Hot Plasma Column Research Articles

Estimate of the Upper Limit on Hot Plasma Differential Emission Measure (DEM) in Non-Flaring Active Regions and Nanoflare Frequency Based on the Mg xii Spectroheliograph Data from CORONAS-F/SPIRIT

The nanoflare-heating theory predicts steady hot plasma emission in the non-flaring active regions. It is hard to find this emission with conventional non-monochromatic imagers (such as Atmospheric Imaging Assembly or X-Ray Telescope), because their images contain a cool temperature background. In this work, we search for hot plasma in non-flaring active regions using the Mg XII spectroheliograph onboard Complex Orbital Observations Near-Earth of Activity on the Sun (CORONAS)-F/SPectroheliographIc X-ray Imaging Telescope (SPIRIT). This instrument acquired monochromatic images of the solar corona in the Mg XII 8.42 \AA line, which emits only at temperatures higher than 4 MK. The Mg XII images contain the signal only from hot plasma without any low-temperature background. We studied the hot plasma in active regions using the SPIRIT data from 18-28 February 2002. During this period, the Mg XII spectroheliograph worked with a 105-second cadence almost without data gaps. The hot plasma was observed only in the flaring active regions. We do not observe any hot plasma in non-flaring active regions. The hot plasma column emission measure in the non-flaring active region should not exceed $3 \times 10^{24}$ cm$^{-5}$. The hot Differential Emission Measure (DEM) is less than 0.01 % of the DEM of the main temperature component. Absence of Mg XII emission in the non-flaring active regions can be explained by weak and frequent nanoflares (delay less than 500 seconds) or by very short and intense nanoflares that lead to non-equilibrium ionization.

Guiding of laser pulses in plasma waveguides created by linearly-polarized femtosecond laser pulses

We experimentally demonstrate that plasma waveguides produced with ultra-short laser pulses (sub-picosecond) in gas jets are capable of guiding high intensity laser pulses. This scheme has the unique ability of guiding a high-intensity laser pulse in a plasma waveguide created by the same laser system in the very simple and stable experimental setup. A hot plasma column was created by a femtosecond class laser that expands into an on-axis parabolic low density profile suitable to act as a waveguide for high intensity laser beams. We have successfully guided ~1015 W cm−2 laser pulses in a 8 mm long hydrogen plasma waveguide with a 35% guiding efficiency.

Quantitative Determination of Density of Ground State Atomic Oxygen from Both TALIF and Emission Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Two experimental techniques have been used to quantify the atomic oxygen density in the case of hot air plasma generated by a microwave (MW) resonant cavity. The latter operates at a frequency of 2.45 GHz inside a cell of gas conditioning at a pressure of 600 mbar, an injected air flow of 12 L/min and an input MW power of 1 kW. The first technique is based on the standard two photon absorption laser induced fluorescence (TALIF) using xenon for calibration but applied for the first time in the present post discharge hot air plasma column having a temperature of about 4500 K near the axis of the nozzle. The second diagnostic technique is an actinometry method based on optical emission spectroscopy (OES). In this case, we compared the spectra intensities of a specific atomic oxygen line (844 nm) and the closest wavelength xenon line (823 nm). The two lines need to be collected under absolutely the same spectroscopic parameters. The xenon emission is due to the addition of a small proportion of xenon (1% Xe) of this chemically inert gas inside the air while a further small quantity of H2 (2%) is also added in the mixture in order to collect OH(A-X) and NH(A-X) spectra without noise. The latter molecular spectra are required to estimate gas and excitation temperatures. Optical emission spectroscopy measurements, at for instance the position z=12 mm on the axis plasma column that leads to a gas measured temperature equal to 3500 K, an excitation temperature of about 9500 K and an atomic oxygen density 2.09×1017±0.2×1017 cm−3. This is in very good agreement with the TALIF measurement, which is equal to 2.0×1017 cm−3.

