Effect of detachment on Magnum-PSI ELM-like pulses: direct observations and qualitative results

Characteristics of edge-localized mode (ELM) energy and particle losses in type-I ELMy H-mode plasmas on ASDEX Upgrade are investigated by a large data set with as much as possible independent variations in engineering parameters. A statistically unbiased estimate for ELM energy and particle losses has been employed to remove the diagnostic noise as much as possible. The data set shows that the ELM energy loss decreases with the pedestal density or collisionality. However, the role of plasma shape, especially, triangularity, as an explicit parameter is revealed. Elevated triangularity leads to lower ELM frequency and larger ELM energy loss significantly exceeding that simply expected from the increased pedestal pressure in high triangularity plasmas at a fixed power. The observed larger ELM energy drop at higher triangularity involves the ELM perturbations of the electron temperature profile across an ELM that extend radially more inward, suggesting that there is a direct effect of plasma shape on ELM energy losses. It is found that the fraction of ELM loss power does not remain constant but the increased pedestal collisionality enhances the transport level between ELMs and reduces the ELM loss power. The ELM particle flux at fixed power rises with increasing gas fuelling rate. On the other hand, at fixed gas puff, an inverse proportionality between ELM frequency and ELM particle loss roughly holds despite of a large variation of power. When the particle flux near the separatrix is enhanced, the increase of ELM frequency and the reduction of ELM loss power caused by the increased collisionality lead to the reduction of ELM energy loss so that the energy balance could be sustained.

High repetition rate injection of deuterium pellets from the low-field side (LFS) of the DIII-D tokamak is shown to trigger high-frequency edge-localized modes (ELMs) at up to 12× the low natural ELM frequency in H-mode deuterium plasmas designed to match the ITER baseline configuration in shape, normalized beta, and input power just above the H-mode threshold. The pellet size, velocity, and injection location were chosen to limit penetration to the outer 10% of the plasma. The resulting perturbations to the plasma density and energy confinement time are thus minimal (<10%). The triggered ELMs occur at much lower normalized pedestal pressure than the natural ELMs, suggesting that the pellet injection excites a localized high-n instability. Triggered ELMs produce up to 12× lower energy and particle fluxes to the divertor, and result in a strong decrease in plasma core impurity density. These results show for the first time that shallow, LFS pellet injection can dramatically accelerate the ELM cycle and reduce ELM energy fluxes on plasma facing components, and is a viable technique for real-time control of ELMs in ITER.

The properties of grassy edge-localized modes (ELMs) in EAST in the favorable and unfavorable B t are statistically studied. Statistical analysis indicates that there is no systematical difference in the frequencies of grassy ELMs under the two different magnetic configurations in the similar parameter spaces. The high-frequency grassy ELM (f ELM > 1 kHz) in unfavorable B t is dependent on the high poloidal beta β p and high triangularity δ u, while the high-frequency grassy ELM (f ELM > 1 kHz) in favorable B t appears to rely on the high plasma density. A frequently occurring phenomenon in favorable B t defined as ‘clustered ELM’ seems to be the most evident difference in ELM behavior between favorable and unfavorable B t. Statistical analysis shows that larger plasma-wall outer gap, longer plasma elongation, lower low-hybrid wave heating power and electron cyclotron resonance heating power favor the occurrence of clustered grassy ELMs. Further studies indicate that the generation of clustered grassy ELMs could be correlated with the lower electron temperature in the bulk plasma.

