Neutral Particle Flux Research Articles

An analysis of the influence of impurities on fast particle attenuation and on fast ion spectral shape in LHD

Neutral particle fluxes measured by neutral particle analyzers can provide information about the ion temperature as well as the non-Maxwellian anisotropic ion distribution tails from neutral beam injection and ion cyclotron radio frequency heating. In the case of multidirectional diagnostics employing high resolution atomic energy spectrometers, the neutral atomic flux source is not localized in contrast to pellet charge exchange or diagnostic neutral beam methods. The correct interpretation of such measurements from plasma in a complex toroidally asymmetric geometry, like that of LHD, requires careful numerical modeling of the neutral flux formation. Previously a measured neutral flux calculation scheme was developed and was used for the LHD geometry and a suitable analytic expression for ionization cross sections sigma(s)(z)(E,n(e),T(e),Z(eff)) of impurities was formulated by Janev et al. [Nucl. Fusion 29, 2125 (1989)]. In this paper, the attenuation of fast particles by impurities is incorporated into the neutral flux calculation scheme and the influence of impurities on the calculated neutral flux spectra is shown. Finally, the behavior of the calculated and experimental suprathermal particle distributions is compared for pure hydrogen and for argon impurity seeded plasmas.

Energy confinement and magnetic field generation in the SSPX spheromak

The Sustained Spheromak Physics Experiment (SSPX) [Hooper et al., Nuclear Fusion 39, 863 (1999)] explores the physics of efficient magnetic field buildup and energy confinement, both essential parts of advancing the spheromak concept. Extending the spheromak formation phase increases the efficiency of magnetic field generation with the maximum edge magnetic field for a given injector current (B∕I) from 0.65T∕MA previously to 0.9T∕MA. We have achieved the highest electron temperatures (Te) recorded for a spheromak with Te>500eV, toroidal magnetic field ∼1T, and toroidal current (∼1MA) [Wood et al., “Improved magnetic field generation efficiency and higher temperature spheromak plasmas,” Phys. Rev. Lett. (submitted)]. Extending the sustainment phase to >8ms extends the period of low magnetic fluctuations (<1%) by 50%. The NIMROD three-dimensional resistive magnetohydrodynamics code [Sovinec et al., Phys. Plasmas 10, 1727 (2003)] reproduces the observed flux amplification ψpol∕ψgun. Successive gun pulses are demonstrated to maintain the magnetic field in a quasisteady state against resistive decay. Initial measurements of neutral particle flux in multipulse operation show charge-exchange power loss <1% of gun input power and dominantly collisional majority ion heating. The evolution of electron temperature shows a distinct and robust feature of spheromak formation: A hollow-to-peaked Te(r) associated with q∼1∕2.

Is astronomy possible with neutral ultrahigh energy cosmic ray particles existing in the standard model?

The recently observed correlation between HiRes stereo cosmic ray events with energies E ∼ 1019 eV and BL Lacertae objects occurs at an angle that strongly suggests that the primary particles are neutral. We analyze whether this correlation, if not a statistical fluctuation, can be explained within the Standard Model, i.e., assuming only known particles and interactions. We have not found a plausible process that can account for these correlations. The mechanism that comes closest—the conversion of protons into neutrons in the IR background of our Galaxy—still underproduces the required flux of neutral particles by about two orders of magnitude. The situation is different at E ∼ 1020 eV, where the flux of cosmic rays at the Earth may contain up to a few percent of neutrons, indicating their extragalactic sources.

High Heat Flux Performance of Plasma Facing Materials and Components Under Service Conditions in Future Fusion Reactors

The first wall and the divertor in present-day or next step thermonuclear fusion devices are exposed to intense fluxes of charged and neutral particles, in addition the plasma facing materials and components are subjected to radiation in a wide spectral range. These processes, in general referred to as ’plasma wall interaction’ will have strong influence on the plasma performance, and moreover, they have major impact on the degradation and on the lifetime of the plasma facing armour and the joining interface between the plasma facing material and the heat sink. Beside physical and chemical sputtering processes, thermal fatigue damage due to cyclic heat fluxes during normal operation and intense thermal shocks caused by severe thermal transients are of serious concern for the engineers which develop reliable wall components. In addition, the material and component degradation due to high fluxes of energetic neutrons is another critical issue in D-T-burning fusion devices which requires further extensive research activities. This paper represents a tutorial focussed on the development and characterization of plasma facing components for thermonuclear fusion devices.

