Ohmic Input Power Research Articles

Preliminary design of an AXUV photodiode-based bolometer camera for MT-I spherical tokamak

The MT-I is a spherical tokamak at the Pakistan Tokamak Plasma Research Institute, and has a major radius of 15 cm and a minor radius of 9 cm. This paper presents the design and computational analysis of the Absolute eXtreme Ultra-Violet (AXUV) photodiode-based bolometer camera, which will be used for the first time in the MT-I tokamak. The bolometer camera contains a single array of 16 AXUV photodiodes placed behind an aperture to compute the line of sight (LOS) and field of view (FOV) of the system. This array is located at the midplane of the vessel, which provides a radial resolution of about 16 mm. The individual FOV of each photodiode and the collective FOV of the array are 4.1° and ±34.5°, respectively. The impurity densities and their cooling rates for elements like carbon, oxygen, and iron are used to compute the plasma emissivity distribution, the power incident on the photodiode array, and signal-to-noise ratio (SNR) analysis. The analysis reveals that the radiated power loss is about 10 % of the ohmic heating input power. The geometrical etendue falls within the range of 2.2 × 10−5 to 3.2 × 10−5 Sr cm2. The bolometer exhibits a frequency response bandwidth of 10 kHz, and its temporal resolution is around 0.1 milliseconds.

Study of Ohmic breakdown and burnthrough phase of ADITYA tokamak

In the ADITYA tokamak, the plasma discharge is initiated through filament pre-ionization assisted breakdown, using the conventional inductively driven electric field. Following the breakdown of the neutral gas, the discharge is sustained by a successful burnthrough phase. The nature of the breakdown and burnthrough phase is studied by varying the parameters influencing it, such as the toroidal electric field, operating pressure, and lithium wall conditioning. The plasma initiation failures in the breakdown and burnthrough phase are identified, and optimized conditions of operating parameters are derived. The value of the Lloyd parameter Eϕ×BT/Bz in the ADITYA for plasma breakdown is found to be in the range of 1200–2100 V/m, which is consistent with other conventional tokamaks. The applied Ohmic input power must overcome the power losses due to fuel ionization and fuel and impurity radiation to achieve the complete burnthrough. The power requirement for the burnthrough phase is obtained experimentally and compared with the estimated values. The required Ohmic input power is found to be ∼60 kW for the successful burnthrough. Furthermore, it has been observed that the vessel wall coating with lithium reduces the impurities influx in the burnthrough phase and, thus, reduces the Ohmic input power consumption.

Bolometer measurements of the radiated power and estimates of the effective ion charge Zeff on the Keda Torus eXperiment

We have measured the power radiated by the plasma in the Keda Torus eXperiment (KTX) using a 20-channel AXUV bolometer. The bolometer has a frequency response bandwidth of 40 kHz, spatial resolution better than 40 mm, and overlap between adjacent channels of about 20%. For an ultra-low q discharge, the radiated power obtained from the bolometer measurements is about 8.0% of the ohmic heating input power. In addition, together with the Hα-emission data, we have used the bolometer data to calculate the effective ion charge Zeff profile, which proves to agree well with a plasma-resistivity calculation and which is hollow in the central region of a poloidal cross-section of the KTX.

Neutral beam heating of a RFP plasma in MST

Electron temperature is observed to rise due to neutral beam injection (NBI) in the Madison Symmetric Torus (MST). Heating is observed to be 100 ± 50 eV in the core of 200 kA plasmas. This is the first definitive measurement of auxiliary heating of a reversed-field pinch (RFP). This heating is consistent with a 1D classical model which was developed. This 1D model calculates the evolving thermal conductivity and ohmic power input profiles during enhanced confinement, and can calculate NBI deposition and classical fast ion diffusion and slowing. The predicted temperature change is consistent with measured beam heating both during and after enhanced confinement, which is consistent with previous observations that fast ions are well confined and behave roughly classically in the RFP.

