Injection Heating Power Research Articles

Bus Design for the Poloidal Field Coils of the NSTX-Upgrade Fusion Device

The National Spherical Torus eXperiment (NSTX) has undergone a major upgrade to NSTX-U at Princeton Plasma Physics Laboratory (PPPL). NSTX upgrade (NSTX-U) will double the toroidal field, plasma current, and neutral beam injection heating power, as well as significantly increase the pulse duration. NSTX-U uses three poloidal field (PF) coils at the vessel top and three at the bottom near the divertor areas to control the local plasma shape there. These shaping coils operate at a maximum terminal voltage of 2 kV, corresponding to a maximum current about 20 kA. To supply the power to the PF coils, hard copper bus bars are typically used near the vacuum vessel, while water cooled flexible cables are used away from the vessel. The detailed design of the hard copper bus bars will be covered in this article. During operation, these hard bus bars are subject to high electromagnetic (EM) forces, thermal displacement loads, as well as plasma halo loads due to disruption. The EM, thermal, and structural analysis were performed, and the results revealed that, with the worst loads combined, the new design meets the NSTX-U thermal, structural, and fatigue cycle requirements. The manufacturing and installation process of the bus work will be discussed at the end of this article.

Center Stack Casing Bakeout BUS Design for the NSTX-Upgrade Fusion Device

The National Spherical Torus eXperiment (NSTX) has undergone a major upgrade to NSTX-U at the Princeton Plasma Physics Laboratory. NSTX-U will double the toroidal field, plasma current, and neutral beam injection heating power, as well as significantly increase the pulse duration. The plasma-facing components (PFCs) in the NSTX-U vacuum vessel are mainly graphite, which has a total surface area of about 41 m2. To achieve high vacuum and reduce impurity from PFCs during operation, it is important to bake the graphite parts and remove most of the moisture absorbed by graphite during installation. Typical bakeout for NSTX-U lasts about 3 to 4 weeks. The NSTX-U inner vacuum vessel, i.e., the center stack casing, will be heated to about 450°C by passing 8 KA direct current through it during bakeout. The design of the bakeout bus directly attached to the casing flanges at vessel top and bottom are covered in detail in this paper. At the vessel top, the water-cooled bus terminal is subject to high thermal growth (about 18 mm in the vertical direction and 3 mm in the radial direction). At the vessel bottom, the bakeout bus must withstand 120 KA of halo current during disruption, as well as dislocation from thermal growth. This paper covers the design to address all these challenges. A machined CuCrZr terminal with internal water-cooling channels was used to prevent any brazing work in high stress areas. Detailed analysis will also be covered to show that the proposed design can satisfy thermal, structural, and fatigue requirements during bakeout and operation.

Wall Conditioning Systems for the NSTX-U Fusion Device

The National Spherical Torus eXperiment (NSTX) has undergone a major upgrade to NSTX-U at Princeton Plasma Physics Laboratory (PPPL). NSTX-U will double the toroidal field, plasma current, and neutral beam injection heating power, as well as significantly increase the pulse duration. Control of the influx of impurity gases and fuel recycling from plasma facing components (PFCs) are critical for further improving NSTX-U plasma performance. To achieve high quality plasmas during operations, wall conditioning technologies are needed to limit impurities in the plasma by removing the impurities from PFCs before plasma operation, and reducing recycling and outgassing during discharges via reactive coating materials. In this article, the design of a bakeout system, a glow discharge cleaning (GDC) system, a deuterated trimethylboron (dTMB) boron coating system, and a lithium evaporation (LITER) coating system for NSTX-U will be briefly described. Designed for enhanced operational safety, fully automated process control, and easy equipment maintenance and serviceability, these systems have been successfully used to condition the PFCs for NSTX and NSTX-U. Operational sequence and procedure of using these wall conditioning methods for NSTX-U will be covered in this article. Wall conditioning results will also be briefly discussed at the end of this article.

Turbulent transport in TCV plasmas with positive and negative triangularity

Local gyrokinetic simulations with the GENE code are used to investigate the turbulent transport for different levels of neutral beam injection heating power in the Tokamak à Configuration Variable (TCV) for plasmas with both positive and negative edge triangularity. The sensitivity of the heat fluxes with respect to the main plasma parameters, including background gradients, impurity content, and electron temperature, is systematically studied. The experimentally measured transport levels are recovered with variations of profiles compatible with experimental error bars. When considering experimental conditions, trapped electron modes are the dominant instabilities for all heating powers and both types of shapes, whereas ion temperature gradients (ITGs) are not found. A numerical experiment, considering plasma profiles that strongly destabilize ITG modes, is thus performed. Negative triangularity is found to reduce the transport level also in this regime.

