Central Ion Temperature Research Articles

Charge Exchange Neutral Particle Measurement in Heliotron E

The absolute measurements of charge exchange neutral flux in the Heliotron E device are described. The dependence of the neutral density is investigated on the electron density and ohmic current. The neutral density decreases as the electron density increases in accordence with the Monte Carlo calculation. For the fixed electron density, the neutral density is found to increase with increasing the ohmic current. The three kinds of T i profile are discussed. The scaling of central ion temperature is also studied. For low current plasma, it shows good agreement with the neoclassical value. In high current plasma, however, T io is about 1/3 of that predicted by the neoclassical theory, which is caused by anomalous ion heat and particle transport driven by ohmic current.

Coupled radial and thermal fluctuations of ignited tokamak plasmas

The coupling between radial motion and temperature fluctuations of an ignited D-T tokamak plasma is studied in terms of a one-dimensional transport model. The energy balance equations are linearized about the thermal equilibrium (i.e. ignition) point and an eigenvalue analysis is used to determine the growth rate of perturbations. If the density dynamics is relatively slow, it is found that there is, at most, one growing eigenmode. The growth rate of temperature perturbations and the extent of radial movement depend upon a parameter ηr which describes the elasticity of radial motions (ηr = T/R dR/dT, where R is the major radius and T is the plasma temperature) . A simple MHD model is used to determine the dependence of ηr upon the vertical field index (η = - R/BvdBv/dR) and the poloidal beta. The implications of different empirical scaling laws for the electron energy confinement time are examined. Similar values of ηr (ηr ∼ 0.2) can be obtained for representative ETF/INTOR and ignition test reactor (ITR) parameters. For ηr ∼ 0.2 the effect of radial motion is to reduce the central ion temperature needed for thermal stability from ∼ 50 to ∼ 35 keV if the electron energy confinement time scales as τe ∼ na2; at lower temperatures (central ion temperatures ∼ 10 – 20 keV) , the thermal runaway time is increased by a factor ∼ 2 to 3. It is pointed out that if the plasma edge is determined by some type of limiter at the midplane, small temperature fluctuations can significantly change the plasma minor radius; active burn control may be needed in some reactor designs to prevent unacceptably large radial movement in addition to providing constant plasma pressure and temperature.

Results of hydrogen pellet injection into ISX-B

High-speed pellet fuelling experiments have been performed on the ISX-B device in a new regime characterized by large global density rise in both Ohmically and neutral-beam heated discharges. Hydrogen pellets of 1 mm in diameter were injected in the plasma midplane at velocities exceeding 1 km·s−1. In low-temperature Ohmic discharges, pellets penetrate beyond the magnetic axis, and in such cases a sharp decrease in ablation is observed as the pellet passes the plasma centre. This behaviour can be accounted for by an ablation model that includes dynamic cooling of the target plasma while the ablation proceeds. Complete penetration can be prevented by operation in low-density regimes where runaway electrons are thought to be responsible for high ablation. A similar effect is observed with moderate to large amounts of neutral-beam injection. There is a strong enhancement of the ablation rate in the outer 10-cm plasma region even for short heating intervals, which can be explained by the presence of multi-kilo-electron volt ions in the discharge. Density increases of ∼300% have been observed without degrading plasma stability or confinement. Energy confinement time increases in agreement with the empirical scaling τE ∼ ne and central ion temperature increases as a result of improved ion-electron coupling. Laser-Thomson scattering and radiometer measurements indicate that the pellet interaction with the plasma is adiabatic. The low level of power emission from the pellet-plasma interaction region is consistent with negligible charge-exchange losses; within the experimental accuracy, nearly all of the pellet mass can be accounted for in the initial plasma density rise. Penetration to r/a ∼ 0.15 is optimal, in which case large-amplitude sawtooth oscillations are observed and the density remains elevated. Gross plasma stability is dependent roughly on the amount of pellet penetration and can be correlated with the expected temporal evolution of the current density profile.

High-power neutral injection and ion power balance in TFR

A fast-neutral power of up to 570 kW has been injected into TFR. At low densities (n̄e ≅ 2–3 × 1013 cm−3), the central ion temperature was doubled to 2 keV as a result of the injection. The ion temperature increases linearly with the injected power. Macroscopic perturbations due to the injected power yield information on the ion behaviour and the ion energy confinement time. There are indications, that the ion heat conduction might be anomalous. Effects of neutral injection on equilibrium, stability, and impurity concentration are discussed.

Ion Temperature Characteristics of Upgraded DIVA

The radial profile and scaling law of ion temperature were examined on hydrogen plasmas in the DIVA tokamak device with its toroidal magnetic field upgraded from 10 to 20 kG. The radial profile of ion temperature was measured from the Doppler broadening of various impurity lines in vacuum uv and ultraviolet regions which were experimentally confirmed to emit from a localized position in the minor cross-section and from the energy spectrum of charge-exchanged fast hydrogen-atoms. The radial profiles obtained from these two methods were in agreement in the inner region (( r / a )\({\lesssim}0.6\) with the minor radius a of the device). The central ion temperatures were investigated in the wide parameter ranges: toroidal magnetic fields 6≦ B t ≦20 kG; plasma currents, 10≦ I p ≦48 kA; and averaged electron densities \(0.6{\leqq}\overline n_{\text{e}}{\leqq}3.8\times10^{13}\) cm -3 . The obtained central ion temperatures were found to be in good agreement with Artsimovich's scaling law.

Reduced fusion-neutron production in non-axisymmetric tokamak devices

In tokamak plasmas subjected to substantial toroidal-field ripple, sufficiently energetic members of the bulk-ion population can diffuse rapidly away from the hot central region by the superbanana process. The ion velocity distribution is truncated at the velocity above which the escaping fast ions cannot be replaced sufficiently rapidly by Coulomb collisions. This tail-depletion effect causes a marked reduction in the fusion reactivity and can give rise to an anomalously low central ion temperature as deduced from the thermonuclear neutron flux. The application of this phenomenon to the Alcator and TFR plasmas is discussed.

The ion temperature and energy balance in stable discharges in the LT-3 Tokamak

The ion temperature in the Tokamak LT-3 has been deduced from measurements of the thermal Doppler broadening of oxygen impurity line emission from the hydrogen plasma. The profiles of lines in the UV region of the spectrum were analysed with a polychromator which measures simultaneously 7 contiguous spectral bands separated by 0.25 AA in the wavelength. With this instrument, ion temperatures as low as 10 eV could be measured reliably. In stable discharges, which occur in LT-3 only for a sufficiently high q-value at the plasma column boundary, the central ion temperature rise to a maximum of about 30 eV after 4 ms from the initiation of the discharge. The ion energy confinement time at the centre of the discharge reaches a maximum value of about 400 mu s.