Helium Density Profile Research Articles

Simulation of the time-dependent propagation of leak-triggered helium density gradients in cryogenic and nonisothermal accelerator vacuum systems

A novel simulation code for the calculation of the time-dependent evolution of helium density profiles in tubular-shaped beam vacuum systems of particle accelerators is presented. The code called TransVac was written in the statistics programming language R using the noncommercial development software RStudio and is based on an analytical approach. In contrast to earlier simulation tools based on analytical computational methods, the new code does not only master the profile calculation in conventional vacuum systems operated at room temperature, but also in fully cryogenic and in nonisothermal systems composed of cryogenic and warm sections. Data of helium adsorption isotherms are used to calculate gas densities profiles in cold vacuum systems with the cryosorption-based wall-pumping effect. The article discusses how the simulation code works and which mathematical algorithm is used. Comparisons between experimental and theoretical data confirm that the software developed provides sufficiently reliable predictions on the propagation behavior of leak-triggered helium pressure waves in cryogenic and room-temperature vacuum systems as well.

Understanding helium transport: experimental and theoretical investigations of low-Z impurity transport at ASDEX Upgrade

The presence of helium is fundamentally connected to the performance of a fusion reactor, as fusion-produced helium is expected to heat the plasma bulk, while He ‘ash’ accumulation dilutes the fusion fuel. An understanding of helium transport via experimentally validated theoretical models of the low-Z impurity turbulent transport is indispensable to predict the helium density profile in future fusion devices. At ASDEX Upgrade, detailed, multi-species investigations of low-Z impurity transport have been undertaken in dedicated experiments, resulting in an extensive database of helium and boron density profiles over a wide range of parameters relevant for turbulent transport (normalised gradients of the electron density, the ion temperature, and the toroidal rotation profiles, the collisionality and the electron to ion temperature ratio). Helium is not found to accumulate in the parameter space investigated, as the shape of the helium density profile follows largely that of the electron density. Helium is observed to be as peaked as the electron density at high electron cyclotron resonance heating fraction, and less peaked than the electron density at high neutral beam heating fraction. The boron density profile is found to be consistently less peaked than the electron density profile. Detailed comparisons of the experimental density gradients of both impurities with quasilinear gyrokinetic simulations have shown that a qualitative agreement between experiment and theory cannot always be obtained, with strong discrepancies observed in some cases.

Modeling of helium transport and exhaust in the LHD edge

Experimental results from LHD show a reduction of helium concentration in the plasma with the introduction of a magnetic island on the m/n = 1/1 resonant surface in the plasma edge. Simulations of the plasma with and without the island are carried out with the coupled code EMC3-EIRENE and compared to charge exchange recombination spectroscopy measurements of ionized core helium, and visible spectroscopy measurements of edge neutral helium. The numerical simulations indicate that the experimental parameters lie in a high density regime where the impurity transport is dominated by the outward directed friction force. The EMC3-EIRENE simulations capture the reduction in helium transport well and indicate that: (1) the reduction in core helium is a result of increased outward transport caused by the magnetic island and an increased opening of the edge-surface layer to the divertor plates; (2) the dominant source of neutral helium is best modeled by recycled helium at the targets; and (3) ionized helium density profiles are best matched in the simulations when there is a large core helium source in addition to a smaller edge source.

Helium transport in the core and stochastic edge layer in LHD

Radial profiles of the density ratio of helium to hydrogen ions are measured using charge exchange spectroscopy with a two-wavelength spectrometer in the large helical device. Helium transport at the last closed flux surface (LCFS) and stochastic magnetic field layer outside the LCFS as well as in the core plasma is studied for a wide range of helium fractions, i.e. from hydrogen-dominated plasmas up to helium-dominated plasmas. The helium density profile becomes more peaked and inward convection velocity increases in the hydrogen-dominant plasma, while it becomes flat or hollow and the convection velocity is in the outward direction in the helium-dominant plasmas. The density gradient of helium at the LCFS is twice that of hydrogen and becomes steeper as the hydrogen becomes more dominant.