Neutron emission from KSTAR Ohmically heated plasmas

An un-calibrated sensitive 3He detector was used to monitor the KSTAR neutron rate during Ohmically heated discharges. Neutrons were detected for every shot. If the neutrons were from D–D reactions then there should be a dependence on the ion temperature. We obtained ion temperature, electron density, Ohmic heating input power measurements, etc. from standard diagnostics. The study shows no observable dependence on the Ohmic power, ion temperature, or calculated source strength. It appears that the neutrons are from sources other than D–D reactions. The most probable source is high-energy runaway electrons.

Majority Ion Heating by Neutral Beam Injection and Confinement of Fast Ions in the Madison Symmetric Torus Reversed Field Pinch

A new 1 MW neutral beam injector (START-20F) is in operation on the Madison Symmetric Torus (MST) reversed field pinch. The beam, consisting of two arc discharge plasma generators, an optimized ion optical system and an integrated neutralizer/injector tank, operates at 25kV and up to 40A of neutrals for a 20 msec pulse (compared to a typical MST pulse length of 60 msec). The injected 1 MW of hydrogen neutrals (with approximately 85% in the full energy component) is significant compared to the 3-4 MW of ohmic input power in a typical target discharge. At this beam energy and a background electron density of about 1x1019 m–3 and temperature ≤1keV, roughly 90% of the injected power is deposited within the plasma. Initial experiments with the high power NBI show a large heating of the bulk ions: the fit of the width of energy spectrum as measured by Rutherford scattering (which is generally related to core ion temperature) quickly increases from 180eV to 230eV. This apparent significant and rapid heating of bulk ions is difficult to explain by classical collisions only, as modeling predicts 75% of the injected power is deposited on electrons and 15% on ions. The confinement of the fast ions (measured by the persistence in time of fusion neutrons due to a small fraction of deuterium in the beam fuel) is much greater than the canonical 1 msec confinement of particles and energy in the MST. The fast particle confinement is measured to increase with magnetic field strength. Further recent experiments document fast particle confinement time versus direction of injection (parallel or antiparallel to central magnetic field), beam energy, and background plasma properties.

Measurement of Electrical Conductivity of Weakly Nonideal Multicomponent Plasma Mixtures Generated From Dielectric Materials

Nonideal complex multicomponent plasmas generated from dielectric compound materials are of crucial importance to many critical technologies, and the need to measure and determine the electrical conductivity of these plasmas is imperative. In this paper, we present preliminary successful measurements of the electrical conductivity of weakly nonideal partially ionized complex plasma mixtures generated from dielectric materials. The complex multicomponent partially ionized vapors were generated using an electrothermal plasma source operated in the ablation-controlled arc regime, where the compound dielectric materials were used as the liner of the capillary wall serving as the source of plasma species. The measured discharge current was used in conjunction with the active or pure resistive part of the recorded discharge voltage to calculate the electrical conductivity as a function of time. A comprehensive 1-D time-dependent computer code with radiation transport, which uses the recovered ohmic input power as the only driving force of the computations, was used to report the corresponding plasma state. Measurements in the temperature range of 11000-16200 K and density range of 0.1-25 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> were performed, and the results were presented, discussed, and compared with theoretical predictions.

Improved modelling of electrothermal plasma source with radiation transport

An improved one-dimensional, time-dependent model with radiation transport is developed and used to simulate the evolution and flow of Ohmically heated nonideal plasma in the capillary of an electrothermal (ET) plasma source operated in the ablation stabilized arc regime. The model uses the Ohmic input power, recovered from the experimentally measured impedance, as the sole driving force of the computations to circumvent depending on arguable theoretical models of the resistivity of nonideal plasma. A consistent model of the equation of state and thermodynamic functions of weakly nonideal multi-component plasma mixtures is implemented and used to calculate the thermodynamic properties of the ET plasma generated from the ablation of the capillary wall. The flow velocity at the bore exit is not allowed to exceed the local sound speed and no prior assumptions of choked-flow are made at the bore exit. The frequency-dependent and frequency-averaged opacities of the plasma have been calculated considering the basic atomic processes of photoionization, inverse bremsstrahlung, resonant photoabsorption as well as electron scattering. Radiation transport has been implemented by modelling the plasma column as a grey gas the emissivity of which is calculated using an effective beam length Leb. The radiative heat flux escaping the surface of the grey plasma column is used to calculate the rate of the ablated mass improving the temporal behaviour of the calculated plasma parameters. Particular attention is devoted to investigating the ablation process and the evaluation of the so-called heat of ablation. Computational results are presented and discussed.

