Electron Thermal Flux Research Articles

Nonlocal heat transfer in nanostructures

Kinetics of electrons in a degenerate conductor heated up by absorption of a high-frequency field localized in a region of about hundred nanometers has been studied. A new law for nonlocal electron thermal flux has been predicted.

Response of multiscale turbulence to electron cyclotron heating in the DIII-D tokamak

Small-scale density turbulence (k⊥ρi∼4–10) and electron thermal flux are both observed to increase during electron cyclotron heating (ECH) of a high-temperature tokamak plasma (k⊥ is the turbulent wavenumber and ρi the ion gyroradius). In contrast, large- (k⊥ρi≤1) and intermediate-scale (k⊥ρi∼1–3) turbulence ñ∕n levels and ion thermal transport remain effectively constant. This implies that the small-scale turbulence is not a remnant or tail of the ubiquitous, large-scale or intermediate-scale turbulence, and also indicates a potentially important role in determining anomalous electron thermal transport. Radial scans of small-scale turbulence during ECH indicate decreased fluctuations in the deep core compared with increased levels towards the edge. This trend is consistent with linear gyrokinetic growth rate predictions for electron temperature gradient driven instabilities.

Radiation hydrodynamics with backscatter and beam spray in gas filled hohlraum experiments at the National Ignition Facility

Several experiments using either CO 2 or propane gas filled halfraums [i.e. hohlraums with a single laser entry hole (LEH)] have been shot at the National Ignition Facility (NIF) in a joint LosAlamos/Livermore collaboration. The experiments have been modeled by the Lasnex code. The possibility of beam spray due to filamentation of the incident laser beam is assessed through simulations which parametrically decrease the f-number of the beam at times of high intensity. The uncertainty in heat transport is evaluated through parametric variations in the electron thermal flux limit (fe). Each calculation in the resulting two parameter set is post-processed to simulate outputs which can be compared with Dante detector results for the soft X-ray flux through the LEH, and gated, framed images of hard X-rays (FXI) through the hohlraum side walls. Simulations which well match the data for both gases indicate that the laser energy is penetrating the gas filled hohlraum even towards the end of the pulse. This suggests that the gas fill is useful in keeping the hohlraum open to laser energy throughout the pulse.

Temperature gradient driven electron transport in NSTX and Tore Supra

Electron thermal fluxes are derived from the power balance for Tore Supra (TS) and NSTX discharges with centrally deposited fast wave electron heating. Measurements of the electron temperature and density profiles, combined with ray tracing computations of the power absorption profiles, allow detailed interpretation of the thermal flux versus temperature gradient. Evidence supporting the occurrence of electron temperature gradient turbulent transport in the two confinement devices is found. With control of the magnetic rotational transform profile and the heating power, internal transport barriers are created in TS and NSTX discharges. These partial transport barriers are argued to be a universal feature of transport equations in the presence of invariant tori that are intrinsic to non-monotonic rotational transforms in dynamical systems.

Electron transport and the critical temperature gradient

Tore Supra electron thermal fluxes, analyzed over a range of heating powers and plasma densities, are shown to vary parametrically according to the small-scale electron temperature gradient (ETG) model, rather than the ion inertial scale electrostatic gyro-Bohm model. Steady-state power balance analysis and time-varying interpretative transport simulations, performed on the Tore Supra Fast Wave Electron Heating database, validate the ETG thermal flux-versus-gradient relation and the existence of a critical electron temperature gradient. The critical gradient length R/Lc and the parametric dependence of the thermal flux, qe(ne,Te,∇Te,q,s), agree well with the ETG model.

Two-dimensional simulations of the transition from Townsend to glow discharge and subnormal oscillations

The transition from Townsend to glow discharge is studied via two-dimensional simulations of discharges with moderate pd values corresponding to the right branch of the Paschen curve (p is the gas pressure and d the electrode spacing). The discharge model is coupled to the external circuit model enabling simulations of subnormal oscillations during the discharge transition from the Townsend to normal regime. This transition is accompanied by radial constriction of the discharge and formation of a normal cathode spot. The electron thermal flux backscattered to the cathode is observed to be important in the Townsend regime, which contributes significantly to discharge instability by virtue of the secondary emission coefficient γ becoming an increasing function of E/N. The simulations are carried out for different gas pressures and different parameters of the external circuit. The predicted results are in agreement with experimental observations and previously published semi-analytical models.

Time-dependent electron thermal flux inhibition in direct-drive laser implosions.