Charge dependence of neoclassical and turbulent transport of light impurities on MAST

Carbon and nitrogen impurity transport coefficients are determined from gas puff experiments carried out during repeat L-mode discharges on the Mega-Amp Spherical Tokamak (MAST) and compared against a previous analysis of helium impurity transport on MAST. The impurity density profiles are measured on the low-field side of the plasma, therefore this paper focuses on light impurities where the impact of poloidal asymmetries on impurity transport is predicted to be negligible. A weak screening of carbon and nitrogen is found in the plasma core, whereas the helium density profile is peaked over the entire plasma radius. Both carbon and nitrogen experience a diffusivity of the order of 10 m2s−1 and a strong inward convective velocity of ∼40 m s−1 near the plasma edge, and a region of outward convective velocity at mid-radius. The measured impurity transport coefficients are consistent with neoclassical Banana-Plateau predictions within . Quasi-linear gyrokinetic predictions of the carbon and helium particle flux at two flux surfaces, and , suggest that trapped electron modes are responsible for the anomalous impurity transport observed in the outer regions of the plasma. The model, combining neoclassical transport with quasi-linear turbulence, is shown to provide reasonable estimates of the impurity transport coefficients and the impurity charge dependence.

Feasibility of non-thermal helium measurements with charge exchange spectroscopy on ITER

The use of active charge exchange recombination spectroscopy (CXRS) as a diagnostic for fusion-produced alpha particles on ITER is constrained by the signal-to-noise ratio, which is determined by the intensity of the line of interest, the optical throughput of the diagnostic, the neutral beam penetration, and the intensity of bremsstrahlung radiation. The CX spectral line for fast ions has been modelled together with the expected background emission and we present the signal-to-noise ratios calculated as a function of the diagnostic design parameters. Combining the CXRS data from both the heating and the diagnostic neutral beams on ITER, information on fast ions with energies up to 1 MeV can be obtained for the parameters of the ITER core CXRS diagnostic design. To achieve this, energy binning of the signal is used (100 keV bins or larger), in order to improve the signal-to-noise ratio, with a time resolution of 2 s. The time resolution of the measurement can be improved using a higher throughput spectrometer, but this is ultimately limited by the amount of light from the neutral beam that can be collected. Despite the challenges and the fact that the results are not as optimistic as previously assumed, it is concluded that useful information on fast helium density profiles can be obtained using CXRS on ITER.

Simulations of ITER with combined effects of internal and edge transport barriers

Predictive simulations of ITER with the presence of both an edge transport barrier (ETB) and an internal transport barrier (ITB) are carried out using the BALDUR integrated predictive modelling code. In these simulations, the boundary is taken at the top of the pedestal, where the pedestal values are described using theory-based pedestal models. These pedestal temperature models are based on three different pedestal width scalings: magnetic and flow shear stabilization (Δ ∝ ρis2), flow shear stabilization () and normalized poloidal pressure (). The pedestal width scalings are combined with a pedestal pressure gradient scaling based on the ballooning mode limit to predict the pedestal temperature. A version of the semi-empirical Mixed Bohm/gyroBohm (Mixed B/gB) core transport model that includes ITB effects is used to compute the evolution of plasma profiles. In this model, the anomalous transport in the core is stabilized by the influence of Er × B flow shear and magnetic shear, which results in the formation of ITB. The combination of the Mixed B/gB core transport model with ITB effects, together with the pedestal model, is used to simulate the time evolution of plasma current, temperature, and density profiles for ITER standard type I ELMy H-mode discharges. It is found that ITER fusion performance using the BALDUR code with Mixed B/gB transport model without the presence of ITB is quite pessimistic (Fusion Q ∼ 3). The presence of ITB is crucial and can result in a significant improvement, which is needed for achieving a target Fusion Q of 10. The improvement due to the presence of ITB is almost the same for all simulations with those three pedestal temperature models. This is caused by the predicted pedestal temperature from each pedestal temperature model varying just slightly. The presence of ITB has a strong impact on both temperature profiles, especially near the centre of the plasma, but has a small impact on electron, deuterium, tritium and carbon density profiles, except the helium density profile. The formation of ITB does not impact on the pedestal. It is also found that during a sawtooth crash, the temperature profiles drop significantly, but there is a small change in the density profiles. However, the sawtooth oscillation has no impact on the pedestal. When the auxiliary heating power is turned off, it is found that significant fusion power is sustained.

