Good Energy Confinement Research Articles

Physical design of JT-60 super upgrade

The JT-60 Super Upgrade (JT-60SU) is an upgraded tokamak device of JT-60U for developing the steady-state reactor and advanced tokamak operation in the International Thermonuclear Experimental Reactor. The device is planned to utilize the JT-60 facilities fully and to minimize the needed modification. The major radius is 4.8 m and the maximum plasma current is 10 MA. Neutral beam injection with 750 keV beam energy is the primary heating method. The machine is capable of steady-state operation with high density up to 8.8 × 10 19 m −3 at 5 MA plasma current. The high operating density, over the Greenwald et al. limit, is critically important in order to achieve high bootstrap current fraction. Ballooning mode and low n ideal magnetohydrodynamic (MHD) mode including the bootstrap current were analyzed for steady-state operation. The current profile must be optimized to obtain a normalized beta up to 3. The plasma configuration with high triangularity was adopted in order to get good MHD stability and high energy confinement. A compact divertor was designed in order to get the large plasma space.

Scaling of radiated power to plasma contamination for neon seeded discharges on boronized TEXTOR-94

When neon is injected into a tokamak plasma with the aim of reducing the peak heat flow into the SOL by enhanced edge radiation, both P rad and Z eff increase. The ratio of the incremental P rad to the incremental Z eff , ΔP rad ΔZ eff , can change on TEXTOR from 0.1 MW to about 2 MW depending on n e , on T e ( a) and on the transport regime. The measured ΔP rad ΔZ eff is higher for I-mode auxiliary heated (higher diffusion) than for OH discharges (lower diffusion), consistent with model calculations. For I-mode discharges ΔP rad ΔZ eff is a decreasing function of T e ( a) and it scales approximately as n e 2 . The I-mode regime on TEXTOR, which displays very good energy confinement scaling, can represent, for suitable conditions, a satisfactory compromise between a high τ E and not excessive particle confinement.

Impurity feedback control for enhanced divertor and edge radiation in DIII-D discharges

Radiated power fractions above 90% have been achieved in DIII-D using impurity feedback control. Heat flux reductions up to a factor of 6 and confinement enhancements of 1.6 X ITER-89P L-mode scaling have been obtained with a radiated power fraction above 90%. When the outer separatrix strike point was positioned for pumping with the DIII-D cryopump, ELMing H-mode (Type III ELMs) was triggered with neon feedback injection in conjunction with a constant deuterium gas puff rate. Changes in the ELM frequency, energy confinement and central neon concentrations as a function of radiated power are presented. Observations of detached discharges with H-mode confinement similar to the ASDEX-U CDH-mode are also discussed.

ICRH power absorption measurements from analysis of RF sinusoidal low-frequency modulation experiments in TEXTOR

Modulation of the ICRH power amplitude has been performed sinusoidally in TEXTOR over a wide modulation frequency range in an (H)-D plasma. From an analysis of reproducible discharges in which the ICRH is modulated at a relatively high modulation frequency Omega /2 pi (50-300 Hz), we observe that only a fraction (about 50%) of the ICRH power is transferred to the plasma with a good energy confinement time. We also show that the analysis of a single modulated discharge already provides part of the information. The analysis is supported by a careful investigation of the non-stationary energy response obtained from the diamagnetic look and from the equilibrium measurement. The eigenfunction expansion of the transport equation for global energy in which each successive mode has a particular absorption coefficient and a decreasing confinement time indicates that about 50% of the ICRH power is coupled to the fundamental mode with the best confinement time. The remainder of the power goes to other modes with decreasing confinement times. Such a time-dependent analysis allows us therefore to distinguish between the fraction of the power absorbed by the core of the plasma (good confinement) and the fraction absorbed by the edge (bad confinement). The conclusions of this modulation analysis are qualitatively similar for high-field side (HFS) antennae with predominantly direct electron heating and for low-field side (LFS) antennae with predominantly minority-ion heating. In the second scenario, a detailed analysis has been provided to account for the anisotropic contribution to the energy of the minority ions.

