S. V. Lebedev

Recent progress on the development and analysis of the ITPA global H-mode confinement database

This paper describes the updates to and analysis of the International Tokamak Physics Activity (ITPA) Global H-Mode Confinement Database version 3 (DB3) over the period 1994–2004. Global data, for the energy confinement time and its controlling parameters, have now been collected from 18 machines of different sizes and shapes: ASDEX, ASDEX Upgrade, C-Mod, COMPASS-D, DIII-D, JET, JFT-2M, JT-60U, MAST, NSTX, PBX-M, PDX, START, T-10, TCV, TdeV, TFTR and TUMAN-3M. The database now contains 10382 data entries from 3762 plasma discharges, including data from deuterium–tritium experiments, low-aspect ratio plasmas, dimensionless parameter experiments and plasmas. DB3 also contains an increased amount of data from a range of diverted machines and further data at high triangularity, high density and high current. A wide range of physics studies has been performed on DB3 with particular progress made in the separation of core and edge behaviour, dimensionless parameter analyses and the comparison of the database with one-dimensional transport codes. The errors in the physics variables of the database have also been studied and this has led to the use of errors in variables fits. A key aim of the database has always been to provide a basis for estimating the energy confinement properties of next step machines such as ITER, and so the impact of the database and its analysis on such machines is also discussed.

Scaling of the energy confinement time with β and collisionality approaching ITER conditions

The condition of the latest version of the ELMy H-mode database has been re-examined. It is shown that there is bias in the ordinary least squares regression for some of the variables. To address these shortcomings three different techniques are employed: (a) principal component regression, (b) an error in variables technique and (c) the selection of a better conditioned dataset with fewer variables. Scalings in terms of the dimensionless physics variables, as well as the standard set of engineering variables, are also derived. The new scalings give a very similar performance for existing scalings for ITER at the standard beta(n) of 1.6, but a much improved performance at higher beta n.

H-mode studies on TUMAN-3 and TUMAN-3M

The focus of the TUMAN-3 and TUMAN-3M tokamaks programme is on issues of improved confinement. The transition from an ordinary ohmic regime into improved confinement mode has been found in circular limiter configuration in a vessel with all-metallic walls and limiters. The signatures of the H-mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the radial electric field was found in experiments with internal probe biasing. Other techniques were demonstrated to trigger H-mode: short increase of the working gas puffing rate, minor radius magnetic compression and pellet injection. The scaling of the energy confinement time in ohmic H-mode was obtained, which differs dramatically from the scaling for the ordinary ohmic regime. A strong dependence of on plasma current was found. The scaling for the ohmic H-mode is consistent with the scaling proposed for devices with powerful auxiliary heating (JET/DIII-D H-mode scaling). The result shows that H-mode physics is universal in tokamaks with different geometries and heating methods. In 1994 a new vacuum vessel was installed in the TUMAN-3 tokamak. The modified device, TUMAN-3M, is able to produce higher and , up to 2 T and 0.2 MA, respectively. During the first operational period a plasma current of 0.15 MA was achieved at T, which corresponded to . The impact of the quality of wall coating on confinement was asserted. The longest energy confinement time (30 ms) was observed under the conditions of best boronization.

Evolution of geodesic acoustic mode in ohmic H-mode in TUMAN-3M tokamak

The behavior of a geodesic acoustic mode (GAM) in the TUMAN-3M tokamak has been experimentally studied using the heavy-ion beam probing technique. Oscillations of the electric potential under the action of a GAM localized at the plasma periphery have been detected. The GAM was observed in the regime of low confinement (L-mode) with low plasma density (∼0.8 × 1019 m−3) and disappeared upon the transition to a high confinement regime (H-mode). The possible role of GAM as a precursor of the improved confinement (LH-transition) is discussed.

Confinement of energetic ions in a tokamak plasma at magnetic field in the range of 0.7–1.0 T

We have experimentally studied the influence of toroidal magnetic field (BT) and plasma current (Ip) on the capture and confinement of energetic ions (EIs) formed upon ionization of a neutral beam injected in a tokamak. Based on the results of measurements of the flux of 2.45-MeV fusion neutrons, it is concluded that the amount of EIs significantly grows with increasing BT from 0.7 to 1.0 T and Ip from 140 to 180 kA. In addition to the classical Coulomb slowing down, a supplementary channel of EI losses is found that accounts for a 15% decrease in their confinement time.

