Fast 3He Research Articles

Study on Fast Deuteron Diagnostics Method Using Fast <sup>3</sup>He Visible Spectra in the Large Helical Device Deuterium Plasma

Fast ion diagnostic is one of the most crucial plasma diagnostics for nuclear fusion investigation. A new diagnostic method for fast ions has been proposed using visible spectra of 3He produced by a deuteron-deuteron reaction. This diagnostic method has a better energy resolution than methods using neutron/γ-ray and is superior to conventional spectroscopy in measuring high energy (MeV order) ions. This diagnostic method has been predicted using numerical analysis for ITER, but no verification experiments have been performed yet. In this study, we examined the measurability of this diagnostic method in the large helical device (LHD) deuterium plasma. Although very dependent on the measurement geometry and the spectrometer performance, it may be possible to measure the fast 3He visible spectrum.

Characteristics of fast 3He ion velocity distribution exciting ion cyclotron emission on JT-60U

Ion cyclotron emission (ICE) is probably excited by non-thermal ion velocity distribution. Identifying characteristics of the ion velocity distribution that excites the ICE can contribute to understanding its emission mechanism. The characteristics of fast helium-3 (3He) ion velocity distribution that excites 3He ICE [ICE(3He)] on JT-60U are investigated. First, fast deuterium ion distributions, which mainly contribute to deuterium–deuterium fusion reactions, are calculated with a fast ion orbit following Monte-Carlo code (OFMC code) under realistic conditions to evaluate birth spatial and velocity distributions of the 3He ions. Then, the fast 3He ion distributions are evaluated with the OFMC code using the birth distributions and compared between cases when the ICE(3He) is observed and not. The evaluated fast 3He ion distributions at the midplane edge of the plasma on the low field side have strong pitch-angle anisotropy in both cases. In the case with the ICE(3He) excitation, a relatively peaked bump-on tail structure in the energy direction is formed in the distributions. The formation of the relatively peaked bump-on tail structure is localized at the plasma edge on the low field side. On the other hand, the distributions at the plasma edge on the low field side have a broader structure in the energy direction in the case without the excitation. The comparison results show that the formation of the relatively peaked bump-on tail structure in the fast 3He ion distribution makes a key contribution to the excitation of the ICE(3He) on JT-60U.

Comparison of Dispersion Model of Magneto-Acoustic Cyclotron Instability with Experimental Observation of 3He Ion Cyclotron Emission on JT-60U

The Magneto-acoustic Cyclotron Instability (MCI) is a possible emission mechanism for Ion Cyclotron Emissions (ICEs). A dispersion model of the MCI driven by a drifting-ring-type ion velocity distribution has been proposed. In this study, the model was compared with the experimental observations of 3He ICEs [ICEs(3He)] on JT-60U. For this purpose, at first, velocity distributions of deuterium–deuterium fusion produced fast 3He ions at the time of an appearance of the ICE(3He) were evaluated by using a fast ion orbit following code under a realistic condition. The calculated distribution at the edge of the plasma on the midplane on the low field side is shown to have an inverted population and strong anisotropy. This distribution can be reasonably approximated by the drifting-ring-type distribution. Next, dispersions of the MCIs driven by the drifting-ring-type distribution were compared with those of observed ICEs(3He). The comparison shows that toroidal wavenumbers and frequencies of the calculated MCIs ...

Fast ions in mode conversion heating (3He)–H plasmas in JET

Fast ions were analysed in experiments focusing on fundamental 3He minority and mode conversion (MC) in the ion cyclotron resonance range of frequencies (ICRF) in H plasmas and on second harmonic heating of 3He ions at 2.65 T mimicking D–T plasma heating in ITER at half its nominal toroidal magnetic field. Gamma-ray spectrometry, neutral particle analysers and fast-ion loss diagnostics provided information on the generation of fast-ion populations and on the distribution of ICRH power among the species in various heating scenarios and for a large range of 3He concentrations. In the scenario with the fundamental 3He minority and MC wave heating at BT(0) = 3.41 T and f ≈ 32 MHz, fast 3He ions accelerated by ICRH in the MeV energy range were detected in discharges with low 3He concentration. In the experiments with a 3He concentration scan it was found that at a 3He concentration of ≈2.2% the 3He ion losses disappeared while a population of energetic D ions gradually built up due to a redistribution of the ICRH power between species on reaching the first MC regime. Under those conditions the ICRF-heated D beam ions effectively absorbed the wave power at their Doppler shifted resonance, which was close to the plasma centre. In discharges with second harmonic heating of 3He ions at BT(0) = 2.65 T and f ≈ 52 MHz, the confined energetic 3He ions were found in the MeV energy range. There is some evidence that the D ions were also accelerated by ICRF. This paper also demonstrates that the synergy of the various fast ion diagnostics allows making a broad picture of the physics of the redistribution of the absorbed ICRH power in complicated heating scenarios of JET.