Integrated edge-localized-mode (ELM)-free or small ELM scenarios for the demonstration fusion power plant (DEMO) are investigated in ASDEX Upgrade using argon seeding for radiative power removal mainly in the pedestal region. An important aspect is the modification of the electron pressure in the pedestal by the additional radiative power losses. Full ELM suppression could be achieved in a no-ELM H-mode scenario featuring an edge electromagnetic quasicoherent mode up to a heating power of 5 MW, where argon radiation allowed the extension of the heating power operational space. At higher powers up to 12 MW (reaching the beta limit), ELMs of reduced size prevail and detachment is obtained by the argon seeding. Control of the position of a radiating zone localized inside the X-point was found to be favorable compared to the control of the separatrix power for low /. Integration of pedestal cooling with Ar and ELM suppression by resonant magnetic perturbations (RMP) allowed an increase of the core radiation and a partial recovery of normalized confinement to H98 = 1. This favorable behavior is finally limited by the loss of the RMP density pump-out effect, followed by an ELM-free H-mode phase and re-occurrence of ELMs. For an active tailoring of the pedestal pressure profile, precise knowledge of the radiation profile is required. Modeling of the argon radiation profile with the STRAHL code showed good agreement with bolometry only if charge exchange of neutral deuterium with argon ions was taken into account, highlighting the importance of the neutral density in the pedestal region. In view of best integration of a no-ELM scenario with divertor detachment and high heating power, the divertor compression and enrichment of argon and neon were compared using dynamic gas puff experiments. Argon shows more than a factor of 3 higher divertor enrichment compared to neon, but the absolute values decrease with higher neutral deuterium pressure.

The application of resonant magnetic perturbations (RMPs) with a toroidal mode number of n = 3 to connected double null plasmas in the MAST tokamak produces up to a factor of 9 increase in edge-localized mode (ELM) frequency and reduction in plasma energy loss associated with type-I ELMs. A threshold current for ELM mitigation is observed above which the ELM frequency increases approximately linearly with current in the coils. The effect of the RMPs is found to be scenario dependent. In one scenario the mitigation is only due to a large density pump out event and if the density is recovered by gas puffing a return to type-I ELMs is observed. In another scenario sustained ELM mitigation can be achieved irrespective of the amount of fuelling. Despite a large scan of parameters complete ELM suppression has not been achieved. The results are compared with modelling performed using either the vacuum approximation or including the plasma response. The requirement for a resonant condition, that is an optimum alignment of the perturbation with the plasma, is confirmed by performing a scan in the pitch angle of the applied field.

Real-Time Control of Tokamak Plasmas: from Control of Physics to Physics-Based Control

We use data from four tail seasons (2001–2004) of the Cluster spacecraft to study statistical features of energetic electrons (40–400 keV) in the magnetotail plasma sheet in combination with plasma parameters and indices of geomagnetic activity. We find that the horizontal magnetic field magnitude best orders the energetic electron observations. Using this method we observe a statistical flux gradient toward the neutral sheet. The fluxes also increase with an increase in the vertical magnetic field component (dipolarization). The intensity of supra‐thermal electrons is observed to increase strongly as function of plasma temperature. Although the energetic electron fluxes also appear to increase strongly with geomagnetic activity this is found to be mainly due to the increase in plasma temperature with increasing Kp. Investigation of neutral sheet fluxes and phase space densities at fixed first adiabatic invariant (μ) indicates that adiabatic heating can explain observed flux increases during field dipolarization. The spectral slope in the supra‐thermal range is observed to be independent of geomagnetic activity (Kp), while there is a significant local time dependence, with harder spectra observed at dawn compared to the dusk side.

The enhancement of radiative output of the pulsed high-pressure sodium discharge in the blue region of the visible spectrum is due principally to radiation from the nd2D→3p2P transitions of the sodium atom where n=4–15. Excitation of these upper energy levels results from a transient non-steady-state increase in plasma temperature to about 5500 °K in the center of the arc. This temperature decays to a value near that encountered in steady-state excitation conditions in a time comparable to the empirically optimized pulse duration and to the previously determined time for plasma temperature relaxation.

