Ohmic Heating Power Research Articles

Soft x-ray diagnostics system for electron temperature measurement in the integrated commissioning phase of JT-60SA.

A soft x-ray (SX) diagnostic system has been designed and installed in JT-60SA for the first plasma. For quantitative measurement, the etendue of each viewing chord was evaluated analytically and numerically. The electron temperature is evaluated from the detected bremsstrahlung emission ratio between two detector arrays with two different thicknesses of Be filters. The two filter thicknesses were optimized to be 7 and 50μm for the expected electron temperature range of 0-3keV. The one-dimensional profile of SX emission is reconstructed from line integrated emission by the elliptic Abel inversion scheme. For a plasma discharge with a plasma current of 1 MA, a peaked electron temperature profile with about 800eV at the center is obtained. The total bremsstrahlung power was also evaluated using the electron temperature profile and the absolutely evaluated etendue of each viewing chord. In this evaluation, the bremsstrahlung power is around 10% of ohmic heating power.

Modelling of ohmic startup and runaway electron formation in support of JT-60SA initial operation

The plasma startup with ITER-relevant sub-mPa neutral pressures in the JT-60SA superconducting tokamak is studied using a zero-dimensional (0D) power balance model for the plasma burn-through phase. In the ohmic startup scenario assumed here, the lower limit of the operational neutral pressure is determined by the condition for the Townsend avalanche at a high limit, whereas the upper limit depends on the available ohmic heating power for the burn-through with the base power supply (PS) of the superconducting coils, corresponding to 0.2–0.3 V m−1. The operational conditions that lead to RE generation are evaluated to mitigate the risk of RE discharges. The simulation shows that startup REs are generated during the temperature rise immediately after the burn-through under relatively low-density conditions, and then the RE current value is gradually increased in balance with the RE loss processes on the timescale of I p ramp-up. Therefore, it is expected that REs can be avoided or mitigated if the electron density during the early build-up phase is tuned by a combination of prefill gas and additional fuelling. However, a startup with an excessive prefill gas will lose the margin of the density control, and a full RE discharge can be triggered when the plasma burn-through fails owing to a high concentration of impurities such as oxygen, if the operational point is marginal to the burn-through limit. The present modelling work implies that preparing the operation with sufficient margins against the failure of impurity burn-through is important for mitigating the risk of unintended plasma termination. Although the carbon wall environment may have a lower risk of RE generation, to alleviate the operational uncertainties of the new device, strategies for expanding the operational window are also discussed in support of JT-60SA initial operation.

Key designs of a short-bore and cryogen-free high temperature superconducting magnet system for 14 T whole-body MRI

A 14 T whole-body magnetic resonance imaging (MRI) magnet is designed with Bi2223 high temperature superconducting (HTS) wire. Owing to the large critical current and large critical tensile strength of the wire type HT-NX at ultra-high field, the magnet is optimized in a compact shape with length of 1.9 m and outer diameter of 1.3 m. It is in the form of stacked double pancakes (DPs) with total wire consumption of 455 km. The DPs with various inner and outer radii enable the electromagnetic design to obtain highly homogeneous field of 1.5 ppm over 400 mm diameter of spherical volume (DSV). Hoopstress by winding process, thermal contraction and Lorenz force is calculated and below the wire strength specification. Considerable reduction of screening current effect in MRI magnet application is proved by a calculation example. Field homogeneity as it is influenced by various systematic tolerances and random geometric tolerances is numerically estimated, for further design optimization and design of shimming system. Iron room for passive shielding with dimension of 5 × 5 × 10 m is found to be with wall thickness of around 30 cm to effectively contain the 5-Gauss field to the wall surface. Cooling strategy for cryogen-free technique is proposed and the magnet coils temperature is calculated as below 8 K based on the estimation of joint Ohmic heating power of 1.0 W. Quench protection adopts circuit subdivision and dumping resistors to restrict the maximum voltage below 1.9 kV. No-insulation technique for the quench protection is also discussed. Some other wire candidates like REBCO and Nb3Sn for development of such ultra-high field MRI magnets are commented.