We simulate direct-drive CH target implosions with square laser pulses by a one-dimensional Fokker-Planck solver combined with a hydrodynamic code, and compare the results with those simulated by the flux-limited Spitzer-Härm model. We find that the electron thermal flux inhibition is time dependent, resulting in longer density scale length, larger laser absorption, and smaller growth of Rayleigh-Taylor instability. The time of peak neutron production calculated from Fokker-Planck simulations agrees with experiments for both 1-ns and 400-ps pulses.

Electronic thermal properties of the interface between a normal metal and a high-temperature superconducting material

The electron thermal flux and electron thermal conductivity of the interface between a normal metal and a high-temperature superconductor (HTSC) are calculated using quasiclassical equations. Calculations are made for various values of the interface transparency and various orientations of the axes of a HTSC crystal. It was shown that compared with an interface between a normal metal and an “ordinary” superconductor (s-type symmetry, isotropic order parameter), the thermal conductivity of an HTSC-normal-metal interface is substantially higher and has a nonactivation dependence. The thermal properties were calculated for various interface models, including mirror and diffuse, and also for various potential barrier profiles. An analysis is made of the possibility of using devices based on normal-metal-HTSC interfaces for bolometric and microrefrigerator applications.

Dissipative models of colliding stellar winds -- I. Effects of thermal conduction in wide binary systems

The influence of electron thermal conduction on the 2D gas dynamics of colliding stellar winds is investigated. It is shown that, as a result of the non-linear dependence of the electron thermal flux on the temperature, the pre-heating zones (in which the hot gas in the interaction region heats the cool winds in front of the shocks) have finite sizes. The dependence of the problem of the structure of the flow in the interaction region on the dimensionless parameters is studied, and a simple expression is derived for the size of the pre-heating zones at the axis of symmetry. It is shown that small values of the thermal conductivity do not suppress the Kelvin–Helmholtz instability if the adiabatic flow is subject to it. Further studies, both numerical and analytical, in this direction will be of great interest. The influence of thermal conduction on the X-ray emission from the interaction region is also estimated.

Ambipolar magnetic fluctuation-induced heat transport in toroidal devices

The total magnetic fluctuation-induced electron thermal flux has been determined in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] from the measured correlation of the heat flux along perturbed fields with the radial component of the perturbed field. In the edge region the total flux is convective and intrinsically ambipolar constrained, as evidenced by the magnitude of the thermal diffusivity, which is well approximated by the product of ion thermal velocity and the magnetic diffusivity. A self-consistent theory is formulated and shown to reproduce the experimental results, provided nonlinear charge aggregation in streaming electrons is accounted for in the theory. For general toroidal configurations, it is shown that ambipolar constrained transport applies when remote magnetic fluctuations (i.e., global modes resonant at distant rational surfaces) dominate the flux. Near locations where the dominant modes are resonant, the transport is nonambipolar. This agrees with the radial variation of diffusivity in MST. Expectations for the tokamak are also discussed.

Anomalous transport due to the ion acoustic drift mode in tokamaks

In toroidal geometry, the ion Landau resonance is suppressed by the ion magnetic drift for modes propagating in the electron diamagnetic drift, and long wavelength modes with (k⊥ρ)2 ≃ \U0001d4aa(0.01) remain unstable. Coupling to the ion acoustic transit mode becomes predominant in such long wavelength regime allowing the mode frequency to exceed the diamagnetic drift frequency by a large margin. Finite β has a destabilizing influence on the toroidal ion acoustic mode particularly in the long wavelength regime. The quasilinear electron and ion thermal fluxes both increase with β partly through finite β modification of the mode frequency and/or growth rate, and partly through magnetic fluctuations. Even the trapped electron thermal flux exhibits an increase with β. In strong turbulence regime, the thermal diffusivity scales as Bohm corrected for finite β, χ ∝ (r/R)2(cT/eB)q2β.

Electromagnetic ion acoustic ballooning mode in tokamaks

It is shown that in toroidal geometry, extremely long-wavelength modes with the ion finite Larmor radius parameter (kperpendicular to rho )2 approximately=O(10-3) and mode frequency of the order of the ion acoustic transit frequency can be strongly destabilized by a finite beta (the ratio of plasma to magnetic pressure) well below the MHD ballooning limit. Ion Landau resonance is suppressed by the ion magnetic drift for modes propagating in the electron diamagnetic drift. The quasilinear electron and ion thermal fluxes both increase with beta . In the strong turbulence regime, a Bohm-type thermal diffusivity corrected for finite beta , chi alpha (r/R)2(cT/eB)q2 beta , emerges.

