Peak Electron Temperature Research Articles

Comparison of electron internal transport barriers in the large helical device and JT-60U plasmas

Plasmas with an electron internal transport barrier (ITB), which is characterized by peaked electron temperature profiles, are obtained in the JT-60U tokamak and in the large helical device (LHD) when the electron cyclotron heating (ECH) is focused on the magnetic axis. The maximum values of , where R is the major radius and is the scale length of the electron temperature gradient, are similar for the LHD and JT-60U ITB plasmas. However, there is a clear jump of observed in LHD but not in JT-60U in the ECH power scan. This result is consistent with the fact that the trigger mechanism of the electron ITB is the fast transition of the radial electric field from a small negative Er to a large positive Er in LHD and a change of the magnetic shear from positive to negative is required for the formation of the electron ITB in JT-60U. There are also differences in the electron temperature profiles inside the ITB. The flattening of the electron temperature profile inside the strong ITB could be explained by the sharp increase of q values observed in JT-60U, while no flattening of the electron temperature profile is observed in LHD, where the central q values stay low.

Role of trace impurities in large-volume noble gas atmospheric-pressure glow discharges

A computational study of capacitively coupled atmospheric-pressure glow discharges in high-purity helium is presented, and the role of trace impurities in such noble gas plasmas is established. Trace impurities result in generation of significant amounts of charged species through Penning ionization and charge exchange reactions. The altered charged species balance in the plasma causes a large change in the discharge impedance when compared to a completely pure noble gas discharge. Peak electron temperatures of the order of 50 000 K and electron densities of order 1011 cm−3 are predicted. Significant electron generation is observed inside the sheath region due to the highly collisional nature of the plasma. This study underscores the importance of modeling the effect of even trace amounts of impurities in noble gas atmospheric-pressure glow discharges.

Transport of the plasma with neoclassical internal transport barrier on CHS

The plasma with neoclassical internal transport barrier (N-ITB)has been observed for electron cyclotron heated (ECH) plasma in the Compact Helical System Heliotron/Torsatron. The N-ITB is characterized by the peaked electron temperature,with a large temperature gradient (0.43 keV cm-1) and a large electric field shear at the barrier location.The transport analysis shows that the electron thermal conductivity in plasma with N-ITB is lower than that without N-ITB at the transport barrier. The neoclassical transport is calculated from the analytical model for the neoclassical particle flux.In the ECH plasma with N-ITB, the electron thermal conductivity χe is at the level of the neoclassical χe at the barrier transport which demonstrates that the anomalous transport is reduced to a level well below the neoclassical transport.

Bootstrap current analysis for neoclassical internal transport barrier discharge of CHS

The bootstrap current analyses have been carried out for two different kinds of discharges of compact helical system (CHS) heliotron/torsatron. One is the electron cyclotron resonance heating (ECRH) discharge with moderate density and without internal transport barrier (ITB) (ne(0)>0.5×1019 m-3, Te(0)<1.2 keV). Another is the neoclassical ITB (NC ITB) discharge, which has been achieved in the rather low electron density condition (ne<0.4×1019 m-3) with ECRH and co-NBI. The NC ITB mode is characterized by steeply peaked electron temperature profile (Te(0)>2.0 keV) in the core domain and large positive radial electric field (Er~ + 10 kV m-1). The dependence of toroidal net current on magnetic axis position Rax was investigated for ECRH discharges without ITB. There is good agreement in the tendency of Rax dependence of the net toroidal current between experiment and neoclassical calculation. In the NC ITB discharges, the experimental net current including both bootstrap and beam-driven current is largely reduced compared with that in the L-mode and it drops to almost zero. It seems as if the reversal of bootstrap current takes place in the NC ITB phase but such a view is inconsistent with neoclassical prediction. The observed reduction of the plasma current in the ITB phase is not explained by the neoclassical theory.

Two-dimensional modeling of long-term transients in inductively coupled plasmas using moderate computational parallelism. II. Ar/Cl2 pulsed plasmas

Quantifying transient phenomena such as pulsed operation is important to optimizing plasma materials processing. In particular, pulsed electronegative plasmas are promising candidates for reducing notching and charge buildup in features during microelectronics fabrication. In this article, a two-dimensional plasma equipment model is employed to investigate pulsed inductively coupled plasmas in Ar/Cl2 gas mixtures. The consequences of varying pulse repetition frequency (PRF), duty cycle, power, pressure, and Cl2 mole fractions on plasma properties are quantified. The nonmonotonic temporal dynamics in Cl− density observed in experiments are well captured by the model. We found that for constant peak power, a lower duty cycle resulted in higher peak electron temperatures at the leading edge of the power pulse due to a lower initial electron density at the end of the afterglow. Increasing the PRF produces an increase in the time averaged electron density due to a lower rate of attachment in the afterglow. The inertia of Cl− ions produces a sluggish response to rapid changes in plasma potential which results in “islands” of higher Cl− density in the periphery of the reactor. The results show that as the Cl2 fraction increases, the transition from electron–ion to ion–ion plasma is more pronounced.