Neutral beam prompt loss in LTX- β

Prompt loss of beam injected fast ions approaches 100% in lithium tokamak experiment-beta (LTX-β) discharges, though significantly improved confinement is expected for the higher current plasmas made available by a recent upgrade to the Ohmic heating power supply. Modeling of fast ions using TRANSP/NUBEAM finds a maximum coupled beam fraction of 76% at the near-term limits of the LTX-β operating space. The full ion orbit code POET is employed to validate NUBEAM results against possible non-adiabatic effects on fast ion orbits, but corrections to the prompt loss fraction due to collisionless transport are found to be small. The graphical method code CONBEAM is used to investigate the topology of fast ion phase space as it relates to neutral beam deposition, and counter-injected NBI is considered as a way to access a region of high field side beam deposition. A metric is developed within the CONBEAM using a beam filament model to estimate the prompt loss fraction and shown to agree well with both POET and NUBEAM, enabling near real-time analysis and potential feedback to operators between plasma discharges.

Ionization and transport in partially ionized multicomponent plasmas: Application to atmospheres of hot Jupiters.

We study ionization and transport processes in partially ionized multicomponent plasmas. The plasma composition is calculated via a system of coupled mass-action laws. The electronic transport properties are determined by the electron-ion and electron-neutral transport cross sections. The influence of electron-electron scattering is considered via a correction factor to the electron-ion contribution. Based on these data, the electrical and thermal conductivities as well as the Lorenz number are calculated. For the thermal conductivity, we consider also the contributions of the translational motion of neutral particles and of the dissociation, ionization, and recombination reactions. We apply our approach to a partially ionized plasma composed of hydrogen, helium, and a small fraction of metals (Li, Na, Ca, Fe, K, Rb, and Cs) as typical for atmospheres of hot Jupiters. We present results for the plasma composition and the transport properties as a function of density and temperature and then along typical P-T profiles for the outer part of the hot Jupiter HD 209458b. The electrical conductivity profile allows revising the Ohmic heating power related to the fierce winds in the planet's atmosphere. We show that the higher temperatures suggested by recent interior models could boost the conductivity and thus the Ohmic heating power to values large enough to explain the observed inflation of HD 209458b.

ЭЛЕКТРОИМПУЛЬСНЫЙ НАГРЕВ РЕАКЦИОННОСПОСОБНЫХ СОСТАВОВ ТОНКОСТЕННЫМИ ПРОВОДНИКАМИ-ТЭНАМИ В РЕЖИМАХ МНОГООЧАГОВОГО ЗАЖИГАНИЯ МЕТАТЕЛЬНЫХ ЗАРЯДОВ

The problem of electric pulse heating of a medium by the heater made of metal foil with an ideal thermal contact with the medium is solved. The solution is obtained in the form of rapidly converging series for ohmic heating power, which is given as an arbitrary function of time. Using the obtained solution, a dimensionless criterion is introduced for quasi-homogeneous heating of the heater up to a melting point with intensive heat transfer to environment. It is shown that with a fixed heating duration, the criterion value depends weakly on the type of a heating power function, which allows ignoring parameters of the external electrical circuit of the device while choosing the heater thickness. The definition of a "thin" heater is introduced, which admits of bulk droplet destruction during heating of a reactive medium. In the "thin" heater approximation, quadrature solutions to the thermal problem are obtained under heat exchange according to Newton's law and ideal thermal contact, when the heater power is an arbitrary function of time. Dimensionless similarity parameters of the simulated processes are identified in all solutions, which include thermal and physical characteristics of the heater and medium, as well as the thickness of the heater and the time of its heating up to the melting point. The applicability of the "thin" heater model for calculating the thermal state of a high-energy pyrotechnic coating is shown. As an example, the permissible values of the thickness of the magnesium foil heater are determined, which ensure its uniform heating up to the melting point in 1 and 5 milliseconds under the ideal thermal contact with ignited coating. The obtained quadrature solutions to the thermal problems and the calculated results are applicable in the design of compact electric pulse devices for contact multi-point ignition of various reactive compositions with efficient consumption of electric energy.