Transport and confinement studies in the RFX-mod reversed-field pinch experiment

In the modified RFX experiment (RFX-mod) external magnetic field coils and a close fitting thin conductive shell control radial magnetic fields. In the so-called virtual shell (VS) operation, radial field zeroing at the thin shell radius is stationary provided by the feedback-controlled coils. First experiments on RFX-mod proved the capability of the active scheme to steadily reduce the radial magnetic field. Furthermore it has been found that such edge magnetic field control extends its beneficial effects to the whole plasma. With respect to the old RFX, where magnetohydrodynamic modes amplitude was controlled by the use of a passive thick conductive shell, a stationary 2- to 3-fold reduction of the Br field amplitude in the core is obtained. The reduction of field fluctuations positively reflects on confinement. In fact, a strong reduction of the loop voltage is observed and correspondingly a 3-fold increase in pulse length is achieved by using the same poloidal flux swing. Temperature and particle measurements confirm the improved confinement properties of the VS operation. With a lower ohmic input power, higher electron temperature and lower particle influx are measured. Particle and heat transport have been studied by means of a 1D code. Local power balance was used to compute the heat conductivity profile: for the VS discharges a lower conductivity over a significant region of the plasma is found. The improved properties of RFX-mod VS operation provide a better confinement scaling in terms of plasma current. The results show that compared with the thick shell configuration, a significant confinement improvement can be obtained under stationary conditions by actively controlling the plasma magnetic boundary.

Analysis of electron thermal diffusivity and bootstrap current in ohmically heated discharges after boronization in the HT-7 tokamak

Significant improvements of plasma performance after ICRF boronization have been achieved in the full range of HT-7 operation parameters. Electron power balance is analyzed in the steady state ohmic discharges of the HT-7 tokamak. The ratio of the total radiation power to ohmic input power increases with increasing the central line-averaged electron density, but decreases with plasma current. It is obviously decreased after wall conditioning. Electron heat diffusivity χe deduced from the power balance analysis is reduced throughout the main plasma after boronization. χe decreases with increasing central line-averaged electron density in the parameter range of our study. After boronization, the plasma current profile is broadened and a higher current can be easily obtained on the HT-7 tokamak experiment. It is expected that the fact that the bootstrap current increases after boronization will explain these phenomena. After boronization, the plasma pressure gradient and the electron temperature near the boundary are larger than before, these factors influencing that the ratio of bootstrap current to total plasma current increases from several percent to above 10%.

Compression effects in inductively coupled, high-power radio-frequency discharges for negative hydrogen ion production

In the paper we present a simplified model description of inductively coupled plasma discharges operating at a rather high radio-frequency (rf) power. In this case the induced high plasma currents can cause periodic compressions over a substantial radial distance. Such conditions are obviously given in rf driven 1 MHz/150 kW plasma sources developed at the Institute for Plasma Physics Garching for negative (hydrogen) ion production in future neutral beam injection (NBI) systems for nuclear fusion research, such as the 1 MeV/50 MW NBI system for the International Thermonuclear Experimental Reactor [T. Inoue, R. Hemsworth, V. Kulygin, and Y. Okumura, Fusion Eng. Design 55, 291 (2001)]. The given model describes quite well the compression and other features of the discharge. The results include the Ohmic power input (i.e., electron heating), the resulting density build-up, and—as a new feature—periodical plasma compressions, leading to a direct energy input also into the plasma ions. The model also explains the strange effect of small argon admixtures, which improve the negative ion yield in rf sources by a factor of up to 2–3 (but which have no effect in conventional dc arc sources). With the calculated dependencies from external parameters (e.g., rf-power and frequency, gas pressure, ion mass or the specific geometry), the modeling may help for the further optimization of the rf source.