We present a description of plasma conditions in the Io plasma torus, between 5 and 10 RJ, based on Voyager 1 observations obtained in March 1979. The model includes updated analyses of Plasma Science (PLS) data obtained along the spacecraft trajectory as well as Ultraviolet Spectrometer (UVS) observations of composition made remotely from Jupiter. The plasma characteristics observed along the spacecraft trajectory have been extrapolated along magnetic field lines by numerically solving the equations of diffusive equilibrium to produce radial profiles of plasma properties at the centrifugal equator as well as maps of the densities of the major ionic species in a meridian plane. The diffusive equilibrium distribution of plasma along magnetic field lines depends mainly on T∥. Unfortunately, we only have measurements of T⊥ and must make assumptions about the thermal anisotropy of the plasma. We assume the thermal populations and the suprathermal electrons to be isotropic. The suprathermal ions have probably been recently picked‐up and are expected to be highly anisotropic. Varying the thermal anisotropy of the hot ions between A=T⊥/T∥=1 to 5 has a minor effect on the plasma maps but makes a significant difference to the fraction of hot ions in the plasma when integrated over a complete shell of magnetic flux. We have found that the vertical extrapolation of plasma density is insensitive to the geometry of different magnetic field models except inside 5 RJ (where the plasma scale height is comparable to uncertainties in the location of the centrifugal equator) and outside 8 RJ (where the magnetospheric current sheet significantly perturbs the magnetic field). The radial profile of flux tube content (N L²) exhibits the same “precipice”, “ledge,” and “ramp” features as previous studies as well as confirming small ‐ scale features which indicate local sources of plasma in the cold torus and near the orbit of Europa. The observations of O++ and molecular (SO2+ or S2+) ions inside 5.4 RJ, far from Io, in a region of cold dense plasma, remain difficult to explain, indicating either strong temporal variability in the Io plasma source or a strong source of plasma, possibly from the dissociation of dust, inside Io's orbit. Further evidence of a Europa source are the decrease in the ratio of sulfur to oxygen ions and the increase in plasma temperature outside 8 RJ.

We have investigated the generation of high-power-density thermal plasma jet (TPJ) as a heat source for the annealing of a Si wafer surface in a millisecond period and the formation of an ultrashallow junction. The power density of DC arc discharge thermal plasma jet markedly increased from 11.0 to 32.3 kW/cm2 with increasing spacing between an anode and a cathode (ES) from 1.0 to 3.0 mm. The increase in TPJ power density with ES was mainly due to the increase in plasma temperature from about 16000 to 23000 K. By applying this high-power-density TPJ, the Si wafer was heated by more than 700 K within 10 ms. Using this annealing technique, we demonstrated the dopant activation of an arsenic-implanted Si wafer and successfully obtained a low sheet resistance of 262 Ω/sq at an annealing temperature as high as 1207 K without a significantly enhanced diffusion of the implantation profile.

The authors consider the heating, by magnetic pumping, of a radially non-uniform plasma cylinder of non-uni form radius located in a longitudinal, constant and uniform magnetic field B. An oscillating magnetic field having the form of a travelling wave with phase velocity ω/k of the order of the ionic thermal velocity is produced by means of external azimuthal electric currents. The frequency of the oscillating field ω is significantly less than the reciprocal of the ion-ion collision time 1/τi, and the ion cyclotron frequency ωi, the plasma electrons are magnetized (the electron cyclotron frequency is considerably greater than the reciprocal of the electron-electron collision time), and the plasma pressure is significantly less than the magnetic pressure.Simple expressions are obtained for the energy absorbed per unit time by the plasma as a result of dissipative effects: ion viscosity and longitudinal electronic heat conduction (Joule losses play a minor role in magnetic pumping). Particular attention is paid to the case of acoustic resonance, where the inhomogeneous plasma contains a layer in which the velocity of sound is equal to the phase velocity of the excited wave. In this situation the absorption of energy by the plasma increases sharply and the increase in plasma temperature T is determined to within an order of magnitude by the relation dT/dt ∼ ωT(B∼/B)2, where B∼ is the amplitude of the oscillating magnetic field at the axis.

