Plasma Heat Flow Research Articles

Generation and characteristics of detached recombining plasma and its dynamic behaviour—a bridge between fusion plasmas and low-temperature ionized gases

First, the role of a magnetic divertor in fusion devices is introduced, and it is shown that the energy balance in burning International Thermonuclear Experimental Reactor indicates a strong necessity for reduction of plasma heat load on the divertor target plate. Next, the plasma heat flow to a material surface through sheaths is generally given in terms of the energy transmission factor, in which the energy deposition based on the surface recombination is essential in high-density plasmas. The heat load is independent of any cooling of the plasmas. Then, a variety of interesting characteristics of detached recombining plasmas, which are important in reducing the plasma heat flow, are discussed: generation and structure of plasma detachment, light emission originating from a series of highly excited Rydberg states of atoms, contribution of molecular activated recombination (MAR), transition between conventional electron–ion recombination (EIR) and MAR, relation to dust coagulation processes and the experimental verification of plasma detachment in tokamak fusion devices. Finally, the dynamic behaviour of the detached EIR plasma obtained in the linear divertor plasma simulator NAGDIS-II against the ELM-like heat pulse is shown.

Hot spot formation on electron-emissive target plate with plasma potential variation across magnetic field

Nonlinear dynamic behavior of hot spot formation on tungsten (W) material surfaces is numerically investigated by introducing a transient heat pulse into the local area of the hot W sheet immersed in high heat flux plasma. It is found that the formation and development of hot spot depend not only on plasma parameters (plasma heat flow, plasma potential variation) and the energy and time scale of heat pulse but also sensitively on electron-emission characteristics of the tungsten surface as well as the size of hot spot in a nonlinear way.

Various Phenomena on PSI. PSI in Fusion Devices. Edge Plasma Physics and Atomic-Molecular Processes.

Edge plasma physics and atomic-molecular processes relating to plasma heat flow to material surfaces are discussed. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma, resulting in hot spot formation on the plasma-facing materials due to nonlinear thermal bifurcation induced by thermoelectron emission. Also examined is the recombination enhancement due to the vibrationally excited hydrogen molecule generated from material surfaces in detached plasma with molecular activated recombination.

Experimental Study on Heat Flux from an Argon RF Plasma Using Laser Interferometry Method

The plasma heat flow to a birefringent substrate (LiNbO3) in an argon rf discharge has been investigated with the help of a laser interferometry method. This method is based on monitoring the variation of natural birefringence with temperature by the interferometry. The evaluation of the temperature curve shows that heat flux from the plasma towards the substrate depends on which electrode the substrate is put on. While the value is about 8% of the applied power in the case of the substrate on the powered electrode, the heat flux decreases to less than 1% in the case of on the grounded electrode. Ion fluxes have been obtained from the measured values of plasma potential, electron density and electron temperature. In the case of low pressure the ion flux approximately agrees with the value of the heat flux. However it decreases with an increase in pressure, and is almost 35% of the heat flux at 200 mTorr. Gas temperature in the plasma has been measured using a thermocouple in order to investigate the influence of gas pressure on the heat flux by neutral particles. The result shows that the gas temperature increases by raising the gas pressure.

Heat flows through plasma sheaths

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.

Enhancement of Plasma Heat Flow to the Conductive Divertor Plate Associated with Cross‐Field Potential Variation and Thermoelectron Emission

Enhancement of Plasma Heat Flow to the Conductive Divertor Plate Associated with Cross‐Field Potential Variation and Thermoelectron Emission

Experimental study on plasma heat flow to plasma-facing materials

The plasma heat flow to plasma-facing materials in a well-defined simulated divertor plasma system is investigated experimentally in order to specify the influences of the ion reflection, surface recombination and change in electron energy distribution function on it. A simulated divertor plate (tungsten or carbon), located normally to the magnetic field, was irradiated by hydrogen and helium plasmas. The reduction of the plasma heat flow caused by the ion reflection is clearly shown, to depend on a kind of materials. For the case of tungsten target, the ion energy reflection coefficients obtained experimentally is quantitatively in good agreement with those estimated by the empirical formula.

Reduction of plasma heat flow to a simulated divertor target plate by ponderomotive force

A clear reduction of the particle flux into a simulated divertor target place on which RF voltage pulse is applied is demonstrated experimentally in a hydrogen plasma. An overshooting phenomenon of the particle flux into the target plate is also observed after turning the RF field off. These experimental results can be explained by a simple model taking account of a reduced flow velocity to the target due to the effect of ponderomotive force.