Issues in the plasma wall interactions in RFX

The standard intrinsic nature of transport in reversed field pinches entails that remarkable ohmic power input (20–80 MW) is involved in the all-graphite, MA level RFX device. MHD instabilities restrict the region of plasma-wall interaction (PWI), where power densities >100 MW m −2 may be locally reached. Means for handling such power have been successfully devised to avoid carbon bloom and allow reproducible experiments around 1 MA of plasma current. Improved confinement schemes (quasi-single helicity, pulsed poloidal current drive (PPCD)) corresponding to a more ordered magnetic configuration have opened new operational perspective in which PWI is a less severe problem. However, in general, plasma contamination is not a specific issue. Screening processes have been associated with low ionisation length to Larmor radius ratios and with the presence of an edge radial electric field. Impurities appear to have a role in the formation of the radial electric field by enhancing the ion prompt losses.

Semi-analytical modelling and simulation of the evolution and flow ofOhmically-heated non-ideal plasmas in electrothermal guns

In this paper, the Ohmic power input to the discharge capillary isrecovered and used to analyse the basic processes involved in electrothermal(ET) plasma devices operated in an ablation-controlled-arc regime. Such aninterplay between theory and experiment is necessary to reduce the number ofreasons which might be responsible for the reported discrepancies betweentheory and experiment, as well as to illuminate the subject of ablationcontrolled arcs. A consistent methodology for determining detailed compositionand thermodynamic functions of the non-ideal plasma generated in such devicesis presented and used in the present computations. Different non-idealityeffects, due to Debye-Hückel corrections in the Gibbs free energy, whichhave been ignored in prior publications, have been taken into account. Asemi-analytical model for an ET plasma source with non-ideal effects isdescribed and incorporated into a comprehensive computer code to simulateplasma evolution and flow in the discharge capillary. The model isone-dimensional, time dependent and uses the recovered Ohmic power input inthe source term of the energy equation. The developed code has been used toinvestigate the ablation process and has shown the inappropriateness of awidely used ablation model. Code predictions for different plasma parametersare presented, discussed, and compared to available experimental data.

STUDY OF THE ELECTRON THERMAL CONDUCTIVITY OF THE OHMICALLY HEATED DISCHARGES IN THE HT-7 TOKAMAK

The electron thermal conductivity in the HT-7 tokamak is obtained by analyzing the electron power balance. In the HT-7 device, the electron thermal convection loss can be neglected, compared with ohmic input power, and the thermal conduction loss is the main energy loss mechanism. The electron thermal diffusion coefficient increases with the minor radius, and is about 100 times larger than the neo-classical theory calculation. After siliconization, the electron thermal diffusion coefficient decreases obviously at the outer half region, and the energy confinement time is prolonged.

Stochastic diffusivity and heat transport in the presence of a radial dependence of the perturbed magnetic field in the reversed field pinch

Stochastic diffusivity in the presence of island overlapping can be numerically estimated following the radial evolution of an ensemble of magnetic field lines, starting from a given initial position, when they have travelled for a length of the order of the perturbed magnetic field correlation length along the main magnetic field. In this paper we show that, caused by the radial dependence of the magnetic perturbation and due to the short correlation length in reversed field pinches (RFPs), different `local' diffusion rates at different radii originate.The numerical results describing the stochastic diffusion process, obtained for typical RFP configurations and magnetic field perturbations, are presented and compared with an analytical WKB approximated solution of the relevant one-dimensional Fokker-Planck diffusion equation. The magnetic diffusivity profile is then inserted into a heat diffusion equation for the electrons in which the heat flux is balanced by the ohmic input power. In this way the electron temperature profile is numerically obtained in collisionless and collisional regimes.

