Magnetized Helium Plasma Research Articles

Experimental and theoretical study of bumped characteristics obtained with cylindrical Langmuir probe in magnetized helium plasma

Cylindrical Langmuir probe measurements in a helium plasma were performed and analysed in the presence of a magnetic field. The plasma is generated in the ALINE device, a cylindrical vessel 1 m long and 30 cm in diameter using a direct coupled RF antenna (νRF = 25 MHz). The density and temperature are of the order of 1016 m−3 and 1.5 eV, respectively, for 1.2 Pa helium pressure and 200 W RF power. The axial magnetic field can be set from 0 up to 0.1 T, and the plasma diagnostic is a RF compensated Langmuir probe, which can be tilted with respect to the magnetic field lines. In the presence of a magnetic field, I(V) characteristics look like asymmetrical double probe ones (tanh-shape), which is due to the trapping of charged particles inside a flux tube connected to the probe on one side and to the wall on the other side. At low tilting angle, high magnetic field amplitude, power magnitude and low He pressure, which are the parameters scanned in our study, a bump can appear on the I(V) in the plasma potential range. We then compare different models for deducing plasma parameters from such unusual bumped curves. Finally, using a fluid model, the bump rising on the characteristics can be explained, assuming a density depletion in the flux tube, and emphasizing the role of the perpendicular transport of ions.

Characterization of high flux magnetized helium plasma in SCU-PSI linear device

A high-flux linear plasma device in Sichuan University plasma-surface interaction (SCU-PSI) based on a cascaded arc source has been established to simulate the interactions between helium and hydrogen plasma with the plasma-facing components in fusion reactors. In this paper, the helium plasma has been characterized by a double-pin Langmuir probe. The results show that the stable helium plasma beam with a diameter of 26 mm was constrained very well at a magnetic field strength of 0.3 T. The core density and ion flux of helium plasma have a strong dependence on the applied current, magnetic field strength and gas flow rate. It could reach an electron density of 1.2 × 1019 m−3 and helium ion flux of 3.2 × 1022 m−2 s−1, with a gas flow rate of 4 standard liter per minute, magnetic field strength of 0.2 T and input power of 11 kW. With the addition of −80 V applied to the target to increase the helium ion energy and the exposure time of 2 h, the flat top temperature reached about 530 °C. The different sizes of nanostructured fuzz on irradiated tungsten and molybdenum samples surfaces under the bombardment of helium ions were observed by scanning electron microscopy. These results measured in the SCU-PSI linear device provide a reference for International Thermonuclear Experimental Reactor related PSI research.

Towards a parallel collisionless shock in LAPD

Using a high-energy laser to produce a super-Alfvénic carbon-ion beam in a strongly magnetized helium plasma, we expect to be able to observe the formation of a collisionless parallel shock inside the Large Plasma Device. We compare early magnetic-field measurements of the resonant right-hand instability with analytical predictions and find excellent agreement. Hybrid simulations show that the carbon ions couple to the background plasma and compress it, although so far the background ions are mainly accelerated perpendicular to the mean-field direction.

Dual upper drift waves in RF produced magnetized helium plasma

Self-excited dual upper drift modes have been observed in a magnetized helium plasma, having frequencies less than the ion cyclotron frequency (fci) but higher than that of the conventional low frequency drift waves (f≪fci). The modes propagate mainly in the azimuthal direction with mode numbers m = 1 and m = 2 with frequencies 30 kHz and 50 kHz, respectively, and also have weak axial propagation. They coexist over a wide range of radial and axial locations and the direction of azimuthal propagation coincides with the electron diamagnetic drift. A local dispersion relation obtained from the fluid description of the plasma predicts an instability simultaneously for both m = 1 and m = 2 modes (with different frequencies) over a range of radial locations.

Investigation of particle diffusion and suprathermal electrons in a magnetized helium plasma column

Studying radiative properties of magnetized helium plasma via high-resolution spectroscopy identified close correlations between the particle diffusion and suprathermal electrons for different modes of operation of the MISTRAL installation. The standard diagnostic emission lines in neutral helium (1s3d 3D-1s2p 3P, 1s3s 3S-1s2p 3P, 1s3d 1D-1s2p 1P, and 1s3s 1S-1s2p 1P) show anomalous ratios that are related to enhanced particle diffusion and suprathermal electron generation. The supplementary investigation of singlet/triplet Rydberg series (transitions 1snd 3D-1s2p 3P and 1s5p 1P-1s2s 1S) as well as ionic lines (HeII, transitions n = 3–4 at 469 nm and n = 4–6 at 656 nm) allowed quantitative characterization. Simulations carried out with the atomic physics code SOPHIA demonstrate that simultaneous implementation of diffusion processes and suprathermal electrons matches all experimental findings. Single consideration, however, of either diffusion or hot electrons is in contradiction to the proposed extended set of HeI and HeII emission lines. The high precision achieved with the LSJ-split level structure of SOPHIA coupled to Langmuir probe measurements allowed to conclude to a Bohm type diffusion in MISTRAL.

