Toroidal Rotation Velocity Research Articles

Vertical high-resolution Bragg x-ray spectrometer for the tokamak fusion test reactor

A Bragg x-ray spectrometer of high spectral resolution (λ/Δλ=10 000–20 000) which accommodates three crystals and position-sensitive detectors in the Johann configuration has been installed in the diagnostic basement of the tokamak fusion test reactor (TFTR) for the measurement of radial ion temperature profiles. The ion temperature is derived from the Doppler broadening of Kα-resonance lines of metal impurity ions, e.g., Ti, Cr, Fe, and Ni, in the helium-like and hydrogen-like charge states. The x-ray diffraction plane is almost perpendicular to the magnetic axis, but slightly tilted by an angle of 3.8°, which makes it possible to measure poloidal and toroidal plasma rotation velocities of vΘ>5×103 m/s and vΦ>1×105 m/s, from the Doppler shift of spectral lines. Results obtained from the observation of TiXXl Kα-line spectra with a 220-silicon crystal of a 2d spacing of 3.8400 Å and a curvature radius of 11.05 m are reported.

High time resolution ion temperature profile measurements on PBX

Ion temperature profiles with a time resolution of 2–5 ms have been measured on the Princeton Beta Experiment tokamak (PBX) by charge-exchange recombination spectroscopy (CXRS) and a neutral particle charge-exchange analyzer (NPA). The sightlines of both diagnostics crossed the trajectory of a near-perpendicular heating beam, which enhanced the local neutral density (∝ signal strength) and provided spatial resolution. The time resolution of these two independent techniques is sufficient to see sawtooth oscillations and other magneto- hydrodynamic activity. Effects of these phenomena on the toroidal rotation velocity profile, vφ(r), are clearly observed by CXRS. For example, a sharp drop in the central vφ occurs at the sawtooth crash, followed by a linear rise during the quiescent phase. The NPA results are compared with those from CXRS.

Space potential distribution in the ISX-B tokamak.

Plasma potentials have been measured for the first time in neutral-beam-heated tokamak discharges, with a heavy-ion beam probe. Radial profiles of these potentials are presented for coinjection, counterinjection, and balanced injection, and for plasmas having only Ohmic heating. They are found to be very dependent on the direction of rotation with respect to the plasma current. They are also found qualitatively consistent with those inferred from radial momentum balance with use of the measured toroidal rotation velocity.

Satellite spectra for heliumlike titanium. II.

K\ensuremath{\alpha} x-ray spectra of heliumlike titanium, Ti xxi, from Tokamak Fusion Test Reactor (TFTR) plasmas have been observed with a high-resolution crystal spectrometer and have been used as a diagnostic of central plasma parameters. The data allow detailed comparison with recent theoretical predictions for the Ti xxi heliumlike lines and the associated satellite spectrum in the wavelength range from 2.6000 to 2.6400 A\r{}. Improved values for the excitation rate coefficients of the Ti xxi resonance line, the intercombination lines, and the forbidden line, and new theoretical results on the wavelengths and transition probabilities for berylliumlike satellites due to transitions of the type 1${s}^{2}$2lnl'-1s2p2l''nl''' with n=2--4 have been calculated.These data complement the theory given in an earlier paper [F. Bely-Dubau et al., Phys. Rev. A 26, 3459 (1982)]. The experimental data are in excellent agreement with the theoretical predictions for the entire satellite spectrum. For plasmas with electron temperatures, ${T}_{e}$, in the range from 1.2 to 1.8 keV, there is also good general agreement between theory and experiment for the heliumlike lines of Ti xxi. The observed intensity ratios of the intercombination and forbidden lines to the resonance line are, however, larger than the predicted values by as much as an order of magnitude for electron temperatures ${T}_{e}$1.2 keV. The enhancement of these lines is correlated with the abundance of lithiumlike titanium.The experimental data indicate that still other atomic processes, in addition to those considered by the theory, are important for the excitation of these lines. The experimental results are documented to stimulate further analysis. The observed wavelengths agree to within an uncertainty of 0.5 mA\r{} with predictions from calculations which include relativistic and radiative corrections up to the third order in \ensuremath{\alpha}. The central ion temperature and the central toroidal rotation velocity of TFTR plasmas with ohmic and neutral beam heating were obtained from Doppler measurements of the Ti xxi resonance line.Wavelength shifts of the apparent resonance line profile due to unresolved dielectronic satellites, which can be important corrections to Doppler-shift measurements of plasma rotation velocities, have been determined as a function of the ion and electron temperatures and the fitting limits. Results on the electron temperature and the relative charge-state density of lithiumlike and heliumlike titanium were derived from the satellite spectrum and have been interpreted with plasma modeling. These results are of interest for the determination of the ionization equilibrium and the impurity transport.