Electron energy distribution function in the divertor region of the COMPASS tokamak during neutral beam injection heating

This paper presents the results from swept probe measurements in the divertor region of the COMPASS tokamak in D-shaped, L-mode discharges, with toroidal magnetic field BT = 1.15 T, plasma current Ip = 180 kA and line-average electron densities varying from 2 to 8×1019 m−3. Using neutral beam injection heating, the electron energy distribution function is studied before and during the application of the beam.The current-voltage characteristics data are processed using the first-derivative probe technique. This technique allows one to evaluate the plasma potential and the real electron energy distribution function (respectively, the electron temperatures and densities).At the low average electron density of 2×1019 m−3, the electron energy distribution function is bi-Maxwellian with a low-energy electron population with temperatures 4-6 eV and a high-energy electron group 12-25 eV. As the line-average electron density is increased, the electron temperatures decrease. At line-average electron densities above 7×1019 m−3, the electron energy distribution function is found to be Maxwellian with a temperature of 6-8.5 eV.The effect of the neutral beam injection heating power in the divertor region is also studied.

A Systematic Study of Impurity Ion Poloidal Rotation and Temperature Profiles Using CXRS in the TJ-II Stellarator

A concise systematic study of impurity parameter profiles (impurity ion temperatures and poloidal velocities) measured under different plasma conditions has been carried out in the TJ-II stellarator using an active spectroscopic diagnostic system covering a large section of the central plasma minor radius, from r/a = 0.3 to 0.85, with high spatial resolution. For this, plasmas using hydrogen as the working gas were created and maintained using electron cyclotron resonance heating (from 400 to 500 kW) and line-averaged densities between 0.4 and 0.9 × 1019 m-3 were achieved. In the first instance, it is noted that ion temperature profiles tend to be flat across the accessible plasma radius, whilst a transition in the poloidal velocity from the ion to the electron diamagnetic direction is observed with increasing density. At the same time, the shear point for impurity velocity is seen to propagate outwards towards the plasma edge. Similarly, at a fixed density, a similar tendency is observed when raising the injected heating power. These results together with the behaviour of the impurity temperatures for the same conditions, suggest that collision frequency νEb/e ∼ neTe-3/2 can be considered as an important controlparameter of plasma dynamics.

Thirty-minute plasma sustainment by real-time magnetic-axis swing for effective divertor-load-dispersion in the Large Helical Device

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. A high temperature of approximately 2keV, and steady-state plasma-sustainment operation of the Large Helical Device (LHD) [O. Motojima, K. Akaishi, H. Chikaraishi et al., Nucl. Fusion 40, 599 (2000)] is reported. High-temperature plasmas were created and maintained for more than 30min with a world record injected heating power of 1.3GJ. The three-dimensional heat-deposition profile of the LHD helical divertor was modified and during long-pulse discharges it effectively dispersed the heat load using a magnetic-axis swing technique developed at the LHD. A sweep of only 3cm of the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The modification of the heat-load profile was explained well by field-line tracing. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies. The operation lasted until a sudden increase of radiation loss occurred, presumably because of wall metal flakes dropping into the plasma. The sustained line-averaged electron density was approximately 0.7–0.8×1019m−3. The average input power was 680kW, and the plasma duration was 31min 45s. This successful long operation shows that the heliotron configuration has a high potential as a steady-state fusion reactor.

Status of power balance monitoring for long pulse operation at Tore Supra

A short review of diagnostics and techniques for active power exhaust control is given from the point of view of experiences gained at Tore Supra. The physical and technological aspects are presented with the aim of illustrating strengths, difficulties and potential for future long pulse and steady state fusion devices such as W7X and ITER-FEAT. One of the main features of the Tore Supra approach to power exhaust control is the reliance on infrared (IR) thermography to survey nearly 100% of the high flux target zones. This is applied to the largest elements, the toroidal pump limiter and the heating antennae, by a number of IR endoscopes. For high flux target areas inaccessible to direct view, optical fibres in the IR and the near-IR range are used. The large wall panels, which evacuate the rest of the power, are controlled by calorimetry. Bolometry is used to achieve a complete power balance. These measurements can be inserted in control loops acting typically on the injected heating power or the radiation cooling.

Sensitivity analysis and validation of buildings' thermal models using adjoint-code method

The objective of this study is to present a validation method for simulation codes describing thermal behavior of buildings. Beginning with the classical form of comparison of theoretical and experimental results, it is proposed to improve the procedure by calculating the degree of uncertainty associated with the theoretical results. It can be shown that uncertainties associated with code input data, that is, the simulation parameters, propagate through the calculation and generate a range of uncertainty in the results. After describing the problem and objectives of the study, the computation code analysis will be presented. This concerns use of the CA-SIS (Conditionnement d'Air-Simulation de Systèmes) code, and employs the TRNSYS calculation environment. Experimental validation studies for the evaluation of this code have been carried out using ETNA (Essais Thermiques Naturels ou Artificiels) cells, which have been constructed and are maintained at a site near Paris. These cells and experimental procedures are described. To determine ranges of uncertainty in the numerical results, a sensitivity analysis is first carried out by an “adjoint” method. This method and the relationship linking the uncertainty to calculated sensitivities is presented. Notably, it can be demonstrated that the adjoint method simplifies the calculation of uncertainties. The results presented focus on the cell air temperature. The experimental air temperature evolves during the climatic heating sequence within the range of uncertainty of the theoretical results. It can be deduced that the first step of validation being reached, the developer of CA-SIS code can discuss the complete validation for this configuration. The high sensitivity of the internal air temperature to the parameter of heating power shows the limitation of isothermal air volume hypothesis. From this it can be concluded that it is necessary to improve the numerical modeling of the injection heating power.