Production and diagnosis of energetic particles in FAST

The Fusion Advanced Study Torus (FAST) has been proposed as a possible European satellite facility to study fast-ion physics in deuterium plasmas under conditions relevant to a burning plasma. Energetic minority ions (H or 3He) accelerated by ion cyclotron resonance heating (ICRH), with dimensionless parameters close to those of fusion-born alphas in ITER, can be produced in FAST via 30 MW power ICRH minority heating. This work provides a first assessment of the extent to which the 3He fast-ion population can be diagnosed in FAST with a set of dedicated diagnostics for confined fast particles. Neutron emission spectroscopy (NES), gamma-ray spectroscopy (GRS) and collective Thomson scattering (CTS) diagnostics have been reviewed with a description of the state-of-the-art hardware and a preliminary analysis of the required lines of sight. The results of the analysis, based on numerical simulations of the spatial and energetic particle distribution function of the ICRH-accelerated ions for the standard FAST H-mode scenario, suggest that NES and GRS measurements can provide information on the fast 3He population effective tail temperature, with time resolutions in the range 20–100 ms. The proposed CTS diagnostic can measure the fast-ion parallel and perpendicular temperature with a spatial resolution of 5–10 cm and a time resolution of 10 ms. The paper provides a scientific basis for the prediction of the production and diagnosis of energetic ions in FAST.

Calculated neutron emission spectrum with knock-on effects for RF heated (3He)D plasmas

The neutron emission from (3He)D plasmas with RF heating is calculated using a model that includes supra-thermal (knock-on) components of the deuteron population. The RF generation of fast 3He ions is described and the knock-on components were determined with the help of newly derived 3He + d scattering cross sections. Results are presented on the neutron emission spectrum and its contributions from different deuteron velocity components. It is shown that knock-on leaves an observable feature in the spectrum with a clear dependence on absorbed RF power. The importance of the nuclear interaction in the elastic cross section is demonstrated. The results represent a step forward in the use of neutron emission spectroscopy to diagnose fusion plasmas with minority supra-thermal components in their fuel ion composition.

Comparison of collective Thomson scattering signals due to fast ions in ITER scenarios with fusion and auxiliary heating

Auxiliary heating such as neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) will accelerate ions in ITER up to energies in the MeV range, i.e. energies which are also typical for alpha particles. Fast ions of any of these populations will elevate the collective Thomson scattering (CTS) signal for the proposed CTS diagnostic in ITER. It is of interest to determine the contributions of these fast ion populations to the CTS signal for large Doppler shifts of the scattered radiation since conclusions can mostly be drawn for the dominant contributor. In this study, distribution functions of fast ions generated by NBI and ICRH are calculated for a steady-state ITER burning plasma equilibrium with the ASCOT and PION codes, respectively. The parameters for the auxiliary heating systems correspond to the design currently foreseen for ITER. The geometry of the CTS system for ITER is chosen such that near perpendicular and near parallel velocity components are resolved. In the investigated ICRH scenario, waves at 50 MHz resonate with tritium at the second harmonic off-axis on the low field side. Effects of a minority heating scheme with 3He are also considered. CTS scattering functions for fast deuterons, fast tritons, fast 3He and the fusion born alphas are presented, revealing that fusion alphas dominate the measurable signal by an order of magnitude or more in the Doppler shift frequency ranges typical for fast ions. Hence the observable CTS signal can mostly be attributed to the alpha population in these frequency ranges. The exceptions are limited regions in space with some non-negligible signal due to beam ions or fast 3He which give rise to about 30% and 10–20% of the CTS signal, respectively. In turn, the dominance of the alpha contribution implies that the effects of other fast ion contributions will be difficult to observe by CTS.

Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast 3He and 4He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.