Enhanced heat confinement in the flexible heliac TJ-II

Recent experimental results show that the core electron temperature in the TJ-II stellarator almost doubles previously obtained values for the same heating power. These plasmas, heated with electron cyclotronwaves, are characterized by their low density, and by having highly peaked electron temperature profiles and flat, or even hollow, density profiles. The conditions for obtaining these high electron temperature discharges regarding their density, injected power and dependence on plasma species are described. Neoclassical and experimental transport analyses are performed for these discharges, showing a reduction in the electron heat conductivity at the plasma core. The relations of this observed confinement enhancement to the CHS internal transport barrier and the W7-AS neoclassical electron root feature are discussed.

Overview of TCV results

The TCV tokamak (R = 0.88 m, a<0.25 m, BT<1.54 T) is equippedwith six 0.5 MW gyrotron sources operating at 82.7 GHz for secondharmonic X mode ECH. By distributing the ECCD current sources overthe discharge cross section, fully driven stationary plasmas withIp = 210 kA, ne0 = 2 × 1019m-3, Te0 ≈ 4 keV, were obtained for the fulldischarge duration of 2 s. Highly peaked electron temperature profileswith Te0 up to 12 keV were obtained in central counter-current drivescenarios with off-axis ECH. Absorption measurements using a 118 GHzgyrotron have demonstrated the importance of suprathermal electronsfor third harmonic absorption. A coupled heat-particle transportphenomenon known as `density pumpout', which leads to the expulsion ofparticles from the plasma core, has been linked to the presence of m = 1modes, suggesting that it is due to the existence of locally trappedparticles associated with the loss of axisymmetry. Highly elongateddischarges have been developed with ohmic heating (κ<2.8) and off-axisECH. The latter exhibit considerably improved vertical stability dueto current profile broadening. A `gateway' for ELMy H modes has beendiscovered, which allows stationary ohmic ELMy H mode operation over a wide range of elongation, triangularity and density. Divertordetachment experiments suggest the existence of recombination pathwaysother than three body or radiative processes.

Characterization and conditioning of SSPX plasma facing surfaces

The Sustained Spheromak Physics Experiment (SSPX) will examine the confinement properties of spheromak plasmas sustained by DC helicity injection. Understanding the plasma-surface interactions is an important component of the experimental program since the spheromak plasma is in close contact with a stabilizing wall (flux conserver) and is maintained by a high current discharge in the coaxial injector region. Peak electron temperatures in the range of 400 eV are expected, so the copper plasma facing surfaces in SSPX have been coated with tungsten to minimize sputtering and plasma contamination. Here, we report on the characterization and conditioning of these surfaces used for the initial studies of spheromak formation in SSPX. The high pressure plasma-sprayed tungsten facing the SSPX plasma was characterized in situ using β-backscattering and ex situ using laboratory measurements on similarly prepared samples. Measurements showed that water can be desorbed effectively through baking while the removal rates of volatile impurity gases during glow discharge and shot conditioning indicated a large source of carbon and oxygen in the porous coating.

Equivalent slab thickness and its variability: a study with incoherent scatter measurements

Abstract In order to check its usefulness for the IRI, incoherent scatter (I.S.) radar measurements at Arecibo (18.4° N, 66.7° W), for the solar minimum period of 1974–1977 are used to study equivalent slab thickness (EST) and its variability. While systematic diurnal changes are apparent, day-to-day and hour-to-hour fluctuations can also not be overlooked. This variability is examined in relation to the simultaneously measured F-layer peak height and electron temperature. While during nighttime, EST shows dependence upon the F-layer peak height (hmF2), it shows by day a dependence on electron temperature.

Shafranov shift in the ‘electron root’ feature at the W7-AS stellarator

Plasma discharges at W7-AS in the ‘electron root’ regime show highly peaked electron temperature- and pressure profiles. A switch-off of the ECRH leads to a fast temperature drop with time constants of less than 1 ms and a corresponding change of the Shafranov shift in the center of the plasma. For a temporally resolved comparison between calculated and measured temperature profiles, a fast method of gaining the mapping of the magnetic field to the radius along the ECE sightline is presented. Using these mappings to compare measured ECE signals with modeled temperatures (assuming plausible profiles of the radial electrical field), the time constant for the decay of the electric field after a ECRH power switch-off was found to be 0.6 ms.