Non-spectroscopic (also called matrix effects) and spectroscopic interferences (in particular, from oxide and doubly-charged ions) may compromise the accuracy of inductively coupled plasma mass spectrometry measurements.Dilution is widely used to reduce the matrix effects that depend on the absolute quantity of matrix.However, dilution is a source of errors (especially when performed manually) and takes considerable time.An alternative method of reducing the absolute quantity of matrix is proposed in this article, through a reduction in sample injection volume.In this study, capillary-based mono-segmented flow analysis (MSFA) with sample injection as small as 1 L was compared to the continuous nebulization of sample solutions and flow injection of 50-L aliquots.The injection volume and oxide interference had a positive correlation, with 1-L MSFA reducing the amount of CeO + /Ce + by up to 69%.The concurrent increase in Ba ++ /Ba + as the sample volume decreased suggests an increase in plasma temperature when smaller sample volumes are introduced.Signal suppression induced by the 400-mg L -1 Na matrix significantly decreased as the volume of the injected sample decreased and was virtually eliminated with 1-L MSFA.This decrease translated into a negative correlation between the sample volume and accuracy when a drinking water-certified reference material was analyzed by external calibration without internal standardization or matrix matching.Only 1-L MSFA yielded concentrations within the range of inclusion for all analytes.

One promising approach to keep the residual oxygen level in welding at extremely low levels is to add monosilane to the argon shielding gas. However, its effect on the plasma itself is not clear, and thus this study examines the influence of a silane‐doped argon atmosphere on the chemical components and temperature within a nontransferring, thermal argon plasma in comparison to an atmosphere of pure argon and air. For this purpose, the emitted radiation of the plasma was analyzed using optical emission spectroscopy. The spectra show that the Ar I lines are most prominent in all atmospheres. Compared to the air atmosphere, the plasma in the argon atmosphere exhibits a stronger H I line and an OH emission band. A slight doping of the argon atmosphere with silane (0.01 vol.%) leads to strongly pronounced Si I and H I lines and a clearly recognizable H 2 emission band. The high amount of free electrons resulting from the stepwise ionization and dissociation of the silane molecule leads to an increase in plasma temperature from about 7,600 to 9,000 K.

Electric double layers have been produced in a double-plasma device by the injection of an ion beam into a target plasma. A longitudinal magnetic field is present, of sufficient strength to magnetise the electrons. The double layers occur if the electron density in the target plasma is not sufficient to neutralise the injected space charge. The strength and axial position of the double layers can be controlled by varying the plasma density in the target, the ion-beam density, the neutral pressure, or the magnetic field strength. The double layers seem to form at a position where the ion and electron fluxes satisfy, approximately, the Langmuir condition. The electric fields in the double layers are compared to the values predicted by the original Langmuir theory, later modified by Andrews and Allen to include the effects of reflected particles and initial velocities.

The n = 1 (n is the toroidal mode number) resistive wall mode (RWM) instability and the resonant field amplification (RFA) due to a stable RWM response are numerically investigated for an MAST high-pressure plasma scenario, utilizing the MARS-F Liu et al (2000 Phys. Plasmas 7 3681) and MARS-K Liu et al (2008 Phys. Plasmas 15 112503) codes, for the purpose of understanding the potential RWM behavior in MAST-U plasmas. Ideal kink stability analysis identifies a target plasma, with parameters similar to that of the reference equilibrium reconstructed from an MAST high-pressure discharge, that accesses the RWM regime. The unstable n = 1 RWM for the target equilibrium is subject to strong damping by the plasma toroidal flow and/or the drift kinetic effects from thermal particles. As a result, the mode is found to be stable under the experimental flow conditions, or even without flow stabilization if drift kinetic stabilization is included. The stability prediction is robust against variation of the assumed resistive wall minor radius. Active magneto-hydrodynamic (MHD) spectroscopy modeling, using the magnetic coils designed for controlling the edge localized modes (ELMs) in MAST-U as the antenna, shows strong RFA due to a stable RWM response in the target plasma. Maximal amplification, of 6–7 times larger than the applied vacuum field within the plasma, is obtained assuming a coil phasing of 120–180 degrees between the upper and lower rows of the ELM control coils. The MHD-kinetic hybrid model predicts generally higher RFA than the fluid model, in particular near the low-field and high-field sides of the plasma boundary surface. The MARS-F/K modeling thus shows that the RWM in MAST-U is subject to strong flow and kinetic stabilization, but can nevertheless be detected by active MHD spectroscopy.