Steady-state operation and high energy particle production of MeV energy in the Large Helical Device

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. High temperatures of approximately 1–2 keV, and steady-state plasma sustainment operations have been reported. Recently the steady-state operation regime was greatly extended in the Large Helical Device (LHD). A high-temperature plasma was created and maintained for 54 min with 1.6 GJ in the 2005FY experimental programme. The three-dimensional heat-deposition profile of the LHD helical divertor was modified, and during long-pulse discharges it effectively dispersed the heat load using a magnetic axis swing technique developed at the LHD. A sweep of only 3 cm in the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies and partially by electron cyclotron of fundamental resonance frequency. By accumulating the small flux of charge-exchanged neutral particles during the long-pulse operation, a high energy ion tail which extended up to 1.6 MeV was observed. This is the first experimental evidence of high energetic ion confinement of MeV range in helical devices. The long-pulse operations lasted until a sudden increase in radiation loss occurred, presumably because of metal wall flakes dropping into the plasma. The sustained line-averaged electron density and temperature were approximately 0.8 × 1019 m−3 and 2 keV, respectively, at a 1.3 GJ discharge (#53776) and 0.4 × 1019 m−3 and 1 keV at a 1.6 GJ discharge (#66053). The average input power was 680 kW and 490 kW, and the plasma duration was 32 min and 54 min, respectively. These successful long operations show that the heliotron configuration has a high potential as a steady-state fusion reactor.

Confinement degradation and transport of energetic ions due to Alfvén eigenmodes in JT-60U weak shear plasmas

The confinement degradation and transport of energetic ions due to Alfvén eigenmodes (AEs) destabilized by negative-ion-based neutral beam injection are quantitatively evaluated for the first time. AEs were observed in JT-60U weak shear plasmas with frequencies sweeping up to 30–50 kHz on a time scale of a few hundred milliseconds as the safety factor at the plasma centre, q(0), decreases from 1.75 to 1.5. Then as q(0) decreases from 1.5 to 1.25 in about 600 ms, the AE frequencies are saturated and the AEs are identified as the TAEs. The measured total neutron emission rate (Sn) in the presence of these AEs is compared with that predicted by classical theory. As a result, the confinement degradation of energetic ions is confirmed and the reduction rate in Sn is largest when AEs are destabilized and are located at the large fast ion pressure gradient region in the plasma core. In order to investigate the energetic ion transport due to these AEs, line-integrated neutron emission profiles measured with a large neutron collimator array are compared with the classical confinement calculations. It is found that energetic ions are transported from the plasma core region to the outer region due to these AEs. Furthermore, changes in the energy distribution of charge exchange energetic neutral particle fluxes suggest that the radial energetic ion transport is due to the resonance interaction between the energetic ions and the AEs.

Poloidal Distribution Measurement of Inward Neutral Flux in LHD

Three emission lines of neutral helium, i.e., the λ 667.8 nm (21P-31D), λ 728.1 nm (21P-31S), and λ 706.5 nm (23P-33S) lines, are observed with an array of parallel lines-of-sight which covers an entire poloidal cross section of the plasma in the Large Helical Device (LHD). Their emission locations and intensities are determined from the Zeeman profiles. The electron temperature and density are evaluated from intensity ratios among the observed emission lines at each emission location and the poloidal distribution of ionization flux or its equivalent quantity, inward neutral particle flux, is derived with the help of collisional-radiative model calculations. A relatively strong neutral flux is observed at the inboard-side X-point and this result implies a high particle recycling in the inboardside divertor region. The result is consistent with the ion flux distribution onto the helically located divertor plates measured with Langmuir probes.

Neutron Emission Profile Measurement and Fast Charge Exchange Neutral Particle Flux Measurement for Transport Analysis of Energetic Ions in JT-60U

This paper describes diagnostics for transport analysis of energetic ions in JT-60U. Line integrated neutron emission profiles are measured using Stilbene neutron detectors, which are installed as a neutron emission profile monitor. The flux and energy distribution of charge exchange neutral particles are measured using a natural diamond detector (NDD). These measurements in the presence of bursting instabilities in the frequency range of Alfven Eigenmode induced by negative-ion-based neutral beam injection, which are called Abrupt Large-amplitude Events, indicate that energetic ions are transported from the core region in the plasma due to a resonance interaction between energetic ions and the instabilities. Development of a digital signal processing (DSP) system for a neutron detector and a NDD enabling a sampling rate in the MHz class is proceeding. The DSP system for the neutron detector successfully discriminates between neutrons and gamma-rays in tests using neutron/gamma-ray sources.