A digital feedback control and real-time monitoring system of PF coil power supplies for HL-2A

An upgrade to the magnet power digital feedback system (DFS) for the real time control of the PF coil power supplies is currently being developed and commissioned in HL-2A. The system aims to perform higher control resolution, more stable control cycle, real time monitoring and protection. The DFS has been shown to have better performance than the existing system in the control of the PF coil power supplies. It can control effectively all the PF coil power supplies, including the ohmic heating power supply (OHPS), vertical field power supply (VFPS), radial field power supply (RFPS) and the divertor coil power supply (DCPS). Meanwhile, the high-speed data transmission with reflective memory is used to transfer the control parameters of the PF coil power supplies. Moreover, the DFS also provides the real time monitor and protection based on the experimental physics and industrial control system (EPICS).

Electron thermal confinement in a partially stochastic magnetic structure

Using a high-repetition-rate Thomson scattering diagnostic, we observe a peak in electron temperature Te coinciding with the location of a large magnetic island in the Madison Symmetric Torus. Magnetohydrodynamic modeling of this quasi-single helicity plasma indicates that smaller adjacent islands overlap with and destroy the large island flux surfaces. The estimated stochastic electron thermal conductivity (≈30 m2/s) is consistent with the conductivity inferred from the observed Te gradient and ohmic heating power. Island-shaped Te peaks can result from partially stochastic magnetic islands.

Design of the power supply system for the plasma current modulation on J-TEXT tokamak

In order to further study the influence of current modulation parameters on suppressing tearing instability, the plasma current should be modulated in a wider range. So a modification scheme is designed to improve the performance of ohmic heating power supply system on J-TEXT tokamak. A multilevel cascade circuit with carrier phase-shifted PWM technique has been proposed. Coupled inductors are connected in the form of cyclic symmetry to restrain the circulating current caused by multiple paralleled branches. The simulation proves this proposed current modulation power supply system matches output requirement and achieves good current sharing effect. Finally, a prototype is designed, and the experiment results can verify the correctness of the simulation model well.

Physics and optimization of plasma startup in the RFP

In the tokamak and reversed-field pinch (RFP), inductively driven toroidal plasma current provides the confining poloidal magnetic field and ohmic heating power, but the magnitude and/or duration of this current is limited by the available flux swing in the poloidal field transformer. A portion of this flux is consumed during startup as the current is initiated and ramped to its final target value, and considerable effort has been devoted to understanding startup and minimizing the amount of flux consumed. Flux consumption can be reduced during startup in the RFP by increasing the toroidal magnetic field, Bti, applied to initiate the discharge, but the underlying physics is not yet entirely understood. Toward increasing this understanding, we have for the first time in the RFP employed advanced, non-invasive diagnostics on the Madison Symmetric Torus to measure the evolution of current, magnetic field, and kinetic profiles during startup. Flux consumption during startup is dominantly inductive, but we find that the inductive flux consumption drops as Bti increases. The resistive consumption of flux, while relatively small, apparently increases with Bti due to a smaller electron temperature. However, the ion temperature increases with Bti, exceeding the electron temperature and thus reflecting non-collisional heating. Magnetic fluctuations also increase with Bti, corresponding primarily to low-n modes that emerge sequentially as the safety factor profile evolves from tokamak-like to that of the RFP.

Simulation of Ohmic Heating Power Supply for HL-2M

Simulation of Ohmic Heating Power Supply for HL-2M

Study on Matching a 300 MVA Motor Generator with an Ohmic Heating Power Supply in HL-2M

A new 300 MVA/1350 MJ motor generator (MG) will be built to feed all of the poloidal field power supplies (PFPS) and auxiliary heating power supplies of the HL-2M tokamak. The MG has a vertical-shaft salient pole 6-phase synchronous generator and a coaxial 8500 kW induction motor. The Ohmic heating power supply (OHPS) consisting of 4-quadrant DC pulsed convertor is the one with the highest parameters among the PFPS. Therefore, the match between the generator and the OHPS is very important. The matching study with Matlab/Simulink is described in this paper. The simulation results show that the subtransient reactance of the generator is closely related to the inversion operation of the OHPS. By setting various subtransient reactance in the simulation generator model and considering the cost reduction, the optimized parameters are obtained as x″d = 0.405 p.u. at 100 Hz for the generator. The models built in the simulation can be used as an important tool for studying the dynamic characteristics and the control strategy of other HL-2M PFPSes.