Fivefold confinement time increase in the Madison Symmetric Torus using inductive poloidal current drive

Current profile control is employed in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] reversed field pinch to reduce the magnetic fluctuations responsible for anomalous transport. An inductive poloidal electric-field pulse is applied in the sense to flatten the parallel current profile, reducing the dynamo fluctuation amplitude required to sustain the equilibrium. This technique demonstrates a substantial reduction in fluctuation amplitude (as much as 50%), and improvement in energy confinement (from 1 to 5 ms); a record low fluctuation (0.8%) and record high temperature (615 eV) for this device were observed simultaneously during current drive experiments. Plasma beta increases by 50% and the Ohmic input power is three times lower. Particle confinement improves and plasma impurity contamination is reduced. The results of the transient current drive experiments provide motivation for continuing development of steady-state current profile control strategies for the reversed field pinch.

Increased Confinement andβby Inductive Poloidal Current Drive in the Reversed Field Pinch

With the addition of inductive poloidal current drive for current profile control in the Madison Symmetric Torus (MST) reversed field pinch, the magnetic fluctuation amplitude halves, leading to reduced energy and particle transport. A four- to five-fold increase in the energy confinement time to ${\ensuremath{\tau}}_{E}\ensuremath{\sim}5\mathrm{ms}$ with both decreased Ohmic input power and increased stored thermal energy coincides with record low fluctuation amplitude and record high electron temperature $\ensuremath{\sim}600\mathrm{eV}$ (for MST). $\ensuremath{\beta}[\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2{\ensuremath{\mu}}_{o}〈p〉/B(a{)}^{2}]$ increases from 6% to 8%. Other improvements include reduced electrostatic edge turbulence and plasma impurity content.

Statistical analysis of the global energy confinement time in ohmic discharges in the ASDEX tokamak

In ohmic discharges in all tokamaks at low plasma densities the global energy confinement time, , increases almost linearly with the density (LOC, linear ohmic confinement). In tokamaks with sufficiently large dimensions, saturates at a critical density (ASDEX: ) and is nearly constant at higher densities (SOC, saturated ohmic confinement). In the same density region some experiments report a further confinement regime for deuterium discharges in which exceeds the saturated value and is further increased (IOC, improved ohmic confinement). There the global energy confinement time roughly behaves as in the LOC regime. For both the LOC and the SOC regimes an isotope effect, i.e. the dependence of on the ion mass, is reported as an additional aspect of the ohmic energy confinement. A statistical analysis is performed to identify the parameters which are responsible for the properties of the energy confinement in these discharges in ASDEX. In contrast to earlier reports on confinement time scalings in ASDEX OH, only discharges with a full experimental description of kinetic electron and ion parameters, i.e. profiles of densities, temperatures and , are used to evaluate the energy contents of both species. By means of statistics it is shown that the characteristics of are mainly caused by the behaviour of the electron energy flux and the ohmic input power. The ion energy flux, however, does not play a significant role. Furthermore, the IOC regime is explained as a continuation of the low-density LOC regime. Both the isotope effect and the density dependence of are caused by features of the electron energy transport.

Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

Most Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2–5 ms, corresponding to magnetohydrodynamic (MHD) instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent.

Confinement improvement with RF poloidal current drive in the reversed field pinch

External control of the current density profile in a reversed field pinch (RFP), by means such as RF poloidal current drive, may have beneficial effects well beyond the direct reduction of ohmic input power due to auxiliary heating. Reduction of magnetic turbulence associated with the dynamo, which drives poloidal current in a conventional RFP, may allow operation at lower density and higher electron temperature, for which RF current drive becomes efficient and the RFP operates in a more favourable regime on the n tau -T diagram. Projected parameters for RFX at 2 MA are studied as a concrete example. If RF current drive allows RFX to operate with beta =10% (plasma energy/magnetic energy) at low density (3*1019 m-3) with classical resistivity (i.e. without dynamo enhanced power input), 40 ms energy confinement times and 3 keV temperatures will result, matching the performance of tokamaks of similar size