Fast ion distributions driven by polychromatic ICRF waves on JET

Experiments have been carried out on the JET tokamak to investigate fast 3He and hydrogen minority ion populations accelerated by ion cyclotron range of frequencies (ICRF) waves launched with multiple frequencies (i.e. up to four frequencies separated by up to ≈15%). This ‘polychromatic’ heating is compared with single-frequency, ‘monochromatic’, ICRF heating of reference discharges with similar power levels. Information on the fast ion populations is provided by two-dimensional gamma-ray emission tomography and the measurements are compared with numerical modelling. Polychromatic heating with resonances in the plasma centre (Rres ≈ R0) and on the low magnetic-field side (LFS) (Rres > R0) is found to produce predominantly high-energy standard trapped ions, while resonances on the high magnetic-field side (Rres < R0) increase the fraction of high-energy passing ions. Monochromatic heating with a central resonance produces stronger gamma-ray emission with the maximum emission in the midplane close to, and on the LFS of, the resonance, in agreement with the calculated radial distribution of fast ion orbits. Both the fast ion tail temperature and energy content are found to be lower with polychromatic waves. Polychromatic ICRF heating has the advantage of producing smaller-amplitude and shorter-period sawteeth, consistent with a lower fast ion pressure inside the q = 1 surface, and higher ion to electron temperature ratios.

Chapter 12: FT3 - An Experiment to Study Burning Plasma Physics Issues in Deuterium Plasmas

A conceptual study is presented for a substantial upgrade of the Frascati Tokamak Upgrade (FTU) up to B = 8 T, I = 6 MA, and R [approximately equal to] 1.3 m to study burning plasma (BP) issues in deuterium plasmas operating up to an equivalent DT gain close to Q = 2 in the ELMy H-mode and to Q = 5 with an internal transport barrier (ITB). The effect of alpha particles is simulated by ~1 MeV fast 3He minority heating produced by ion cyclotron resonance heating (20 MW). Thanks to the high-density values ([approximately equal to]4 × 1020 m-3), the FT3 plasmas are characterized by short electron-ion equipartition time (60 ms in the ELMy H-mode scenario) and slowing-down time (44 ms), with respect to the energy confinement time of ~340 ms, a feature characteristic of BP experiments but not always satisfied with present tokamak devices. Advanced scenarios at 5 T with fully noninductive current drive can be investigated with a steady-state current density profile achieved in <5 s. The aim of FT3 is to prepare ITER operation and to provide a test bed for the development of the ITER auxiliary system and diagnostics. Elements of the scientific program are as follows: the investigation of energetic particle collective effects, optimization of H-mode scenarios, development of improved H-mode scenarios and scenarios with ITBs, magnetohydrodynamic and transport studies in ITER-relevant conditions, and study of edge plasma dynamics. FT3 can use all the existing facilities available in Frascati and could be constructed in ~5 yr.

Non-uniform 3He polarization formed by multiple collisions of a fast 3He + ion with polarized Rb vapor in a strong magnetic field

We investigated the spatial distribution of a polarization in 3He beam expected from a novel polarized 3He ion source based on electron pumping, i.e., multiple electron capture and stripping collisions of an incident fast 3He + ion with a polarized Rb vapor in a strong axial magnetic field. For this purpose, a Monte Carlo simulation was carried out for 19 keV 3He + ions with varying Rb vapor thickness, magnetic field, and beam emittance. The calculated results showed a distribution of the 3He polarization that we call a “polarization hole”, which has a low polarization area around the beam axis. The parameters characterizing the polarization hole, i.e., the polarization and radius of the hole, were found to depend on the Rb vapor thickness, the magnetic field, the beam size, and the angular divergence of the initial beam. These parameters were successfully reproduced with analytical functions deduced from a probability density function prescription. This provides a powerful tool to treat complex phenomena of multiple collisions in strong magnetic fields without performing time-consuming Monte Carlo calculations.

D-3He Fusion in the Joint European Torus Tokamak — Recent Experimental Results

A new series of D-3He fusion yield experiment has been performed in the Joint European Torus (JET) using ion cyclotron resonance heating (ICRH) to generate a high-energy 3He tail reacting with a background deuterium plasma. Using recently installed antennas with beryllium screens, radio-frequency power reaching 15 MW can be coupled to the plasma at the fundamental cyclotron resonance of 3He near the magnetic axis. Best results are obtained with 3.5-MA discharges in the double-null configuration with high recycling on the outboard limiters to stay in L mode and to control the plasma density and purity. A record fusion power level of Pfus = 140 kW is obtained, corresponding to a reaction rate of 4.6 × 1016 reaction/s. The amplification factor Q = Pfus/PICRH reaches a maximum of 1.25% at PICRH = 10 MW. The previous best result were Pfus = 700 kW and Q = 1%. Time-resolved measurements show a correlation between fusion power and energy stored in the fast 3He ions in agreement with calculations based on classical slowing down of the 3He ions driven by ICRH to an average energy in the mega-electron-volt range.