Study of the power exhaust and the role of impurities in the Torus Experiment for Technological Oriented Research (TEXTOR)

Controlled application of radiating impurities in the boundary layer can help to solve the problem of power exhaust in a fusion reactor. Experiments in the Torus Experiment for Technological Oriented Research (TEXTOR) [J. Nucl. Mater. 145–147, 3 (1987)] are presented, which show that impurities with sufficiently high atomic number (≥10) are well suited for this purpose. Injection of neon, a gas recycled at the wall, enabled the establishment of a quasistationary radiating boundary layer, from which more than 90% of the input power was emitted. The required neon density was established by means of a feedback control for the neon influx, which was made possible by the toroidal pump limiter Advanced Limiter Technology (ALT-II) [J. Nucl. Mater. 162–164, 115 (1989)]. Alternatively, or in addition silicon was introduced as a condensing element—either by surface reactions from siliconized walls or by silane [SiH(D)4] injection—which revealed self-controlling mechanisms effective with changing plasma parameters. In neither case was a significant increase in central impurity concentration observed and good energy confinement time was maintained up to the highest plasma densities. Based on the information from various refined edge diagnostics, the underlying mechanisms for the buildup of a radiating plasma mantle and the interdependences of neon and silicon on other impurities are discussed.

Measurement of Peripheral Electron Temperature by Electron Cyclotron Emission during the H-Mode Transition in JFT-2M Tokamak

Time evolution and profile of peripheral electron temperature during the H-mode like transition in a tokamak plasma is measured using the second and third harmonic of electron cyclotron emission (ECE). The so called “ H-mode ” state which has good particle/energy confinement is characterized by sudden decrease in the spectral line intensity of deuterium molecule. Such a sudden decrease in the line intensity of D α with good energy confinement is found not only in divertor discharges, but also in limiter dischargs in JFT-2M tokamak. It is found by the measurement of ECE that the peripheral electron temperature suddenly increases in both of such phases. The relalion between H-transition and the peripheral electron temperature or its profile is investigated.

H-Mode Outside Divertor Free from Peripheral Disruption in Doublet III

The heating efficiency and the peripheral disruption of three distinctively different divertor configurtions were studied in Doublet III at a plasma current of 600 kA up to the neutral injection power of 7.5 MW. An outside divertor configuration with a good energy confinement (H-mode), which has a null point at the outside in the major radius and of which separatrix field lines divert to the outside wall, was observed. The discharges of the outside divertor configuration were free from the peripheral disruption characterized by Hα/Dα bursts and the stored energy was nearly proportional to the injection power up to 7.5 MW of NBI.

Thomson scattering diagnostic in the boundary layer of ASDEX

The divertor experiment ASDEX has been equipped with a Thomson scattering system, designed to measure radially resolved electron temperature and density profiles in the boundary layer between the hot plasma core and the wall. Thus, it is possible to obtain detailed information about plasma edge phenomena as a basis of divertor physics. The measured electron temperature at the magnetic separatrix can be described by the Coppi-Mazzucato (1979) scaling law for the electron heat conductivity in ohmically heated plasmas. This is analyzed over a wide range of ASDEX plasma parameters and discharge conditions. During additional heating by neutral injection the Te-profiles show a steepening in the scrape-off area, which can be a consequence of a local decrease in thermal conductivity responsible for the good energy confinement observed for 'H'-regime discharges.

Langmuir probe measurements in beam-heated divertor discharges in D-III

A Langmuir probe array and a thermocouple array installed in the divertor plate were used to measure the edge properties of 4 MW beam-heated divertor plasmas in D-III. Particle recycling near the divertor plate in high-recycling (poor confinement) discharges was approximately twice that of low-recycling (good confinement) discharges. Some low-recycling discharges exhibit ∼ 30% higher edge temperatures than high-recycling discharges. The strong particle shielding of the divertor, however, is capable of making the divertor plasma dense and cold ( T e ≲ 20 eV, n e ∼ 10 14 cm −3) while keeping the recycling near the main plasma low enough to maintain good energy confinement.

High-density, low-q discharges with D-shaped and circular cross-sections in Doublet III

The maximum plasma current in Doublet III is found to be limited by disruptions when the limiter safety factor is approximately two. However, because of the strong toroidal and shaping-field effects on the rotational transform at the outer plasma edge associated with a vertical elongation of 1.5 D-shape, the safety factor estimated from simple geometric considerations for D-shaped plasmas corresponds to values as low as 1.5. These discharges operate stably with considerably higher plasma current than most reactor design studies assume. These low-q discharges show excellent plasma performance: very flat spatial electron temperature profiles, high-density operation with n̄eR/BT up to 7.8, and good energy confinement producing a volume-average β of up to 1% with Ohmic heating only. This operational regime appears to be applicable to future high-β tokamaks with D-shaped cross-sections.