Multi-Functional Diagnostic Method with Tracer-Encapsulated Pellet Injection

In order to obtain a better understanding of impurity transport in magnetically confined plasmas, a Tracer-Encapsulated Soild PELlet (TESPEL) has been developed. The essential points of the TESPEL are as follows: the TESPEL has a double-layered structure, and a tracer impurity, the amount of which can be known precisely, is embedded as an inner core. This structure enables us to deposit the tracer impurity locally inside the plasma. From experiences of developing the TESPEL production technique and its injection experiments, it became clear that various plasma properties can be studied by the TESPEL injection. There are not only impurity transport in the plasma but also transport both outside and inside of the magnetic island O-point, heat transport and high-energy neutral particle flux. Therefore, the TESPEL injection has a favorable multi-functional diagnostic capability. Furthermore a Tracer-Encapsulated Cryogenic PELlet (TECPEL) has been also developed. The TECPEL has an advantage over the TESPEL in terms of no existence of carbons in the outer layer. The TECPEL injector was installed at LHD in December 2005, and the preliminary injection experiments have been carried out.

Local pellet based and line-integrated nonperturbing charge exchange measurements with a compact neutral particle analyzer on Large Helical Device

Experiments on suprathermal ion physics are carried out on the Large Helical Device (LHD) with a 40 channel compact neutral particle analyzer (CNPA). The analyzer is used both in passive nonperturbing chord-integral neutral particle flux measurements and for local probing with an impurity pellet. The analyzer observes trapped particles with ∣v‖∕v∣<0.25 and the pitch angle cosine value v‖∕v changes sign along the diagnostic sight line. The measurable energy range for H0 is 1–170keV. Radially resolved data on local H0 atomic energy spectra have been obtained on LHD by the pellet charge exchange method with CNPA, as well as chord-integral atomic energy distribution data for electron cyclotron heated, neutral beam injected, and high power ion cyclotron heated plasma. The article addresses the diagnostic technical background and the analysis scheme of measurement results taking into account the helical geometry and the species and densities of the charge exchange targets.

Chord integrated neutral particle diagnostic data analysis for neutral beam injection and ion cyclotron radio frequency heated plasma in a complex Large Helical Device geometry

Energy and angle-resolved measurements of charge exchange neutral particle fluxes from the plasma provide information about Ti, as well as non-Maxwellian substantially anisotropic ion distribution tails due to neutral beam injection (NBI) and ion cyclotron radio frequency (ICRF) heating. The measured chord integral neutral flux calculation scheme for the Large Helical Device magnetic surface geometry is given. Calculation results are shown for measurable atomic energy spectra corresponding to heating-induced fast ion distributions from simplified Fokker-Planck models. The behavior of calculated and experimental suprathermal particle distributions from NBI and ICRF heated plasma is discussed in the context of the experimental data interpretation.

Observation of confinement degradation of energetic ions due to Alfvén eigenmodes in JT-60U weak shear plasmas

Confinement degradation of energetic ions due to Alfvén eigenmodes (AEs) has been investigated with negative ion based neutral beam injection at ∼370 keV into JT-60U weak shear plasmas. AEs with a rapid frequency sweeping and then saturation as the minimum value of the safety factor decreases have been observed. This frequency behaviour can be explained by the reversed-shear induced AE (RSAE) and the transition from RSAEs to the toroidicity-induced AEs (TAEs). The associated change in the charge exchange neutral particle flux suggests energetic ion transport due to these AEs. The total neutron emission reduction rate in the presence of these AEs is evaluated by calculation using the orbit following Monte-Carlo code taking into account the change in bulk plasmas. The evaluation indicates confinement of energetic ions is degraded due to these AEs. In particular, it is found that the confinement degradation of energetic ions is maximum during the transition from RSAEs to TAEs and the maximum reduction rate (ΔSn/Sn)MAX is estimated to be ∼45%.

