Main Plasma Ions Research Articles

ICRH-related impurity source and control across experiments in H, D, T plasmas at JET-ILW

The experimental and theoretical analysis were focused on experiments conducted to assess the effect of plasma isotopes, protium (H), deuterium (D), and tritium (T) on ion cyclotron resonance heating (ICRH) related plasma wall interactions. Comparison of L-mode discharges with N = 1 3He and N = 1 H minority ICRH heating scenarios were done for different isotopes. For the selected pulses, the behaviour of high-Z, mid-Z and low-Z intrinsic impurity and radiated power behaviour was investigated based on data from VUV, visible spectroscopy, and bolometry diagnostic at Joint European Torus. It was found that for N = 1 3He scenario during radiofrequency antennas operation, core W, Ni content, Be source and the radiated power are higher for π/2 in comparison to dipole antenna phasing. Lowest core Ni, W content and radiated power is clearly observed for H plasmas in comparison to D and T, where for this ICRH scenario behaviour was similar. However, lower Be photon flux is observed for T in comparison to D plasmas. Be sputtering by He particles is responsible for such an effect. Additionally, several computer simulations were conducted using the COREDIV code. The difference in the electron temperature was due to the difference in the isotope masses. Increased temperature in the central plasma in the case of T plasmas leads to higher radiation in the central plasma in comparison to H plasmas. As a result, the power across separatrix is lower and the temperature on the divertor plate decreases with the increase of the isotope mass. At these temperatures on the divertor plate, W is not sputtered by the main plasma ions H, D and T and by He. For the N = 1 H ICRH scenario clear difference between D and T plasma was observed with higher metallic impurity content for T plasma in comparison to D. Impurity content in the plasmas is found to be sensitive to the power balance between the antenna straps. Its minimum is observed for the maximum of P cen/P tot.

Two-Gaussian Fitting Method for Charge Exchange Spectroscopy on Measurements of Ion Temperature and Toroidal Plasma Flow Velocity in the KSTAR Tokamak

Charge exchange spectroscopy has been widely used in fusion devices to measure ion temperature, and toroidal and poloidal flow velocities of plasma. For local measurement, especially in the core region of the plasma, the spectrum emitted by the charge exchange reaction between the main plasma ions or impurity ions and the intentionally injected neutral beam should be analyzed so that parameters can be accurately deduced. Since the line-integrated spectrum signal through the line of sight of the diagnostic optics usually contains an unnecessary overlapped spectrum signal, referred to as the background signal, that typically originates from the plasma boundary region, a beam modulation technique is commonly applied to separate the background signal from the measured spectrum. Recently, it has been demonstrated in the KSTAR tokamak that a two-Gaussian fitting (TGF) method can be applied to analyze the spectrum and deduce plasma ion temperature and toroidal rotation velocity profiles of reasonable accuracy without beam modulation. It has been realized that the measurement result by the TGF method can be alternatively used to investigate plasma transport dynamics when beam modulation is prohibited to avoid any possible disturbance inhibiting robust plasma control and stable operation of the neutral beam injection system.

Effect of non-axisymmetric perturbations on the ambipolar Er and neoclassical particle flux inside the ITER pedestal region

The transport dynamics of impurities in the pedestal region of ITER plasmas is of crucial interest since this regulates the penetration of impurities from the edge into the core plasma, where an excessive accumulation of impurities can degrade their fusion performance. In the pedestal region of H-mode tokamak plasmas, anomalous transport is highly reduced and impurity transport is found to be well described by the neoclassical theory. Under these conditions, perturbations to the axisymmetric tokamak geometry can strongly affect both the radial electric field and particle transport. In this work, we describe the results of numerical studies performed to quantify the effects on the pedestal ambipolar electric field and radial particle fluxes of the non-axisymmetric fields, associated with both the intrinsic toroidal field ripple and extrinsic fields applied for ELM control, for ITER Q = 10 plasma conditions with emphasis on high Z impurity transport. It is found that the effect of the ITER toroidal field ripple on high Z impurity transport is negligible. On the contrary, extrinsic three-dimensional fields applied for ELM control cause a strong modification of the pedestal ambipolar electric (to less negative values) and the appearance of multi-valued solutions for the pedestal electric field, analogue to core stellarator transport significantly increasing the outward character of neoclassical pedestal transport for both the main plasma ions (D and T in ITER) and high Z impurities, suggesting a strong modification of the background plasma profiles. Finally, it is found that for the Z impurity, its quantitative evaluation has uncertainties (with important implications for the radial flow direction) associated with the high poloidal Mach number ~ 1, due to the high pedestal electric field and large ion mass, which renders the first order neoclassical theory questionable. Detailed ITER MHD 3D equilibria and a benchmark of SFINCS against NEO and NCLASS codes has also been obtained.