Drift-Alfvén wave mediated particle transport in an elongated density depression

Cross-field particle transport due to drift-Alfvén waves is measured in an elongated density depression within an otherwise uniform, magnetized helium plasma column. The depression is formed by drawing an electron current to a biased copper plate with cross-field dimensions of 28×0.24 ion sound-gyroradii ρs=cs∕ωci. The process of density depletion and replenishment via particle flux repeats in a quasiperiodic fashion for the duration of the current collection. The mode structure of the wave density fluctuations in the plane perpendicular to the background magnetic field is revealed using a two-probe correlation technique. The particle flux as a function of frequency is measured using a linear array of Langmuir probes and the only significant transport occurs for waves with frequencies between 15%–25% of the ion cyclotron frequency (measured in the laboratory frame) and with perpendicular wavelengths k⊥ρs∼0.7. The frequency-integrated particle flux is in rough agreement with observed increases in density in the center of the depletion as a function of time. The experiments are carried out in the Large Plasma Device (LAPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the Basic Plasma Science Facility located at the University of California, Los Angeles.

Comparisons of experimental measurements and two-dimensional plasma-fluid numerical simulations of a magnetized plasma column

The density, the electron, ion, and neutral temperatures in dense (ne≳1013 cm−3) and cool (Te<8 eV) magnetized helium plasma columns submerged in a cool neutral gas (n0∼3×1015 cm−3) environment have been measured. These plasmas, of similar collisionality to those expected in the scrape-off layer of ITER (International Thermonuclear Experimental Reactor) [R. Parker, Contrib. Plasma Phys. 34, 422 (1994)], are found to obey the low-recycling behavior of approximate plasma pressure balance along the column axis. Density decreases by a factor of about 2 and Te remains isothermal. Computer simulations using the B2 [R. Schneider et al., J. Nucl. Mater. 196–198, 810 (1992)] plasma-fluid code have been performed and are in good agreement with experimental results. Measured ion temperatures are consistent with classical energy transfer from the electrons. Neutrals within the plasma are heated by elastic and charge-exchange collisions. This set of measurements represents the most detailed comparisons between experimental results and B2 predictions.

Experimental observations of steep temperature steps in dense magnetized plasmas.

Quasi-steady-state steep temperature steps have been experimentally observed in dense $(g{10}^{19}{m}^{\ensuremath{-}3})$ linear magnetized helium plasma columns submerged in a high-density ( $g100$ mT) neutral-gas environment. The formation of such features is found to be favored by a high neutral gas density or a positive bias $(\ensuremath{\ge}{V}_{\mathrm{fl}})$ placed on the end plate. A rapid reduction in plasma density and electron temperature is observed to exist across the steps, as is a dramatic increase in neutral line radiation. Potential steps are inferred. Detailed Langmuir probe measurements have revealed three-dimensional structures. Possible mechanisms of formation are discussed.

Thin-oxide MOS damage caused by wafer charging in magnetized helium plasma

The effect of non-uniform He plasma on the properties of thin oxide (11.5–13 nm) metal-organic-semiconductor (MOS) structures has been investigated by means of C- V and breakdown-voltage histogram techniques. The plasma reactor system used is like a magnetically enhanced reactive ion etcher. The damage creation for MOS structures with various Al gate thicknesses were determined as a function of discharge parameters and plasma exposure time. The results show the vulnerability of the oxide and its interface with Si to the plasma process. The creation of a large amount of defects in the form of a fixed oxide charge and interface states is found; the build-up plasma damage process is very rapid, particularly during the first 5–30 s of the plasma exposure time and it depends essentially on the Al gate thickness (the thinner Al gate exhibits stronger plasma sensitivity). It is established that build-up damage depends on the plasma uniformity, but the non-uniformity is not the only nor the dominating factor. The nature of the process-induced defects and the influence of plasma components are also discussed.

Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

AbstractThe equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece‐Gouldwave. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times τE, τ1 and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In the experiment they are taken from the transient curves of Langmuir‐probe current, optical line radiation and the noise power at the electron cyclotron frequency.The experimental conditions are: Helium‐gas, p = 1 …︁ 5 Pa, Te = 4 eV, n = 1 …︁ 5 · 1010 cm−3, B = 6,5 · 10−2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …︁E = 10 …︁ 15 μs is given by inelastic collision losses. The ionization time constant τ1 is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail formation in the distribution function and enhanced radial losses becoming Bohm‐like in the transition phase.

Ion temperature measurements in the range of 0.1-1 eV by means of the Doppler broadening of the 4686 AA He II line

A method is described to determine the ion temperature in the range 0.1-1 eV from line profile measurements in a magnetized helium plasma of low density.