Modelling of radiative divertor operation towards detachment in experimental advanced superconducting tokamak

In order to actively control power load on the divertor target plates and study the effect of radiative divertor on plasma parameters in divertor plasmas and heat fluxes to the targets, dedicated experiments with Ar impurity seeding have been performed on experimental advanced superconducting tokamak in typical L-mode discharge with single null divertor configuration, ohmic heating power of 0.5 MW, and lower hybrid wave heating power of 1.0 MW. Ar is puffed into the divertor plasma at the outer target plate near the separatrix strike point with the puffing rate 1.26×1020 s−1. The radiative divertor is formed during the Ar puffing. The SOL/divertor plasma in the L-mode discharge with radiative divertor has been modelled by using SOLPS5.2 code package [V. Rozhansky et al., Nucl. Fusion 49, 025007 (2009)]. The modelling shows the cooling of the divertor plasma due to Ar seeding and is compared with the experimental measurement. The changes of peak electron temperature and heat fluxes at the targets with the shot time from the modelling results are similar to the experimental measurement before and during the Ar impurity seeding, but there is a major difference in time scales when Ar affects the plasma in between experiment and modelling.

Spectral emission measurements of lithium on the lithium tokamak experiment

There has been a long-standing collaboration between ORNL and PPPL on edge and boundary layer physics. As part of this collaboration, ORNL has a large role in the instrumentation and interpretation of edge physics in the lithium tokamak experiment (LTX). In particular, a charge exchange recombination spectroscopy (CHERS) diagnostic is being designed and undergoing staged testing on LTX. Here we present results of passively measured lithium emission at 5166.89 A in LTX in anticipation of active spectroscopy measurements, which will be enabled by the installation of a neutral beam in 2013. Preliminary measurements are made in transient LTX plasmas with plasma current, I(p) < 70 kA, ohmic heating power, P(oh) ∼ 0.3 MW and discharge lifetimes of 10-15 ms. Measurements are made with a short focal length spectrometer and optics similar to the CHERS diagnostics on NSTX [R. E. Bell, Rev. Sci. Instrum. 68(2), 1273-1280 (1997)]. These preliminary measurements suggest that even without the neutral beam for active spectroscopy, there is sufficient passive lithium emission to allow for line-of-sight profile measurements of ion temperature, T(i); toroidal velocity and v(t). Results show peak T(i) = 70 eV and peak v(t) = 45 km/s were reached 10 ms into the discharge.

Accelerated texture softening of some root vegetables by Ohmic heating

Effects of different thermal processing methods (Ohmic heating, high and low power, microwave and conventional heating) on textural properties of cylindrical pieces of root vegetables of carrot, red beet and golden carrot were investigated and compared with conventional and microwave processes. The samples were subjected to different processing methods and textural parameters of the processed samples were analyzed using texture profile analysis (TPA) at different processing times. Texture parameters-processing time data were fitted into a previously suggested equation and texture softening rate (K) and residual constant (A) values were calculated. From the K and A values it can be concluded that not only Ohmic heating resulted in greater softening rates but also the final hardness of the samples treated by Ohmic heating was significantly lower than those of other samples treated by either conventional or microwave methods. Negative correlations were found between texture hardness and weight loss of all samples undergone different processing methods.

Ion and Electron Heating Characteristics of Magnetic Reconnection in a Two Flux Loop Merging Experiment

Characteristics of the high-power reconnection heating were measured for the first time directly by two-dimensional measurements of ion and electron temperatures. While electrons are heated mainly inside the current sheet by the Ohmic heating power, ions are heated mainly by fast shock or viscosity damping of the reconnection outflow in the two downstream areas. The magnetic reconnection converts the energy of reconnecting magnetic field B(p) mostly to the ion thermal energy, indicating that the reconnection heating energy is proportional to B(p)(2).