Neutral particle analyzer diagnostics on the TCV tokamak

Experimental apparatus and data analysis techniques used in neutral particle analyzer (NPA) diagnostics on the Tokamak à Configuration Variable (TCV) are described. Two NPAs are used on TCV to measure the energy spectrum of neutral particle fluxes from the plasma. The “five-channel energy analyzer of atomic particles” used in double electrical analysis mode with fast voltage sweeping detect particles without atomic mass discrimination in the energy range of 0.6–8.0keV with a time resolution of 0.5–2.0ms and an energy resolution of 7%–20%. The 28-channel “compact neutral particle analyzer” (CNPA) is an EIIB spectrometer with mass and energy separations designed for medium sized fusion machines featuring a carbon neutral stripping foil, a permanent magnet for dispersion, and channel-electron multiplier detectors. The CNPA simultaneously detects two mass species [hydrogen (H) and deuterium (D) or D and helium (He)] in the 0.5–50keV energy range with a resolution of 60%–10% and a time resolution of 0.5–4.0ms. The CNPA views the plasma across the path of the diagnostic neutral beam and can perform active charge-exchange NPA measurement. Data analysis procedures and numerical algorithms developed for NPA measurement are routinely used on TCV to obtain information on the plasma ion temperature, ion energy distribution function, plasma isotope ratios, and other plasma characteristics.

Interstellar Helium Trapped with the COLLISA Experiment on theMiRSpace Station—Improved Isotope Analysis by In Vacuo Etching

We have redetermined the helium isotopic composition of the local interstellar cloud (LIC) in the framework of the Swiss-Russian Collection of Interstellar Atoms (COLLISA) collaboration. Based on in vacuo etching analyses of foils that have been exposed to the interstellar neutral particle flux on board the Mir space station, we obtain (3He/4He)LIC = (1.62 ± 0.29) × 10-4. This is the most precise determination of the He isotopic composition of the LIC, with errors being lower by more than a factor of 2 compared to an earlier experiment on similar foils and compared to the analysis of interstellar pick-up ions. Comparing our improved result with current models of Galactic chemical evolution reveals that close to 100% of all Galactic low-mass stars must have undergone extra mixing associated with cool bottom processing. The 3He abundance relative to 4He in the LIC is within the uncertainty identical to the value of (1.66 ± 0.06) × 10-4 inferred for the protosolar cloud (PSC), showing that no significant evolution of the 3He abundance took place since 4.6 Gyr ago at the solar distance (~8 kpc) from the Galactic center. Our value sets new rigorous constraints on Galactic evolution models. Since the Big Bang nucleosynthesis theory predicts a value of ≤(1.1 ± 0.2) × 10-4, a noticeable increase of the relative abundance of 3He has apparently occurred in the Galaxy before the formation of the solar system, but, contrary to earlier expectations, only a modest or negligible increase is registered after the formation of the solar system.

High Heat Flux Performance of Plasma Facing Materials and Components under Service Conditions in Future Fusion Reactors

The first wall and the divertor in present-day or next step thermonuclear fusion devices are exposed to intense fluxes of charged and neutral particles, in addition the plasma facing materials and components are subjected to radiation in a wide spectral range. These processes, in general referred to as ‘plasma wall interaction’ will have strong influence on the plasma performance, and moreover, they have major impact on the degradation and on the lifetime of the plasma facing armour and the joining interface between the plasma facing material and the heat sink. Beside physical and chemical sputtering processes, thermal fatigue damage due to cyclic heat fluxes during normal operation and intense thermal shocks caused by severe thermal transients are of serious concern for the engineers which develop reliable wall components. In addition, the material and component degradation due to high fluxes of energetic neutrons is another critical issue in D-T-burning fusion devices which requires further extensive research activities. This paper represents a tutorial focussed on the development and characterization of plasma facing components for thermonuclear fusion devices.

Study of global wall saturation mechanisms in long-pulse ELMy H-mode discharges on JT-60U

Variation of particle absorption at the first wall has been investigated in long-pulse (∼30 s) ELMy H-mode discharges on JT-60U. Quantitative analysis of particle balance indicated that particle inventory at the first wall was globally saturated with a time scale of 10–15 s after several long-pulse discharges. To understand mechanisms of the global wall saturation, distribution of a local wall saturation time on the first wall was calculated from the ion and neutral particle fluxes to the first wall evaluated using a Monte-Carlo neutral particle transport code. The local wall saturation time was estimated to be shorter than 1 s at the divertor plates and the divertor dome, ∼10 s at the lower half of the baffle plates and ∼100 s at the main chamber wall, respectively. This result suggested that the divertor plates, the divertor dome and the lower half of the baffle plates were saturated in a single discharge. On the other hand, the main chamber wall was not saturated in a single discharge. Based on the above result, a model of the global wall saturation was proposed, where dynamic and static inventory regions are defined depending on the wall temperature.