Unraveling the Absorption Spectra of Alkali Metal Atoms Attached to Helium Nanodroplets

The absorption spectra of the first electronic exited state of alkali metal atoms on helium nanodroplets formed of both 4He and 3He isotopes were studied experimentally as well as theoretically. In the experimental part new data on the 2p<--2s transition of lithium on 3He nanodroplets are presented. The absorption spectrum changes drastically when compared to 4He droplets, in contrast to sodium where only marginal differences were observed in former studies. To explain these large differences and to answer some still open questions concerning the interaction of alkali metal atoms with helium nanodroplets, a model calculation was performed. New helium density profiles as well as a refined model allowed us to achieve good agreement with the experimental findings. For the first time the red-shifted intensities in the lithium and sodium spectra are explained in terms of enhanced binding configurations in the excited state displaced spatially from the ground state configurations.

Conceptual design and integration of a diagnostic neutral beam in ITER

In ITER, one of the key issues to achieve 400 s driven-burn operation at Q about 10 (Technical Basis for the ITER-FEAT Outline Design—Section I-3.2.2.IAEA) is helium ash accumulation. As a result, the real-time measurement of the thermalised helium density profile in the confinement region is of fundamental importance. This paper outlines the design of a modulated, 100 keV, hydrogen, diagnostic neutral beam (DNB), together with preliminary calculations of the performance for the measurement of helium ash by charge exchange recombination spectroscopy (CXRS). The DNB uses a negative ion beam as the primary beam to achieve a required performance with acceptable system efficiency (a higher neutralisation efficiency and smaller beam divergence). This uses the same negative ion source as the ITER H&CD injectors, coupled to a single stage accelerator. The design concept and hence the geometry of the beamline components, i.e. the neutraliser and the residual ion dump and the calorimeter, are similar to those utilised for the H&CD injectors with some minor differences due to the lower beam energy and power. An overview of the design is presented giving the basic information on beam transmission and neutralisation, gas flow and pumping. Selected aspects of the design of the high heat flux components are included. Specific layout and safety requirements of the ITER plant require the installation of the DNB within the common secondary confinement of the NB H&CD system (the NB cell); as a consequence, the DNB has to share the vacuum vessel access through a single port with one of the H&CD injectors.

Observation of tritium and helium core transport barriers in reversed shear plasmas on TFTR

Tritium and helium gas-puff experiments have been conducted on TFTR to investigate transport in reverse shear plasmas. Small amounts of tritium and helium gas are puffed into the equilibrium phase of these plasmas. Measurements of the 14 MeV neutron fusion product and the charge-exchange spectrum of helium reveal the transport details of tritium and helium, respectively. The inferred tritium and helium density profiles indicate the formation of a core transport barrier for each species in reverse shear plasmas with enhanced confinement.

Study of helium transport in HL-1M edge plasma region

Radial profiles of the emission intensity for H α and neutral helium line ( λ = 5875.7 A ̊ ) are measured to study helium transport and recycling in the HL-1M edge region in the vicinity of the poloidal graphite limiter (at r = 26 cm) after previous helium glow discharge cleaning and boronization under different LHCD and ECH conditions. A one-dimensional Monte Carlo neutral helium transport code, 1DHET, is utilized to calculate neutral helium density profile in the same region. The comparisons show that the freshly boronized wall with helium glow discharge cleaning exhibits somewhat selective recycling characteristics between the helium and hydrogen atoms, if ECH power is not applied.

Progress toward a microsecond duration, repetitively pulsed, intense-ion beam for active spectroscopic measurements on ITER

We describe the design of an intense, pulsed, repetitive, neutral beam based on magnetically insulated diode technology for injection into ITER for spectroscopic measurements of thermalizing alpha particle and thermal helium density profiles, ion temperature, plasma rotation, and low Z impurity concentrations throughout the confinement region. The beam is being developed to enhance low signal-to-noise ratios expected with conventional steady-state ion beams because of severe beam attenuation and intense bremsstrahlung emission. A 5 GW (e.g., 100 keV, 50 kA) 1 μs duration beam would increase the charge exchange recombination signal by 103 compared to a conventional 5 MW beam.