Screening in a multicomponent plasma by the example of a wet air plasma

A general theory of screening of a dust-particle charge in a multicomponent plasma is developed based on the asymptotic screening theory. In the asymptotic theory, the balance equations for the number of charged plasma particles in the diffusion-drift approximation supplemented with point sinks of plasma particles on a dust particle and the Poisson equation for the self-consistent electric field potential are solved by the linearization method using the three-dimensional Fourier transform. The coefficients of a secular or a characteristic polynomial, whose zeroes determine screening constants in a multicomponent plasma, were calculated by the Leverrier–Faddeev method, allowing one to find these coefficients for any number of initial balance equations for the number of charged plasma particles. By the example of a wet air plasma produced by an external gas-ionization source, the procedure is considered for constructing the truncated system of balance equations for main plasma ions and screening constants are determined. The influence of the number of main ions on screening constants is considered and the physical meaning of all screening constants is found.

Removal of cold alpha particles from helical device for fusion

This paper is devoted to the analysis of the new physics mechanism of the removal of cold alpha particles from the fusion helical reactor. Earlier, we have proposed some mechanisms, such as the drift island motion and estafette of drift resonances to remove the test particles, namely the cold alpha particles from the helical device of a LHD type. In that time, we kept in mind the cold alpha particles with the energy W = 350 keV. Now, we concentrate our study on the removal of the helium ash, i.e. cold alpha particles with the energy W = 35 keV while the main plasma ions are confined.

Dusty photoresonant plasma with coulomb collisions

We have studied the charging of dust particles in a dense photoresonant sodium plasma with electron and ion densities as high as 1016 cm−3 produced by laser pumping of the resonance level of Na, which was a small admixture (up to 1%) in an argon buffer gas. We show that the charge of dust particles with a radius of 10 mm at maximum reaches 3 × 105 electron charges and that the potential of the dust particles at a low electron bulk loss rate agrees well with the orbital motion limited (OML) model data. The behavior of the electric field near a dust particle was found to be nonmonotonic. We established that the distribution of the potential near a solitary charged dust particle agrees well with the Debye one, but the screening length proves to be much larger than even the electron Debye length; the discrepancies are largest at the afterglow stage of the photoresonant plasma, when the sodium ion with a low recombination coefficient is the main plasma ion. We determined the domain of parameters for a dense plasma where an ensemble of dust particles can crystallize.

Multi‐fluid code simulations including anomalous non‐diffusive transport of plasma and impurities in the tokamak SOL

AbstractFast intermittent transport has been observed in the scrape‐off layer (SOL) of major tokamaks including Alcator C‐Mod, DIII‐D, and NSTX. This kind of transport is not diffusive but rather convective. It strongly increases plasma flux to the chamber walls and enhances the recycling of neutral particles in the main chamber. We discuss the anomalous cross‐field convection (ACFC) model for impurity and main plasma ions and its relation to intermittent transport events, i.e. plasma density blobs and holes in the SOL. Along with plasma diffusivity coefficients, our transport model introduces time‐independent anomalous convective velocities across the confining magnetic field. In the discharge modeling, diffusivity coefficients and ACFC velocity profiles are adjusted to match a set of representative experimental data. We use this model in the edge plasma physics code UEDGE to simulate the multi‐fluid two‐dimensional transport for these three tokamaks. We present simulation results suggesting the dominance of anomalous convection in the far SOL transport. These results are consistent with the hypothesis that the chamber wall is an important source of impurities and that different impurity charge states have different magnitude and sign of their anomalous radial convective velocities. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of carbon and main ion radiation profiles in matched helium and deuterium plasmas in JET

This paper examines the radiation profiles and corresponding ionization source profiles of carbon and main plasma ions in matched helium and deuterium L-mode plasmas in JET. The radiation intensities from C 1+, C 2+ and C 3+ in the helium plasmas showed reduction by factors of 8, 10 and 25, respectively along the inner SOL and divertor leg compared with the deuterium cases. However, the emission in the outer divertor leg was less than a factor of 2 lower in helium. Using photon efficiencies calculated by the UEDGE code for the spectrometer lines of sight, the calculated source rates of C 1+ and C 3+ along the inner SOL and inner divertor were reduced by factors of 4 and 20, respectively in the helium plasmas. In the outer divertor leg the source rate of C 3+ was reduced by a factor of 10 but the C 1+ source rate did not change in helium. These measurements are consistent with a model that chemical sputtering of carbon dominates the source from the inner wall and inner divertor in deuterium L-mode plasmas while physical sputtering appears to dominate the source from the outer divertor.

Analysis of DIII-D Radiative Edge Discharges

Coupled plasma and multi-charge state impurity transport simulations are performed to analyze recent DIII-D discharges with a strong radiating edge resulting from the injection of non-intrinsic impurities. Good agreement between the simulations and the experiment is obtained using a Bohm model for the main plasma ion and energy transport and a simple fixed-shape model for the transport of the impurity charge states.

Analysis of DIII-D Radiative Edge Discharges

Coupled plasma and multi-charge state impurity transport simulations are performed to analyze recent DIII-D discharges with a strong radiating edge resulting from the injection of non-intrinsic impurities. Good agreement between the simulations and the experiment is obtained using a Bohm model for the main plasma ion and energy transport and a simple fixed-shape model for the transport of the impurity charge states.