The Temperature of the Extended Solar Corona

We use the Coronal Diagnostic Spectrometer instrument on board the Solar and Heliospheric Observatory to analyse coronal helmet streamer structures observed close to the solar minimum / maximum on the 1996 July 8 / 1999 August 4-5th. The radial variation of peak electron temperature is extracted out to 2 solar radii. These are found to agree well with Yohkoh observations close to the solar maximum, but are found to be reduced by around half a million close to the solar minimum.

Experimental study of laser beam transmission and power accounting in a large scale length laser plasma

It is shown that the measured laser power transmission through a large scale length, high temperature plasma (which emulates an indirect drive ignition-scale plasma) is in approximate agreement with the simulated transmission provided the simulations account for the power loss due to scattering from laser-plasma instabilities. Detailed accounting of the incident, transmitted, scattered, and absorbed powers is used to infer the likely location in the target where most of the scattering occurs along the incident beam trajectory. This location is near the incident laser side of the target at peak electron temperatures for a range of laser intensities. As a result, the backscattered light measurements at peak electron temperature do not require significant adjustment to account for attenuation of the backscattered light as it propagates out through the plasma.

Measurement of the energy penetration depth into solid targets irradiated by ultrashort laser pulses

The energy penetration depth of a short (100 fs) Ti-sapphire laser pulse (0.8 &mgr;m) of intensity 3x10(16) W/cm(2), in solid density materials has been measured. High-Z (BaF2) and low-Z (MgF2) solid layers targets were used. The penetration depth was determined from the measurement of the x-ray emission spectra, as a function of the target thickness. The investigation of these spectra showed that in the low-Z case, solid density material to a depth of 50 nm was heated to a peak electron temperature of approximately 150 eV. For the high-Z material, the penetration depth corresponding to this temperature exceeded 100 nm. This is evidence of a larger heat penetration depth in a high-Z material in comparison to a low-Z material. A model based on electron heat conduction is used to estimate the energy penetration depth. It is suggested that the larger heat penetration in high-Z material is due to heating of the material, caused by the radiation flux, generated by the electron heat conduction.

Ultrafast plasma studies by phase and amplitude measurements with femtosecond spectral interferometry

Using the technique of frequency-domain interferometry, we study the dynamics of ultrafast laser-produced plasmas at moderate laser irradiances. From the complex reflection coefficient measured with the two in-quadrature probe pulse polarizations, we determine the peak electron temperature during the pump laser pulse, the gradient scale length at critical density, the variation of the real and imaginary part of the dielectric constant of the plasma, and, using the Drude model, the collisionality of the plasma as a function of time and temperature. We find electron relaxation collision frequencies which depart from those predicted by the model of Lee and More (Phys. Fluids 1984;27:1273.) by no more than a factor of two.

Ablation characteristics of Au, Ag, and Cu metals using a femtosecond Ti:sapphire laser

Femtosecond laser ablation of metallic bulk crystals of Au, Ag and Cu was experimentally studied with laser pulse widths ranging from 120 fs through 800 fs at a center wavelength of 780 nm for micro-machining applications. Two different ablation regimes were found in terms of the laser fluence. The characteristic length of different ablation regimes was explained in terms of the optical skin depth and thermal diffusion length; it was determined by the peak electron temperature in the two-temperature model. The lateral feature of the two ablation regimes is discriminated by the amount of particles accumulated by the evaporation process. Ablated particle was observed less in the lower fluence regime than in the higher fluence regime, but there was no significant difference on the ablated surface. The parameters used in the two-temperature model, are discussed in order to model the ultrashort pulsed laser ablation process theoretically. It is shown that the obtainable range of the lower fluence regime is enhanced with the shorter pulse lasers, because the ablation etch rate is decreased with longer pulse width.

Titanium K-shell x-ray production from high velocity wire array implosions on the 20-MA Z accelerator

The advent of the 20-MA Z accelerator [R. B. Spielman, C. Deeney, G. A. Chandler et al., Phys. Plasmas 5, 2105 (1997)] has enabled implosions of large diameter, high-wire-number arrays of titanium to begin testing Z pinch K-shell scaling theories. The 2 cm long titanium arrays, which were mounted on a 40 mm diameter, produced between 75±15 to 125±20 kJ of K-shell x rays. A mass scan indicates that, as predicted, higher velocity implosions in the series produced higher x-ray yields. Spectroscopic analyses indicate that these high velocity implosions achieved peak electron temperatures from 2.7±0.1 to 3.2±0.2 keV and obtained a K-shell emission mass participation of up to 12%.