Simulation and Design of Biological Shield for the 115 kJ IR-MPF-100 Plasma Focus Device Using MCNP Code

The paper reports on the design of biological neutron shielding for IR-MPF100 plasma focus device which recently has been designed and constructed in plasma physics and nuclear fusion research institute. Plasma focus devices are known as pulsed intense sources of ionizing radiations such as hard X-ray and fast neutrons as a result of the formation of a hot dense plasma column then acceleration of energetic ions and electrons in the opposite directions. Therefore, taking into account a biological shield particularly for the operators of the PF device as radiation workers is crucial. Analytical calculations on the maximum permissible effective dose for radiation workers (for whole-body exposure) allow below 200 shots/year for IR-MPF100 operating at its nominal 115 kJ capacitor bank energy without any shielding wall. In order to decrease the personnel absorbed radiation dose and increase the maximum allowed shot per year the design considerations for a biological shield has been recognized using MCNP4C code. Our calculations was based on the effect of ordinary concrete, polyethylene mixed with 30 % natural boron and solid boric acid on the decrement of the absorbed dose. These calculations represent that using a double layer shield consists of 30 cm width of pure polyethylene as well as 10 cm lead, ends in appropriate decrement of the effective dose per shot from 0.1 mSv to 1.2 µSv, therefore increases the allowed usage of the device up to 15,600 annual shots.

Study of erosion and deposition characteristics of Li during liquid Li limiter experiment in HT-7

In the magnetic confinement fusion device, the first wall as plasma facing components will directly affect the performance of high temperature plasma. And the interaction of plasma and materials also affect the life of the first wall. Liquid lithium first wall receives more and more attention due to the properties of repairing itself and effectively inhibiting boundary particle recycling. So the research of the interaction between liquid lithium wall and plasma is particularly important. Erosion and deposition characteristics of lithium and its influence on the performance of plasma during lithium limiter experiment in HT-7 device are studied in-depth in this paper. Experimental results show that when the interaction between Li and plasma is weak, Li enters into the plasma mainly by weak surface evaporation and sputtering. During this process, Li line emission is strengthened, impurity and hydrogen recycling is decreased resulting in the improvement of plasma performance. When the interaction between Li and plasma becomes extremely strong, it is found so many big scale Li droplets ejected from liquid lithium surface to cause intense Li efflux into plasma, leading to plasma discharge disruption. Li atoms coming from Li limiter are ionized in the scrape-off layer (SOL), and entered into hot plasma column as ions (Li+, Li2+, Li3+) and transported in plasma. After the experiment, it can be found that a lot of white spots distributed in the vacuum chamber wall, with its main composition being Li2CO3 by XPS analysis. Through observing Li spot distribution and analyzing the lithium film thickness by scanning electron microscopy (SEM) in different samples, it is observed that the lithium is primarily deposited around the limiter, but the number of Li spots is more at the low field side than that at the high field side of the device, and the Li film gradually becomes thinner along the toroidal direction of the HT-7 device, leading to the non-uniformity of impurity and hydrogen recycling. The experiment may provide a reference for studying the interaction of plasma and liquid lithium first wall and the application of liquid lithium first wall in future tokamak device.

Kinetic effects and nonlinear heating in intense x-ray-laser-produced carbon plasmas.

The x-ray laser-matter interaction for a low-Z material, carbon, is studied with a particle-in-cell code that solves the photoionization and x-ray transport self-consistently. Photoionization is the dominant absorption mechanism and nonthermal photoelectrons are produced with energy near the x-ray photon energy. The photoelectrons ionize the target rapidly via collisional impact ionization and field ionization, producing a hot plasma column behind the laser pulse. The radial size of the heated region becomes larger than the laser spot size due to the kinetic nature of the photoelectrons. The plasma can have a temperature of more than 10000 K (>1eV), an energy density greater than 10^{4} J/cm^{3}, an ion-ion Coulomb coupling parameter Γ≥1, and electron degeneracy Θ≥1, i.e., strongly coupled warm dense matter. By increasing the laser intensity, the plasma temperature rises nonlinearly from tens of eV to hundreds of eV, bringing it into the high energy density matter regime. The heating depth and temperature are also controllable by changing the photon energy of the incident laser light.

Plasma expansion into a waveguide created by a linearly polarized femtosecond laser pulse

We demonstrate the efficient generation of 4 mm and 8 mm long plasma waveguides in hydrogen and helium. These waveguides have matching spots sizes for 13 to 34 μm laser beams. The plasma waveguides are created by ultra-short laser pulses (sub-picosecond) of moderate intensities, ∼1015–1016 W cm−2, that heat the plasma to initial temperatures of tens of eV in order to create a hot plasma column that will expand into a plasma waveguide. We have determined that the main heating mechanism when using fs laser pulses and plasma densities ∼1018–19 cm−3 is Above Threshold Ionization. Detailed time and space electron density measurements are presented for the laser produced plasma waveguides.