Energetic ion transport by abrupt large-amplitude event induced by negative-ion-based neutral beam injection in the JT-60U

To investigate energetic ion transport induced by bursting modes in the frequency range of Alfvén eigenmodes, which is called abrupt large-amplitude events (ALEs) driven by negative-ion-based neutral beam (N-NB) injection, neutron emission profile measurement and charge exchange (CX) neutral particle (flux) measurement using a natural diamond detector have been performed simultaneously in JT-60U. It is found from the CX neutral particle (flux) measurement that energetic neutral particles in a limited energy range (100–370 keV) are enhanced due to ALEs, and the neutron radial profile is flattened. The change in the energetic ion density profile inferred from these measurements indicates that ALEs expel energetic ions from the core region of the plasma and induce both redistribution and loss of energetic ions. It has been shown that the energy range of transported energetic ions is consistent with a resonance condition between energetic ions and ALEs, and the energetic ion transport results from the resonant interaction between energetic ions and ALEs. Further, a fraction of the energetic ion loss has been quantitatively estimated to be ∼4%.

Integrated exhaust scenarios with actively controlled ELMs

An integrated radiative high performance scenario has been established at ASDEX Upgrade based on simultaneous feedback control of the average divertor neutral particle and power flux in combination with a high, pellet induced frequency of edge localized modes (ELMs). This approach is fully compatible with the present tungsten wall coating covering about 65% of the plasma facing components and is intended for application in the envisaged full-tungsten experiment. In these experiments, divertor recycling and effective divertor temperature (derived from thermoelectric currents) were tuned by acting on fuel gas puff and argon injection rates. The ELM frequency (fELM) was kept high by repetitive injection of small cryogenic deuterium pellets to avoid the radiative instabilities seen at low fELM and high radiated power, and to control the ELM energy. No confinement loss is observed in this radiative type-I ELMy scenario with relatively flat density profiles. In contrast, similar type-III ELM scenarios achieved in hydrogen show a confinement loss of 25% as compared to the type-I phase. In parallel to pellets, alternative ELM trigger techniques have been investigated as well. Fast vertical plasma oscillations are able to synchronize the ELM frequency to values higher and lower than the intrinsic fELM, but remain to be tested in the integrated scenario. Supersonic gas injection showed better fuelling efficiencies than usual gas puffing but instantaneous ELM release has not been achieved. A particular experimental challenge for AUG conditions is to obtain a high pace making frequency, to establish scalings of confinement and energy loss as a function of controlled ELM frequency.

Charge exchange neutral particle measurements with natural diamond detector under the deuterium–deuterium neutron field on JT-60U tokamak

A natural diamond detector (NDD) has been installed on the JT-60U tokamak to measure the flux and the energy distribution of charge exchange (CX) fast neutral particles. A NDD has many important advantages to be used as a CX neutral particle analyzer, for example very compact size, high energy resolution, and high radiation hardness etc., while the neutrons and γ rays are a large noise source in the deuterium plasma. The shield was set up around the NDD to reduce those noises. Time-resolved energy distribution of CX neutral particles corresponding to injected beam energy have been successfully obtained under high intensity neutron yield Yn>1015 n/s. Further enhanced neutral particle fluxes during sawtooth oscillation and Alfvén eigenmodes were observed with the NDD. The performance of the NDD as CX neutral particle spectrometer under high intensity neutron yield was demonstrated for the first time on JT-60U in this work.

Energy-Resolved Neutral Particle Fluxes in TJ-II ECRH Plasmas

Energy-resolved neutral particle fluxes are studied in the TJ-II stellarator by using measurements from a neutral particle analyzer. The average ion energy can be deduced up to positions outside the last closed magnetic surface because of an upgrade of the diagnostic. The results suggest that the average ion energy profile is flat, even at positions outside the last closed magnetic surface, which implies the existence of hot ions well outside the plasma. Such a flat profile may be related to wide ion orbits connecting distant areas of the plasma.