Vibrational frequency shift of HF in helium clusters: Quantum simulation and experiment

We report accurate variational and diffusion quantum Monte Carlo calculations for the size dependence of the vibrational frequency shift of HF molecules embedded in helium clusters with up to n=198 helium atoms. The frequency shift exhibits a strong initial size dependence and saturates at a redshift of about 2.7 ± 0.1 cm−1 for clusters with over 100 atoms. This value is in good agreement with our experimental redshift of 2.65 ± 0.15 cm−1 for clusters with over 1000 atoms. The helium cluster is found to undergo significant structural changes upon embedding of HF. The density in the nearest neighbor shell exceeds the bulk helium density by a factor of two. A second nearest neighbor density maximum and a peripheral density plateau very close to the bulk helium value is found. In spite of the anisotropic interaction between HF and helium all clusters have almost perfectly spherical helium density profiles and indicate close to free rotor behavior of HF inside the cluster. The cluster size dependence of the redshift can be qualitatively described by an induced dipole model.

Dopant location in SF6He39,40

Recent quantum Monte Carlo studies of doped helium clusters have yielded different results for the location of the SF6 impurity, despite good agreement on helium density profiles, thus raising the question of wave function bias on structural properties. We present here a systematic analysis of the effect of the trial function on variational and diffusion Monte Carlo (VMC and DMC) results for the ground state of SF6HeN (N=39 and 40). Four different sets of wave functions are used, together with isotropic pairwise potentials. Use of a two-peak term in the He–SF6 wave function to describe the extensive helium structuring induced by the impurity greatly improves the VMC energies and helium densities. For all of the wave functions, the impurity SF6 distribution has its maximum at the cluster center in both VMC and DMC. This result agrees with the conclusion previously presented by Barnett and Whaley, but it contradicts the recent DMC result of Chin and Krotscheck. To explain this discrepancy, we analyze the amount of sampling necessary to ensure a reliable description of the SF6 density. We find that considerably more sampling is required for the impurity than for the helium density or the energy. Similar considerations are shown to affect the extent of structure seen in pure HeN density profiles.

Helium transport and exhaust studies in enhanced confinement regimes in DIII-D

A better understanding of helium transport in the plasma core and edge in enhanced confinement regimes is now emerging from recent experimental studies on DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159]. Overall, the results are encouraging. Significant helium exhaust (τHe*/τE∼11) has been obtained in a diverted, H-mode plasma with edge-localized modes (ELM’s) simultaneous with a central source of helium. There is no evidence of central peaking of the helium density profile even in the presence of this central source. Detailed analysis of the helium profile evolution indicates that the exhaust rate is limited by the exhaust efficiency of the pump (∼5%) and not by the intrinsic helium transport properties of the plasma. Perturbative helium transport studies using gas puffing have shown that DHe/χeff ∼1 in all confinement regimes studied to date (including H mode and VH mode). Furthermore, there is no evidence of preferential accumulation of helium in any of these regimes.

Diagnostics for the study of helium exhaust in DIII-D (abstract)

Recent studies have shown that fairly weak dilution of the D-T fuel mixture by the thermal alpha particle population may quench ignition in ITER. The alpha particle exhaust issue is not well understood experimentally, especially in H mode. On DIII-D, a large set of diagnostics for studying alpha particle thermalization and exhaust are being developed. Helium density profiles in the core plasma are inferred from data obtained by the existing DIII-D charge-exchange recombination spectroscopy system which has 32 chords that span the plasma cross section and has been absolutely calibrated so that absolute emission intensities can be inferred from the measured data. Relative chord-to-chord intensity calibrations have been made by injecting neutral beams into neutral gas with no magnetic fields. A multifiber system viewing the divertor floor has been absolutely calibrated to measure the hydrogen and helium recycling profile across the divertor floor. Other diagnostics in development include a Penning discharge gauge to measure helium partial pressure under the divertor baffle in DIII-D. Preliminary measurements from these diagnostics will be presented. This work supported by U.S. Department of Energy (DOE) Contract DE-AC03-89ER51114 and by appointment to the DOE Fusion Energy Postdoctoral Research Program administered by Oak Ridge Associated Universities.