Hard TiCx/SiC/a-C:H nanocomposite thin films using pulsed high energy density plasma focus device

Thin films of TiCx/SiC/a-C:H were synthesized on Si substrates using a complex mix of high energy density plasmas and instability accelerated energetic ions of filling gas species, emanated from hot and dense pinched plasma column, in dense plasma focus device. The conventional hollow copper anode of Mather type plasma focus device was replaced by solid titanium anode for synthesis of TiCx/SiC/a-C:H nanocomposite thin films using CH4:Ar admixture of (1:9, 3:7 and 5:5) for fixed 20 focus shots as well as with different number of focus shots with fixed CH4:Ar admixture ratio 3:7. XRD results showed the formation of crystalline TiCx/SiC phases for thin film synthesized using different number of focus shots with CH4:Ar admixture ratio fixed at 3:7. SEM results showed that the synthesized thin films consist of nanoparticle agglomerates and the size of agglomerates depended on the CH4:Ar admixture ratio as well as on the number of focus shots. Raman analysis showed the formation of polycrystalline/amorphous Si, SiC and a-C for different CH4:Ar ratio as well as for different number of focus shots. The XPS analysis confirmed the formation of TiCx/SiC/a-C:H composite thin film. Nanoindentation results showed that the hardness and elastic modulus values of composite thin films increased with increasing number of focus shots. Maximum values of hardness and elastic modulus at the surface of the composite thin film were found to be about 22 and 305GPa, respectively for 30 focus shots confirming the successful synthesis of hard composite TiCx/SiC/a-C:H coatings.

RELAXATION OF PULSAR WIND NEBULA VIA CURRENT-DRIVEN KINK INSTABILITY

We have investigated the relaxation of a hydrostatic hot plasma column containing toroidal magnetic field by the Current-Driven (CD) kink instability as a model of pulsar wind nebulae. In our simulations the CD kink instability was excited by a small initial velocity perturbation and developed turbulent structure inside the hot plasma column. We demonstrated that, as envisioned by Begelman, the hoop stress declines and the initial gas pressure excess near the axis decreases. The magnetization parameter "σ", the ratio of the magnetic energy to the thermal energy for a hot plasma, declined from an initial value of 0.3 to about 0.01 when the CD kink instability saturated. Our simulations demonstrated that axisymmetric models strongly overestimate the elongation of the pulsar wind nebulae. Therefore, the previous requirement for an extremely low pulsar wind magnetization can be abandoned. The observed structure of the pulsar wind nebulae do not contradict the natural assumption that the magnetic energy flux still remains a good fraction of the total energy flux after dissipation of alternating fields.

THREE-DIMENSIONAL RELATIVISTIC MAGNETOHYDRODYNAMIC SIMULATIONS OF CURRENT-DRIVEN INSTABILITY. II. RELAXATION OF PULSAR WIND NEBULA

We have investigated the relaxation of a hydrostatic hot plasma column containing toroidal magnetic field by the current-driven (CD) kink instability as a model of pulsar wind nebulae. In our simulations, the CD kink instability is excited by a small initial velocity perturbation and develops a turbulent structure inside the hot plasma column. We demonstrate that, as envisioned by Begelman, the hoop stress declines and the initial gas pressure excess near the axis decreases. The magnetization parameter σ, the ratio of the Poynting to the kinetic energy flux, declines from an initial value of 0.3 to about 0.01 when the CD kink instability saturates. Our simulations demonstrate that axisymmetric models strongly overestimate the elongation of the pulsar wind nebulae. Therefore, the previous requirement for an extremely low pulsar wind magnetization can be abandoned. The observed structure of the pulsar wind nebulae does not contradict the natural assumption that the magnetic energy flux still remains a good fraction of the total energy flux after dissipation of alternating fields.