Simultaneous measurements from the Millstone Hill radar and the Active satellite during the SAID/SAR arc event of the March 1990 CEDAR storm

Abstract. During a nearby passage of the Active satellite above the Millstone Hill radar on 21 March 1990 at local sunset, the satellite and the radar performed simultaneous measurements of upper ionospheric parameters in nearly the same spatial volume. For this purpose the radar carried out a special azimuth-elevation scan to track the satellite. Direct comparisons of radar data and in situ satellite measurements have been carried out quite rarely. In this case, the coincidence of co-ordinated measurements and active ionospheric-magnetospheric processes during an extended storm recovery phase presents a unique occasion resulting in a very valuable data set. The measurements show generally good agreement both during quiet prestorm and storm conditions and the combination of radar and satellite observations gives a more comprehensive picture of the physical processes involved. We find a close relationship between the rapid westward ion drift peak at subauroral latitudes (SAID event) and the occurrence of a stable auroral red (SAR) arc observed after sunset by an all-sky imager and reported in an earlier study of this event. The SAID electric field is caused by the penetration of energetic ions with energies between about 1 keV and 100 keV into the outer plasmasphere to a latitude equatorward of the extent of the plasmasheet electrons. Charge separation results in the observed polarisation field and the SAID. Unusually high molecular ion densities measured by the satellite at altitudes of 700-870 km at subauroral and auroral latitudes point on strong upward-directed ion acceleration processes and an intense neutral gas upwelling. These structures are collocated with a narrow trough in electron density and an electron temperature peak as observed simultaneously by the radar and the satellite probes.Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma temperature and density); Magnetospheric physics (plasmasphere).

Simultaneous measurements from the Millstone Hill radar and the Active satellite during the SAID/SAR arc event of the March 1990 CEDAR storm

During a nearby passage of the Active satellite above the Millstone Hill radar on 21 March 1990 at local sunset, the satellite and the radar performed simultaneous measurements of upper ionospheric parameters in nearly the same spatial volume. For this purpose the radar carried out a special azimuth-elevation scan to track the satellite. Direct comparisons of radar data and in situ satellite measurements have been carried out quite rarely. In this case, the coincidence of co-ordinated measurements and active ionospheric-magnetospheric processes during an extended storm recovery phase presents a unique occasion resulting in a very valuable data set. The measurements show generally good agreement both during quiet prestorm and storm conditions and the combination of radar and satellite observations gives a more comprehensive picture of the physical processes involved. We find a close relationship between the rapid westward ion drift peak at subauroral latitudes (SAID event) and the occurrence of a stable auroral red (SAR) arc observed after sunset by an all-sky imager and reported in an earlier study of this event. The SAID electric field is caused by the penetration of energetic ions with energies between about 1 keV and 100 keV into the outer plasmasphere to a latitude equatorward of the extent of the plasmasheet electrons. Charge separation results in the observed polarisation field and the SAID. Unusually high molecular ion densities measured by the satellite at altitudes of 700-870 km at subauroral and auroral latitudes point on strong upward-directed ion acceleration processes and an intense neutral gas upwelling. These structures are collocated with a narrow trough in electron density and an electron temperature peak as observed simultaneously by the radar and the satellite probes.Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma temperature and density); Magnetospheric physics (plasmasphere).

A compact velocity-overshoot model for deep-submicron bipolar devices considering energy transport

This paper reports a compact velocity-overshoot model for deep-submicron bipolar devices considering energy transport based on a qualitative analysis. As verified by two-dimensional (2-D) simulation results, the analytical velocity-overshoot model predicts that in a bipolar device with a very short base width, its peak electron velocity, which can exceed the saturated velocity, occurs at a location ahead of its peak electron temperature location.

Measurements of planar target heating by an intense lithium ion beam

The heating of a planar multilayer (CH-Au-Al-CH) foil target by an intense lithium ion beam is measured using time-integrated spectroscopy of $K$\ensuremath{\alpha} x-ray satellite emission from the Al layer. The time-resolved beam irradiance, kinetic energy, and focal spot size are simultaneously measured using lithium ions Rutherford scattered from the foil. The approximately 20 ns full width at half maximum, 10 MeV peak kinetic energy, Li${}^{3+}$ ion beam deposited up to 435 TW/g (\ifmmode\pm\else\textpm\fi{}30%) in the Al layer. The peak electron temperature reached in the Al layer is estimated to be 38--43 eV by comparing the relative emission intensities from the He-like and Li-like Al with collisional-radiative-equilibrium (CRE) calculations. The data are used to examine one-dimensional radiation-hydrodynamic simulations that calculate the target plasma temperature and density using the measured beam parameters as input. It was found that a detailed CRE treatment of the atomic level populations and the line transport is essential for accurate calculations of the radiation loss from the ion heated target. Simulations that include such a treatment embedded within the radiation-hydrodynamic calculations are in good agreement with the data, within the experimental uncertainties. The methods developed here for observation and analysis of $K$\ensuremath{\alpha} spectra provide a sensitive tool suitable for the more stringent examinations of intense ion beam-matter interaction models that will ultimately be required for light-ion driven fusion.