Target dependence of equilibrium mean charge of fast ions emerging from solids

Equilibrium mean charges of H and 4He ions emerging from solids were measured for many kinds of target materials, and were found to show an oscillatory dependence on target atomic number. This oscillation was found to be the reflection of electron shell structures of the target atoms. The electron capture cross sections of very fast 3He 2+ ions were also measured and interpreted on the same physical ground.

Analytical expressions for fusion spectra produced in ‘‘beam-target’’ fusion reactions

For ‘‘beam-target’’ fusion reactions, collimated measurements of the energy spectrum of one of the reaction products can provide information on the degree of anisotropy of the reacting beam ions. Analytical expressions relating the fusion spectrum to the velocity distribution of the beam ions are given. Application of the expressions to measurements of the spectrum of 15-MeV protons produced by reactions between energetic 3He ions and relatively cold deuterons during ICRF heating in the PLT tokamak indicate that the velocity distribution of fast 3He ions is peaked perpendicular to the tokamak magnetic field.

A Technique for Measuring the Fast3He++Distribution During3He++Minority Icrf Heating

AbstractA technique for measuring the fast 3He++ distribution during 3He++ minority ion cyclotron resonance frequency heating is discussed. The technique involves the use of 10- to 100-keV neutral helium beams to neutralize the fast 3He++ ions by double charge-exchange (3He++ + 4He0 → 3He0 + 4He++). The neutralized fast 3He atoms then escape from the plasma and are detected by conventional neutral particle analyzing apparatuses. By the use of such a technique, the effectiveness of the coupling of the ion cyclotron waves to the 3He++ minority can be measured.

Alpha particle simulation and diagnostics using 3He++ minority ICRF heating

We have investigated the theoretical feasibility of using fast 3He++ ions, produced by ICRF minority heating, to simulate the behavior of fusion-produced 4He++ ions in large fusion experiments. This proposed technique would allow the study of ‘‘alpha particlelike’’ heating in near term experiments, such as TFTR, JET, and JT-60, without the necessity of high levels of neutron production and the subsequent activation. The results of our calculations study show that ICRF minority heating should be able to produce confined fast 3He++ distributions in TFTR-sized experiments with an energy distribution and density similar to those expected for fast alpha particles in fusion reactor experiments, such as INTOR/FED/NET.

An associated particle neutron time of flight system

A fast 3Hen timing system having significantly improved timing resolution has been developed for the D(d, n) 3He reaction at incident deuteron energies below 400 keV. The associated particle technique is used to define a neutron beam at an angle in the laboratory between 40° and 120°, determined by the angle at which the associated 3He particles are detected. Unambiguous 3He particle detection, resulting from the use of a magnetic analyser to shield the surface barrier 3He particle detector from scattered deuterons, allows the absolute neutron flux in the associated neutron beam to be precisely determined. The derivation of a fast timing signal from the detector voltage pulse, enables a constant fraction pulse height discriminator to be triggered on 3He pulses of equivalent energy less than 500 keV without spurious triggering on noise. The 3He-n timing resolution obtained after compensation for the energy dependent 3He flight times is 2.8 ns (fwhm) in the particular case of a 2.40 MeV neutron beam defined at 102°, and 3.2 ns (fwhm) in the case of a 2.80 MeV neutron beam at 50°. The effect of instrumental timing jitter and walk in the system is virtually negligible. Stable neutron beams having intensities of over 120 neutrons sec −1 msterad −1 averaged over the beam profile are maintained for long periods. The random background count rates outside the neutron beam typically correspond to a neutron flux of less than 2 × 10 −5 that of the direct beam.

A 90° 3He Neutron Spectrometer

A fast 3He neutron spectrometer has been developed that is free of recoil spectrum effects. Positioning solid-state detectors outside the neutron beam at 90° results in this advantage and avoids nuclear reactions in the detectors themselves. Two modes of operation are examined: In conjunction with a pulse height analyzer, and in a high-event-rate, current-signal regime. The spectrometer was used in the latter mode to measure the neutron spectrum produced by a nuclear explosive in the Los Alamos time-of-flight neutron cross-section measurements. ? Present address: Rensselaer Polytechnic Institute, Troy, New York