Feedback suppression of the radiative condensation instability

A new scheme of feedback suppression of the radiative condensation instability in the tokamak edge plasma is presented. The physics basis of this scheme is the modulation of the parallel electron thermal flux at the instability frequency via a feedback circuit. The latter consists of a temperature sensor, amplifiers, phase shifters, and a suitable form of insulated segmented poloidal limiter sections used as suppressors. The necessary feedback power is the order of a kilowatt for small tokamaks and a few megawatts for large reactor-type machines.

Plasma Processes in Solar Flares

A rationale is presented for a conception that appearance of flares in active regions is due to the interaction of large-scale convective elements. Such an interaction gives rise to shear motions in the vicinity of the inverse polarity line of the photospheric magnetic field which generate vortical motions leading to non-equilibrium state of the magnetic configuration. Modern concepts of manifestations of turbulent plasma processes are described in terms of theoretical models for solar flares. Plasma effects arising at propagation of electron beams and thermal fluxes in the solar atmosphere are considered. Their role in the interpretation of hard X-ray and type III radio bursts is pointed out. The role of the turbulent Stark effect for diagnostics of collective plasma processes in solar flares is emphasized.

Anomalous heat flux due to magnetic electron-drift vortex waves in a magnetized plasma.

The magnetic electron-drift vortex mode in a magnetized plasma is investigated using the Braginskii transport equations for the electrons. In a sufficiently inhomogeneous external magnetic field, the mode can become unstable, causing enhanced local magnetic fluctuations. The latter give rise to an anomalous electron thermal flux that significantly affects the transport characteristics of the plasma. Using the conventional quasilinear approach, the corresponding anomalous thermal conductivity is obtained.

Extraction of volume-produced H− ions from a hybrid magnetic multicusp source

The negative ion density n− is studied using the photodetachment technique in the extraction region of a 5.7-l hybrid magnetic multicusp source, and is correlated with the extracted negative ion current I−. It is found that biasing positive the plasma electrode (PE) of the extractor affects in a similar way both n− and I−: both attain a maximum for a PE bias of +1.5 V, while the electron density ne and extracted current Ie exhibit a monotonic reduction. At optimum PE bias, n−/ne attains a value of 0.7. Ie is found to be comparable to the electron thermal flux through the extraction slit; this indicates that there is no potential barrier for the negative particles at the plasma border in front of the extraction slit. At optimum PE bias both Ie and I− vary in proportion to n0.8e. Two regimes characterize the variation of I− vs n−; at high density I− varies in proportion to n1− and appears to be larger than the thermal negative ion flux through the extraction slit, calculated under the assumption that the negative ion temperature is close to the neutral molecule temperature. The discrepancy is the largest (a factor of ten) at the lowest gas pressure.

Anomalous drift-wave transport analysis of tokamak discharges

The drift-wave model of anomalous transport is used to interpret electron energy balance in discharge data from five tokamaks. A consistent interpretation requires the inclusion of impurity radiation losses which, in some cases, become comparable with the Ohmic power input. For discharges with a well-defined radial electron thermal flux the value of the coefficient in the drift-wave transport formula is determined along with bounds on the variation of the coefficient that are consistent with the interpretation of the data.

Spontaneous Generation of Electromagnetic Waves in Plasmas with Electron Thermal Flux

Spontaneous generation of propagating electromagnetic fields due to a microinstability is investigated for plasmas which convey electron thermal fluxes. The following two cases are examined: 1) Electromagnetic fields spontaneously excited by electrons in a velocity distribution of skewed Maxwellian type. 2) Electromagnetic waves generated by electrons in a velocity distribution which consists of a main part and a high energy part. In this case, the electron thermal flux can be very high. In both cases, induced electromagnetic waves with relatively low frequencies propagate parallel to the direction of Thermal flux

Hydrodynamics and burn of optimally imploded deuterium-tritium spheres

The phenomenology of optimized laser-driven DT sphere implosions leading to efficient thermonuclear burn is reviewed. The optimal laser deposition profile for spheres is heuristically derived. The performance of a 7.5 μg sphere, exposed to its optimal 5.3 kJ pulse, is scrutinized in detail. The timing requirements for efficient central ignition of propagating burn in the sphere are carefully explored. The difficulties stemming from superthermal electron production and thermal flux limitation are discussed. The hydro-burn performance of spheres is characterized as a function of the pulse energy, peak power, time scale, pulse exponent, wavelength, and on the degree of flux limitation. The optimal pulse parameters are determined for spheres with masses ranging from 40 ng to 250 μg, requiring from 50 J to 150 kJ of input energy, and the corresponding optimal performance levels are calculated. Discussion is given to the hydro-burn performance of new structured fusion targets, in which the DT is contained as a gas or frozen as an ice shell inside a high Z pusher-tamper layer.