Counter-NBI assisted LH transition in low density plasmas in the TUMAN-3M

This paper reports observations of the LH transition at very low density in the experiments on counter-current neutral beam injection (NBI) in the TUMAN-3M tokamak. The transition has been found at a target average density of as low as 0.5 ? 1019?m?3, which is lower by a factor of 2.5 than the LH transition density boundary in ohmic and co-current NBI heated plasmas. Relatively low input power in counter-NBI experiments is noticed: Pinput = 130?270?kW. In contrast, in the case of co-NBI the LH transition is difficult at low density. No transition is possible at the above density with co-NBI Pinput up to 500?kW. A model that predicts the generation of a negative radial electric field Er, which is thought to help LH transition during counter-NBI, is suggested. The model conjectures the development of the Er and toroidal rotation V in the presence of large ion orbit losses in the counter-NBI scheme. Measurements of the plasma potential by the heavy ion beam probe technique and edge radial electric field by electrostatic probes indicating negative Er emergence in the counter-NBI scenario are presented. Doppler spectroscopy of B3+ impurity ions has shown an increase in the V of 16 ? 6?km?s?1 after counter-NBI switch on. The measured V agrees with the above model estimations.

New features of the energy confinement from TUMAN-3 ohmic H-mode experiments

New features of the energy confinement were observed in the Ohmic regime with reduced transport - Ohmic H-mode. Parametric dependencies as well as absolute values of the energy confinement time are in agreement with scaling proposed for description of the ELM-free H mode in bigger devices with powerful auxiliary heating (DIII/JET H-mode scaling). In particular, strong dependencies of tau E on plasma current and input power and weak dependencies on density were found. Energy confinement time is enhanced by a factor of 15 compared to predictions of the usual ohmic scalings (Neo-Alcator, Merezhkin-Mukhovatov). In the small tokamak TUMAN-3 30ms energy confinement time was achieved.

Optimization of geometry of heating neutral beam input into the TUMAN-3M tokamak

We consider the possibility of attaining a maximum contribution of energy to plasma due to variation in the impact parameter of a beam of high-energy neutral atoms and in the slope angle of the injection line to the equatorial plane at different parameters of the discharge. The influence of energy of particles on their confinement is also considered. It is shown that particles can be both passing and trapped depending on energy, which is connected with nonconservation of the adiabatic invariant W⊥/B.

The influence of plasma horizontal position on the neutron rate and flux of neutral atoms in injection heating experiment on the TUMAN-3M tokamak

Horizontal displacement of plasma along the major radius has been found to significantly influence the fluxes of 2.45 MeV DD neutrons and high-energy charge-exchange atoms from neutral beam injection (NBI) heated plasma of the TUMAN-3M tokamak. An inward shift by ΔR = 1 cm causes 1.2-fold increase in the neutron flux and 1.9-fold increase in the charge-exchange atom flux. The observed increase in the neutron flux is attributed to joint action of several factors-in particular, improved high-energy ion capture and confinement and, probably, decreased impurity inflow from the walls, which leads to an increase in the density of target ions. A considerable increase in the flux of charge-exchange neutrals in inward-shifted plasma is due to the increased number of captured high-energy ions and, to some extent, the increased density of the neutral target. As a result of the increase in the content of high-energy ions, the central ion temperature Ti(0) increased from 250 to 350 eV. The dependence of the neutron rate on major radius R0 should be taken into account when designing compact tokamak-based neutron sources.

Resistive loss compensation in the power supply system of the toroidal field winding in the TUMAN-3M tokamak

In the tokamak, in which the winding generating the toroidal field with induction Bt is powered from an inductive energy storage, resistive losses are the main factor that causes Bt to decrease during a discharge. Upgrading of the power supply circuit of the toroidal magnetic field winding (TFW) in the TUMAN-3M tokamak is aimed, primarily, at increasing Bt in the injection heating phase and, second, at maintaining Bt quasi-stationarity during the whole tokamak discharge. To do this, an additional two-section capacitive storage commuted by two thyristor switches has been introduced into the available circuit. Either section of the storage is characterized by a charging voltage of 0.25 kV, a capacitance of 4.32 F, and an energy capacity of 135 kJ. The maximum discharge current of the section is 40 kA. The upgraded circuit compensates for the resistive loss in the TFW and ensures thereby a 50% increase in the magnetic field during injection heating relative to the old circuit: Bt = 1.0 T instead of 0.68 T. In this case, the circuit maintains a TFW current of 110 kA with an accuracy of 10% for ∼60 ms.