Preliminary measurements in Sahand plasma-focus emphasizing on the temporal characteristics of hard and soft X-rays

Plasma Focus has been known as an intense source of soft and hard X-rays (SXR and HXR). In this system a hot and dense plasma column is formed as a result of fast discharging a high current (∼MA) in a given gas. Various radiations are emitted from the pinched plasma. The aim of this paper is to give some information about the temporal behavior of the emitted X-rays from Sahand Filippov type plasma focus device. The optimum conditions for the X-ray production are also investigated. The results show that at each discharge voltage, there is an optimum pressure in which the pinch current is maximum as well as the HXR and SXR yields in the case of argon as the working gas. The optimum pressure increases linearly with the operation voltage. Furthermore the pinch current along with the yield of X-rays increases with the operating voltage at the optimum pressure. The study of the time resolved and time integrated signals of hard and soft X-rays in neon, argon and deuterium as working gases, indicate that the small peaks in the time resolved signals of HXR can be attributed to “micro pinches” which are observed in all above mentioned three gases. Attributing the main peak to the onset of the instability in the plasma column, one can declare that the life-time of the pinch decreases with the atomic number of the gas. The full width at half maximum (FWHM) of the pulses which actually represents the radiation time is maximum for Ne and minimum for Ar.

Self-sustained detachment in the Large Helical Device

Self-sustained detachment has been obtained in the Large Helical Device (LHD). Strong hydrogen gas puffing of ∼200 Pa m3 s−1 after a density feedback phase detaches the plasma from the divertor plates with high reproducibility. High electron density of over 1 × 1020 m−3 is sustained without gas puffing until the heating beam stops and a high-density flat top for 2 s has been demonstrated. Throughout the self-sustained detachment phase, the minor radius of the hot plasma column shrinks to ∼90% of the last-closed-flux-surface, which corresponds to the rational surface. This new state has been named the ‘Serpens mode’, for self-regulated plasma edge 'neath the last-closed-flux-surface. Global energy confinement of the Serpens mode is compared with the international stellarator scaling 1995 (ISS95) and the recently established scaling for high-density LHD plasmas (HD scaling), where shrinking confinement volume and shallow penetration of the heating beams are taken into account. Although the energy confinement of the Serpens mode seems deteriorated compared with ISS95, as in the case of high-density attached plasmas, it is consistent with the HD scaling. This suggests that the energy confinement properties of detached plasmas in LHD are similar to those in high-density attached plasmas.

Density Regimes of Complete Detachment and Serpens Mode in LHD

In the Large Helical Device (LHD), the hot plasma column shrinks at the high-density regime and complete detachment takes place. Hydrogen volume recombination is observed at complete detachment. This phase is self-sustained under specific experimental conditions and called the Serpens mode (self-regulated plasma edge ‘neath the last-closed-flux-surface). The Serpens mode is achieved after either rapid or slow density ramp up, and either by hydrogen or helium gas puffing. The threshold conditions for complete detachment and the Serpens mode are experimentally documented in the parameter space of heating power and density. The threshold density for the Serpens mode transition increases with ∼ 0.4 power of the heating power. The total radiation is shown to be not adequate to describe the threshold conditions, since it mainly includes the information of very edge region outside the hot plasma column. The operational density limit in LHD, which is sustainable in steady state, has been extended to 1.7 times as high as the Sudo density limit, by applying pellet injection to the Serpens plasmas.

Modelling of heat deposition onto the Tore Supra toroidal pumped limiter

The new CIEL (Composantes Internes Et Limiteur) configuration of the Tore Supra tokamak has as its main plasma-facing component (PFC) a Toroidal Pumped Limiter (TPL) [P. Garin et al., in: Proceedings of the 20th Symposium on Fusion Technology, Marseille, vol. 2, 1998, p. 1709], which must sustain the bulk of the energy leaving the plasma. Analysis of the heat deposition pattern on the TPL indicates that perpendicular heat transport may play at least as significant a role as parallel heat transport [F. Saint-Laurent et al., Nucl. Fusion 40 (2000) 1047, R. Mitteau et al., these Proceedings]. We present a new approach for modelling the heat deposited onto the TPL, which follows test ‘heat packet’ trajectories backwards from the TPL towards the hot plasma column. Results are compared with experimental data and trends due to plasma parameters dependencies are described. Because of ripple effects, the limiter is covered by wetted areas with long connection lengths (tens of meters), and shadowed areas with very short connection lengths (centimeters). Sharp transitions between the two are clearly seen in experiment and also reproduced in the model.