Gas target experiments in high heat flux plasma of the TPD-I device

Gas target experiments are performed in the TPD-I device, which produces a high density helium plasma ∼ 6 × 10 19 m −3 with an electron temperature around 10 eV. The maximum plasma heat flow is estimated to be about 20 MW/m 2. A strong reduction of electron density and temperature was observed by feeding a secondary helium gas into the plasma and a stable detached plasma is formed in front of the target plate. The radial profile of the electron temperature is changed from peaked to hollow as the gas pressure is increasing. These experimental results are compared with the plasma simulations performed with the 2D plasma fluid B2 code. Good agreement with the simulated radial profiles of the plasma parameters is found.

Reduction of Plasma Heat Flow by Sheath‐Presheath Modifications with Ponderomotive Force

AbstractA r. f. induced directed force exerted by the application of r. f. voltage on the divertor plate or by introducing r. f. electric field in front of the plate is taken into account for the analysis on sheath‐presheath formation in a very simple manner. The ponderomotive force repels electrons coming to the divertor plate, which results in a reduction of the electron heat flow. The reduction of ionic impact energy to the plate is also expected even for presheath modification combined with positive divertor biasing as well as for successful sheath modification.

Novel technique for edge plasma control by RF ponderomotive force

A new method to reduce plasma heat flow to a limiter head has been developed with the use of a radio frequency (RF) field barrier. The RF current drawn on the limiter surface produces a ponderomotive potential, Ψ rf of the order of the temperature of the plasma near the limiter head in the frequency range, ω ci < ω ≪ ω ce. The limiter head is protected from the plasma by this potential barrier. Experimental proof of the RF limiter concept was attempted using the PISCES-A facility with plasma parameters which are comparable to the edge plasma of typical tokamaks. Production of the RF potential (~20 eV) is confirmed by the reflection of an electron beam injected in vacuum with an RF power of ~1 kW. As a result of the RF field formation, a remarkable suppression of low frequency fluctuations has been observed at the plasma periphery which is expected to reduce the plasma transport in the boundary layer. The experimental results are analyzed using numerical particle orbit and fluid drift calculations. Possible application of plasma flow due to Δ ⊥Ψ rf × B drift to make a diverter configuration in confinement devices is discussed.

Plasma heat flow for a simple collision term

The heat flow between two plates in a plasma of fixed uniform ion density has been studied using a simple collision term which represents isotropic elastic scattering with a constant collision frequency. Results from linear theory, the four-moment and a discrete ordinate calculation are compared. The last two are found to be in good agreement for all the values of the ratio of the length of the system to the mean free path which were studied.

Diffusion‐thermal effects in the topside of the nocturnal equatorial ionosphere

A study is made of the steady state heat balance of the O+, H+, and electron gases in the topside of the nocturnal equatorial ionosphere. The method used is that developed previously by Bailey and others; input plasma concentrations and drift velocities are taken from work by Moffett and Hanson. In the present study, additional terms in the plasma heat flow vectors, as suggested by St.‐Maurice and Schunk, are included. The additional term that depends on relative flow of O+ and H+ gives rise to diffusion‐thermal effects in the O+ and H+ gases. On the other hand, with a zero‐current condition relating the electron, O+, and H+ field‐aligned flows, the electron heat flow vector reduces to the usual thermal conduction form. It is found that for the conditions considered, on the field line with equatorial crossing point at about 800 km at 2200 LT, the H+ temperature is raised by about 50 K and the O+ temperature is raised by about 30 K over the equator in comparison with results obtained when the terms suggested by St.‐Maurice and Schunk are omitted. In the winter hemisphere the H+ temperature may be lowered by about 20 K, and the O+ temperature by about 10 K. It is suggested that diffusion‐thermal effects will become more important with increasing altitude in the equatorial topside ionosphere at night.

Determination of Streaming Velocity and the Flow of Heat and Mass in High-Current Arcs

Arc plasma streaming velocities were calculated from previously unpublished experimental data obtained by H. N. Olsen on total pressure distributions at the anodes in 100- to 300-amp argon and helium arcs. The dynamic streaming component of pressure was calculated by subtracting the magnetic component (due to the self-magnetic field of the arc) from this total pressure. Arc temperatures also measured by Olsen were then used to calculate densities and heat contents of the plasma, and these were used with the streaming pressure to calculate velocities and mass and heat flow distributions. These in turn were integrated to give total mass and heat flows. The results compare reasonably with other measurements and estimates in the literature. Peak velocities range from 16 000 to 60 000 cm/sec, mass flows from 0.04 to 0.23 g/sec. The plasma heat flow distribution shows a close similarity in shape to values of heat transfer intensity at the anode measured by O. H. Nestor. A comparison of the total heat flow in the plasma to the total heat transfer due to streaming at a series of currents shows heat transfer proportional to (heat flow)0.62 which agrees with correlations used for heat transfer from gases and flames. A simplified model of arc pumping is presented which suggests that heat and mass flow should increase as (current)32. A simple analog experiment in mercury is described which illustrates graphically the streaming caused by a divergent current path in a conductor.