Two-temperature chemically non-equilibrium modelling of an air supersonic ICP

In this work, a non-equilibrium mathematical model for an air inductively coupled plasma torch with a supersonic nozzle is developed without making thermal and chemical equilibrium assumptions. Reaction rate equations are written, and two coupled energy equations are used, one for the calculation of the translational–rotational temperature Thr and one for the calculation of the electro-vibrational temperature Tev. The viscous dissipation is taken into account in the translational–rotational energy equation.The electro-vibrational energy equation also includes the pressure work of the electrons, the Ohmic heating power and the exchange due to elastic collision.Higher order approximations of the Chapman–Enskog method are used to obtain better accuracy for transport properties, taking advantage of the most recent sets of collisions integrals available in the literature. The results obtained are compared with those obtained using a chemical equilibrium model and a one-temperature chemical non-equilibrium model. The influence of the power and the pressure chamber on the chemical and thermal non-equilibrium is investigated.

Plasma current and position feedback control in ADITYA Tokamak

Ohmic heating power supply is used in ADITYA Tokamak for plasma breakdown and current drive. Presently the ohmic coils store a magnetic flux of 0.40 V s and high d i/d t is created for the required loop voltage. After the plasma breakdown the plasma current control is made active with the difference of reference plasma current and measured plasma current to drive the PI regulator, which regulates the ohmic falling current. The vertical field current is also made proportional to plasma current by feedback control at appropriate time. The control scheme adopted and the results with the feedback control are described in this paper. We could improve the plasma current as well as plasma current flattop by employing the plasma current feed back control. Further to correct the radial position in fast time scale, four-quadrant transistor switched bridge inverter power supply by plasma position feedback control is proposed. On line ±Δ X (by position coils) position signal is given to PI control and polarity detector accordingly the current is amplified by the bridge inverter and fed to the four set of feedback coils which shifts the plasma position on radial directions.

Measurement of magnetic fluctuation-induced heat transport in tokamaks and RFP

The local electron energy flux produced by magnetic fluctuations has been measured directly in the edge plasma of the Madison symmetric torus (MST) reversed field pinch (RFP), continuous current tokamak (CCT), and the scrape-off layer of the TEXT-U tokamak. The flux produced by electrons travelling parallel to a fluctuating magnetic field is obtained from correlation between the fluctuations in the parallel heat flux and the radial magnetic field. The fluctuations in the parallel heat flux were measured with a fast insertable pyrobolometer. The measurements reveal fundamental differences in the nature of electron energy transport in the RFP and the tokamak. In the RFP the fluctuation-induced energy flux is large (, comparable to the total ohmic heating power) inside the reversal surface where the magnetic field is expected to be stochastic, and small in the edge. The magnetic fluctuation induced radial energy flux Q and radial particle flux (measured independently) are related by a `convective' formula . The electron heat transport is significantly lower than the value predicted by the Rechester - Rosenbluth transport model. This feature of the electron energy transport can be explained using self-consistent calculations that account for clumping of electrons streaming along the magnetic field. In the tokamak the magnetic fluctuations do not contribute to the total energy transport except in the vicinity of the q = 2 magnetic surface, where the transport is associated with large amplitude Mirnov oscillations.

Ohmic Ignition in a Tokamak Reactor with a Confinement Degradation Effect Due to Alpha-Particle Heating

It is shown that a tokamak with a major radius larger than [approximately]6 m and a toroidal field of 10 T can reach ignition by ohmic heating alone at a relatively low peak density [n(o)[approximately]1[times]10[sup 20] m[sup [minus]3]] even with confinement degradation due to alpha-particle heating, provided a confinement enhancement factor of [gamma][sub H] = 2 over the Goldston scaling is assumed. The critical toroidal field and plasma current required for ohmic ignition have been estimated for various sizes of tokamaks with major radii R = 2 m (compact), 6 m [International Thermonuclear Experimental Reactor (ITER) class], and 10 m (large tokamaks). If a broad current profile can be achieved transiently, the critical toroidal field and plasma current can be further reduced by the enhancement in the ohmic heating power. 37 refs., 7 figs.