Experimental study of negative ion profiles in H2-MAR plasmas in divertor simulator MAP-II

The negative hydrogen ion density profile is measured in hydrogen molecular assisted recombination (MAR) plasmas in the steady-state linear divertor/edge plasma simulator material and plasma (MAP)-II device. Fulcher band spectroscopy is also performed for the measurement of the ro-vibrational distribution of hydrogen molecules and the degree of dissociation. We have developed negative ion diagnostics using a combined scheme of an ‘eclipse’ laser photo-detachment system and a double probe. The former is to avoid probe surface ablation phenomena while the latter is to avoid the anomaly in the Langmuir probe characteristics in recombining plasmas, which leads to an overestimation of Te. Negative hydrogen ions are detected in the peripheral regions of the hot plasma column while vibrationally excited molecules exist in the plasma core. Based on the rate equation, the measured negative ion density is compared with the results of model calculations.

Corona-Free Electrical Explosion of Polyimide-Coated Tungsten Wire in Vacuum

We present experimental evidence of corona-free electrical explosion of dielectric-coated W wire in vacuum. A fast current rise of approximately 150 A/ns and a coating of 2 microm polyimide are both needed to achieve the corona-free regime of explosion. Breakdown is absent in corona-free explosion; the wire remains resistive, and this allows anomalously high energy deposition (approximately 20 times atomization enthalpy). MHD simulations reproduce the main differences between corona and corona-free explosions. A corona-free explosion of a wire can be useful for the generation of a hot plasma column by direct energy deposition.

A system for cryogenic hydrogen pellet high speed inboard launch into a fusion device via guiding tube transfer

Particle deposition deep inside the hot target plasma column by cryogenic hydrogen pellet injection is required for efficient particle refueling of fusion devices such as tokamaks. As the ablation plasmoid is subject to a strong outward drift in hot plasmas, pellet launch from the tokamak inboard side is more useful than from the outboard. The depth of the pellet particle deposition depends on density and temperature of the target plasma, and on the pellet mass and velocity. Plasma operation determines density and temperature values, the maximum affordable density perturbation limits the pellet mass. Consequently, the pellet speed remains the only technically variable parameter allowing improvement of the refueling performance. To achieve this an inboard high-speed pellet injection system based on looping type geometry was designed and built at the midsize tokamak ASDEX Upgrade, and first fueling studies had validated the potential for the required injection velocity increase. Throughout the last two years experimental efforts focused on careful step-by-step optimization of the different system hardware components and the operational procedures. Introducing amongst other features a well pumped, rectangular guide track section, the feasibility for the inboard launch scheme up to an injection velocity of 1 km/s was successfully demonstrated. Detailed off-line tests have confirmed that pellets can withstand controlled mechanical and thermal impact even at this high speed, albeit for the sacrifice of increasing and significant mass losses. In a first application to plasma refueling deep penetration into hot target plasmas and hence, high fueling performance was achieved by deeper pellet born particle deposition and hence enhanced particle sustainment times.

Three-temperature model for the dynamics of a plasma produced by exploding metal wires

An MHD model of the implosion of a dense hot plasma column is developed. Because of a better description of radiation transport, this model has a higher spatial resolution compared to the previously developed, simpler, two-temperature model. The new model is applied to calculating the load (a single metal wire or an X-pinch, in particular, a heterogeneous corona-core structure with a sharp boundary) of a nanosecond high-voltage generator. An algorithm of the type previously used to solve the problem in the two-temperature model is supplemented by the iteration procedure for calculating the quasisteady radiation under the assumption that the plasma is optically thick.

XUV emission from an elongated plasma column produced using a high-power laser with a gas puff target

We report the first studies on the formation of an elongated plasma column suitable for X-ray laser experiments, using a gas puff target irradiated with a high-power laser. The gas puff targets, produced by pulsed injection of a small amount of gas from a high-pressure solenoid valve through a nozzle in the form of a slit, have been characterized by optical interferometry and X-ray backlighting. The formation of a hot plasma column up to 30-mm-long is demonstrated. The spatial uniformity of the column was monitored by means of an X-ray pinhole camera. XUV spectra measurements for SF6 gas puff targets show predominant 3–2 line of hydrogenic fluorine (λ = 8.1 nm), however, only linear increasing of its intensity with the target length was observed.

Nonlinear evolution of a wave packet propagating along a hot magnetoplasma column

The method of multiple scales is used to derive a nonlinear Schrödinger equation, which describes the nonlinear evolution of electron plasma ‘slow waves’ propagating along a hot cylindrical plasma column, surrounded by a dielectric medium and immersed in an essentially infinite axial magnetic field. The temperature is included as well as mobile ion effects for ail possible modes of propagation along the magnetic field. From this equation the condition for modulational instability for a uniform plasma wave train is determined.