specifications. This one is made of a scintillator, an optical relay, and a CCD camera shielded from the surrounding. The system was tested on different facilities at CEA/DIF, where a spatial resolution of 120?μm was achieved and gamma dose up to 20 rad effects were measured. Setup and performed test are described.     

Neutron spectroscopy as a fuel ion ratio diagnostic: lessons from JET and prospects for ITER

The determination of the fuel ion ratio n(t)/n(d) in ITER is required at a precision of 20%, time resolution of 100 ms, spatial resolution of a/10, and over a range of 0.016?keV and for n(T)/n(D)<0.6. A crucial issue is the signal-to-background situation in the measurement of the weak 2.5 MeV emission from DD reactions in the presence of a background of scattered 14 MeV DT neutrons. Important experimental input and corroboration for this assessment are presented from the time-of-flight neutron spectrometer at JET where the presence of a strong component of backscattered neutrons is observed. Neutron emission components on ITER due to beam-thermal and tritium-tritium reactions can further enhance the prospects for NES.     

A digital method for separation and reconstruction of pile-up events in germanium detectors

The problem of pulse pile-up is very often encountered in precise measurements of γ-rays using germanium detectors. The standard method of treating the pile-up events is to identify and reject them using an appropriate electronic system. Digital acquisition techniques now allow the recording of waveforms of pile-up events that can be analyzed and the contributing single pulses recovered, rather than simply tolerating the losses associated with pile-up. In this paper, a method for the off-line digital processing of pile-up events from germanium detectors is demonstrated. The method is based on an appropriate fitting of the detector signals, shaped with a suitable digital pulse shaper. It is shown that the method is able to recover the pile-up events with good accuracy even when the constituent signals are in close proximity. The method is very useful for γ-ray spectroscopy in nuclear physics experiments, where the low intensity signals can be lost due to the pile-up in a high-rate environment.     

Energy resolution of gamma-ray spectroscopy of JET plasmas with a LaBr3 scintillator detector and digital data acquisition

A new high efficiency, high resolution, fast γ-ray spectrometer was recently installed at the JET tokamak. The spectrometer is based on a LaBr3(Ce) scintillator coupled to a photomultiplier tube. A digital data acquisition system is used to allow spectrometry with event rates in excess of 1 MHz expected in future JET DT plasmas. However, at the lower rates typical of present day experiments, digitization can degrade the energy resolution of the system, depending on the algorithms used for extracting pulse height information from the digitized pulses. In this paper, the digital and analog spectrometry methods were compared for different experimental conditions. An algorithm based on pulse shape fitting was developed, providing energy resolution equivalent to the traditional analog spectrometry method.     

Characteristics of neutron and gamma radiation in the core of the ΦC-1-4.37.P critical assembly

The characteristics of neutron and γ-ray fields (spectrum, flux density, average energy, cross sections of 23 nuclear reactions, and γ-ray dose rate) and the γ-ray field in the core of the ΦC-1-4.37.P critical assembly of the ΦC-1M critical bench are determined. The results of comparing the characteristics of the neutron fields of the ΦC-1-4.37.P critical assembly and analogous reactors are presented.     

Tritium plasma experiment: parameters and potentials for fusion plasma-wall interaction studies

The tritium plasma experiment (TPE) is a unique facility devoted to experiments on the behavior of deuterium/tritium in toxic (e.g., beryllium) and radioactive materials for fusion plasma-wall interaction studies. A Langmuir probe was added to the system to characterize the plasma conditions in TPE. With this new diagnostic, we found the achievable electron temperature ranged from 5.0 to 10.0 eV, the electron density varied from 5.0 × 10(16) to 2.5 × 10(18) m(-3), and the ion flux density varied between 5.0 × 10(20) to 2.5 × 10(22) m(-2) s(-1) along the centerline of the plasma. A comparison of these plasma parameters with the conditions expected for the plasma facing components (PFCs) in ITER shows that TPE is capable of achieving most (～800 m(2) of 850 m(2) total PFCs area) of the expected ion flux density and electron density conditions.