Viscous effects in a collisional tokamak plasma with strong rotation

The full viscosity tensor for an axisymmetric toroidal plasma in the collisional regime (with strong rotation) is calculated, including gyroviscosity and O(ε) poloidal variations over the flux surface. It is shown that the resulting viscous force is of sufficient magnitude to account for the radial transfer of toroidal momentum that must be inferred in order to explain the rotation measurements in tokamak experiments. The consequences of a viscous force of this form and magnitude on particle transport and on the evolution of toroidal and poloidal rotation velocities are discussed.

Measurements of periodic ripple transport in the ISX-B tokamak

The effect of periodic toroidal field (TF) ripple on ion confinement has been studied in the ISX-B tokamak by comparing neutral-beam-heated plasma performance with 9 and 18 TF coils. Three ripple physics issues were treated by these experiments: (1) enhanced ion thermal conductivity, (2) enhanced loss of energetic ions, and (3) ripple damping of beam-induced toroidal plasma rotation, which may affect the plasma losses. Under a wide variety of plasma conditions, ripple reduced the central-ion temperature by a factor of approximately two (600 eV → 300 eV). Ion temperature was found to be nearly independent of applied neutral-beam power in the large ripple configuration (9 TF coils). These results are shown to be in reasonable agreement with theoretical models of ripple transport. Charge-exchange measurements of the fast-neutral flux indicated no loss of fast passing ions due to ripple, but a large depletion of the fast ions trapped in local ripple wells, as expected theoretically. The central toroidal rotation velocity was reduced by a factor of six by ripple, yielding a momentum confinement time substantially less (factor of about seven) than that expected from standard theoretical expressions for ripple-enhanced ion viscosity.

Plasma rotation in the PDX tokamak

Toroidal and poloidal rotation has been measured in the Poloidal Divertor Experiment (PDX) tokamak in Ohmic and neutral-beam heated plasmas in a variety of discharge conditions and in both circular and diverted configurations. Rotation velocities were deduced from Doppler shifts of magnetic dipole (M1) lines and lines of optically allowed transitions in the visible and UV regions, from Kα emission, and also from an array of magnetic pickup loops. Poloidal and toroidal rotation velocities in Ohmically heated discharges were usually less than 3 × 105 cm·s−1. Near the plasma edge, the toroidal rotation velocity varies with poloidal angle both before and during neutral-beam injection. No systematic poloidal rotation was observed during neutral-beam injection centred about or displaced 10 cm from the horizontal midplane, which implies that the poloidal damping time τθ < 0.5 τii consistent with theoretical estimates. The central toroidal rotation velocity during neutral-beam injection scales linearly with the quantity and is independent of plasma current and toroidal magnetic field. The toroidal rotation velocity is higher in deuterium than in hydrogen plasmas, and also in diverted discharges as compared with circular ones. Toroidal rotation decay times after injection range from 80–100 ms at the centre to 160–180 ms at half the minor radius. Modelling of the radial profile of toroidal rotation indicates a central momentum diffusivity of the order of 8 × 103 cm2·s−1. This is approximately a factor of three higher than the momentum diffusivity obtained from the decay time. All present theories are inadequate in accounting for the observed damping rate of vϕ.

Measurements of plasma ion temperature and rotation velocity using the He II 4686-Å line produced by charge transfer

Spatially and temporally resolved values of the ion temperature and toroidal rotation velocity near the plasma axis have been obtained in neutral beam-heated plasmas in the Doublet III tokamak from measurements of the Doppler broadening and Doppler shift of the He II 4686-Å line, excited by charge transfer reactions between fast hydrogen atoms in the beam and thermal helium ions in the plasma. The measurement technique, which uses visible optics and a multichannel detector, promises to be a straightforward method to obtain ion temperature profiles in beam-heated tokamaks.

Plasma rotation measurements using spectral lines from charge-transfer reactions

The central toroidal rotation velocities of tokamak plasmas have been measured from the Doppler shifts of spectral lines that are excited by charge transfer of the neutral hydrogen heating beams with fully ionized oxygen.