Effects of FinnStim on growth and sea water adaptation of Coho salmon

The effects of FinnStim (betaine plus 3% protein hydrolysate) on the osmotic balance, metabolism and growth in coho salmon Oncorhynchus kisutch (Walbaum) was studied. The fish were fed at levels of 3.5 to 5% of body weight per day with a commercial-type control diet or diet with 1% of FinnStim. The fish were fed experimental diets for 45 days in fresh water and 40 days in sea water; thereafter both groups were fed the control diet and monitored for additional 50 days. Fish weight, length and concentrations of main plasma ions Na +, Cl − did not differ significantly between treatments. FinnStim supplementation significantly decreased muscle K + load one week after SW transfer, and improved specific growth rate (SGR 1.75%/day vs. 1.66%/day) and FCR (1.27 vs. 1.51) in SW. During the whole trial period the FinnStim group showed lower mortality than the control group (2.78% vs. 4.95%).

Effects of neutrals on plasma rotation in DIII-D

Friction due to charge exchange with cold neutral atoms in the edge is investigated as a candidate to govern the poloidal rotation in the edge of a tokamak plasma. The Hirshman and Sigmar neoclassical moment approach is used to determine the rotation velocities of the main plasma ions and of one impurity species, when charge exchange friction is included. It is found that the poloidal rotation of the main plasma ions is controlled by charge exchange friction with neutrals. The impurity ion poloidal rotation is governed by the balance between the impurity viscous force and the main-ion-impurity-ion friction force. The results of the calculation are compared with the measurements obtained in the edge of a DIII-D high (H) mode plasma, using charge exchange recombination (CER) spectroscopy. It is found that the measured main ion poloidal rotation can be accurately predicted by the neoclassical theory including the effect of neutrals, assuming a neutral density ⟨nn⟩ = 3 × 1017 m-3 at the separatrix, decreasing exponentially inside the plasma with an e-folding length of 0.012 m, and peaking near the X point region with a poloidal peaking parameter γ ≡ ⟨nn⟩⟨B2⟩/⟨nnB2⟩ = 1.5. However, for the impurity ions, the neoclassical theory including a single impurity charge state, and regardless of the effect of the neutrals, gives a prediction that has the correct sign, but whose value is a factor of 5 or 6 different from the experimental value

Validity of the impurity entrainment assumption in the thermal stability analysis of multifaceted asymmetric radiation from the edge (MARFEs) in tokamak plasmas

The assumption that impurity ion perturbations are entrained in phase and in fixed proportion with the main plasma ion perturbations is commonly made in analyses of radiative thermal instabilities in the edge of tokamak plasmas, leading to multifaceted asymmetric radiation from the edge (MARFE). An analysis in which the impurity ion perturbations are treated separately from the plasma ion perturbations demonstrates that this assumption does not alter the conclusion of the analysis regarding stability limits or edge density limits for the (m=0, k‖=0) mode, but that inclusion of a separate impurity ion perturbation in the analysis stabilizes the (m>0, k‖≠0) modes.

Monte Carlo Simulation of Edge Ion Dynamics in the Presence of Collisions and a Radial Electric Field

AbstractThe dynamics of ions in the vicinity of the last closed flux surface in tokamaks is a topic of current interest because of its significance to the first wall and divertor surfaces as well as to the LH‐transition. In the presence of a radial electric field, the ion trajectories and the structure of the ion phase space are altered. The fields can have secondary effects e. g. through the radial current resulting from the direct ion orbit loss. A Monte Carlo method for studying the edge ion behaviour is presented here. The direct ion orbit loss of the main plasma ions is analyzed under various plasma conditions, and the effect of boundary conditions on direct ion orbit loss is studied.

Comment on ‘‘Effects of neutral beam injection on poloidal rotation and energy transport in tokamaks’’ [Phys. Fluids B 5, 3012 (1993)

A comment on effects of neutral beam injection on poloidal rotation and energy transport in tokamaks plasma is presented. The enhanced neoclassical effects such as poloidal mass flow, viscous heating and the convective flow of main plasma ions which is inward for coinjection should be taken into account in the interpretation of experimental data. (AIP)

Direct chemical ionization: Dependence of the CI plasma on the emitter temperature

The absolute intensity of the main plasma ion and the relative intensities of the various ionic species (both positive and negative) present in a DCI plasma vary with the emitter temperature. This phenomenon is shown to be due to the dependence of the depth of penetration of the primary electrons into the reactant gas, which in turn is a function of the gas temperature.

Second‐order effects related to a model for a parallel shock

Second‐order effects are calculated for a low‐frequency electromagnetic instability due to an ion beam in a plasma. This instability is a characteristic of a parallel shock model that includes an ion reflecting electrostatic subshock. The analysis is similar to that done by S. P. Gary, but a counterstreaming configuration is chosen that is homogeneous in time and that has specially growing modes. Among other effects, energy is transferred from the incoming main plasma ions to the beam ions. Electron pressure effects are calculated and are small for low frequencies. The application of this model to the earth's bow shock is discussed.