Toroidal Rotation Velocity Research Articles

Momentum transport and plasma rotation profile in toroidal direction in JT-60U L-mode plasmas

The characteristics of momentum transport and plasma rotation in the toroidal direction are studied, using near-perpendicular neutral beam injection (PERP-NBI), co tangential and counter (CTR) tangential NBI in JT-60U. Diffusive and non-diffusive terms of momentum transport are evaluated from the transient analysis by using the momentum source modulation. Fast ion losses due to the toroidal field ripple, which locally induces the edge CTR rotation, are used as a novel momentum source. Parameter dependence of these transport coefficients i.e. the toroidal momentum diffusivity (χϕ) and the convection velocity (Vconv), and the relation between momentum and heat diffusivities (χi) are investigated in L-mode plasmas systematically. The toroidal momentum diffusivity increases with increasing heating power and decreases with increasing plasma current. The relation of χϕ and χi to some non-dimensional parameters is investigated. A clear dependence of χϕ/χi on normalized plasma pressure (βN) is observed. It is also found that toroidal rotation velocity profiles in the case with and without external torque input can be almost reproduced by χϕ and Vconv estimated from the transient momentum transport analysis at low β (βN < 0.4).

Development of advanced x-ray imaging crystal spectrometer utilizing a large area segmented proportional counter for KSTAR

An advanced x-ray imaging crystal spectrometer (XICS) for KSTAR tokamak has been developed by utilizing a segmented two dimensional (2D) position-sensitive multiwire proportional counter. The XICS for the KSTAR tokamak provides time-resolved measurements of the radial ion and electron temperature profiles, toroidal plasma rotation velocity, and ionization equilibrium. The segmented 2D detector with delay-line readout and supporting electronics has been adopted to improve the photon count rate capability. The current fabrication status of the XICS for the KSTAR tokamak and the first performance test results of the prototype segmented 2D detector are presented.

Certain considerations concerning the nature of radial electric field and toroidal rotation velocity profile in tokamak plasma

A new and simple approach aimed at understanding the nature of the radial electric field in some types of tokamak plasmas is presented. It is shown that the radial electric field at some point on a magnetic surface in a tokamak plasma can be divided physically into two parts. The first part is related to a very small difference between densities of electrons and ions of two component plasmas on a magnetic surface in the corotating frame. The second part is related to toroidal rotations of current layers on magnetic surfaces. A new integral equation is calculated. The efficiency of this approach is shown by means of an illustrative example of a Tore-Supra experiment.

Spontaneous Toroidal Rotation in Alcator C-Mod Plasmas with No Momentum Input

Spontaneous toroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input. The magnitude of the rotation ranges from -60 km/s (countercurrent) in limiter L-mode discharges to +140 km/s (cocurrent) in ion cyclotron range of frequencies-heated H-mode plasmas. The core rotation in L-mode plasmas is generally countercurrent and is found to depend strongly on the magnetic topology; in near double null discharges, the core rotation changes by 25 km/s with a variation of a few millimeters in the distance between the primary and secondary separatrices. In H-mode plasmas, the rotation increments in the cocurrent direction with the toroidal rotation velocity increase proportional to the corresponding stored energy increase, normalized to the plasma current. These discharges exhibit a positive Er in the core. Immediately following the transition from L-mode into enhanced Dα (EDA) H-mode, the cocurrent rotation appears near the plasma edge and propagates to the center on a time scale similar to the energy confinement time but much less than the neoclassical momentum diffusion time, indicating both the role of the plasma boundary in the dynamics of the H-mode transition and the anomalous nature of momentum transport. Rotation velocity profiles are flat in EDA H-mode plasmas and centrally peaked for edge-localized mode-free H-modes, demonstrating the effects of an inward momentum pinch. In EDA H-mode discharges that develop internal transport barriers, the core toroidal rotation inside the barrier foot is observed to drop on a time scale similar to the core pressure profile peaking (hundreds of milliseconds), indicating a negative Er well in the core region.

NCSX Plasma Heating Methods

The National Compact Stellarator Experiment (NCSX) has been designed to accommodate a variety of heating systems, including ohmic heating, neutral beam injection, and radio-frequency (rf). Neutral beams will provide one of the primary heating methods for NCSX. In addition to plasma heating, neutral beams are also expected to provide a means for external control over the level of toroidal plasma rotation velocity and its profile. The experimental plan requires 3 MW of 50-keV balanced neutral beam tangential injection with pulse lengths of 500 ms for initial experiments, to be upgradeable to pulse lengths of 1.5 s. Subsequent upgrades will add 3 MW of neutral beam injection (NBI). This paper discusses the NCSX NBI requirements and design issues and shows how these are provided by the candidate PBX-M NBI system. In addition, estimations are given for beam heating efficiencies, scaling of heating efficiency with machine size and magnetic field level, parameter studies of the optimum beam injection tangency radius and toroidal injection location, and loss patterns of beam ions on the vacuum chamber wall to assist placement of wall armor and for minimizing the generation of impurities by the energetic beam ions. Finally, subsequent upgrades could add an additional 6 MW of rf heating by mode conversion ion Bernstein wave (MCIBW) heating, and if desired as possible future upgrades, the design also will accommodate high-harmonic fast-wave and electron cyclotron heating. The initial MCIBW heating technique and the design of the rf system lend themselves to current drive, so if current drive became desirable for any reason, only minor modifications to the heating system described here would be needed. The rf system will also be capable of localized ion heating (bulk or tail), and possibly IBW-generated sheared flows.

Plasma rotation measurement in small tokamaks using an optical spectrometer and a single photomultiplier as detector

The method for plasma rotation measurement in the tokamak TCABR is reported in this article. During a discharge, an optical spectrometer is used to scan sequentially spectral lines of plasma impurities and spectral lines of a calibration lamp. Knowing the scanning velocity of the diffraction grating of the spectrometer with adequate precision, the Doppler shifts of impurity lines are determined. The photomultiplier output voltage signals are recorded with adequate sampling rate. With this method the residual poloidal and toroidal plasma rotation velocities were determined, assuming that they are the same as those of the impurity ions. The results show reasonable agreement with the neoclassical theory and with results from similar tokamaks.

Role of Te/Ti and ∇vtor in ion heat transport of ASDEX Upgrade H-mode plasmas

Experiments in H-mode plasmas have shown that both heat and particle transport are sensitive to the ratio between electron and ion temperature (Te/Ti). While decreasing Te/Ti is beneficial for confinement, an increased electron heating in these so called ‘hot ion plasmas’ deteriorates the confinement. H-mode plasmas with low Te/Ti are often accompanied by high toroidal rotation velocity (vϕ). Its gradient (∇vϕ) can destabilize the ion temperature gradient mode (ITG) through its parallel component in the parallel velocity shear, but it has also stabilizing effects since it produces an E × B shearing rate (ωE × B). In this paper, the effects of electron heating on the ion heat transport is investigated in H-mode plasmas heated by neutral beam injection (NBI) and electron cyclotron heating (ECH). In particular, the correlation on Te/Ti and ∇vϕ is studied and compared with calculations made with GLF23 and GS2. Experimentally it is shown that the normalized gradient length of the ions () is correlated with both Te/Ti and ∇vϕ: peaked ion temperature profiles are only obtained with low Te/Ti and high ∇vϕ, and vice-versa. When ECH is added, both ion heat and momentum transport are enhanced, leading to a drop of both the Ti and vϕ profiles. The effective growth rate γeff = γ−ωE × B is calculated, with the mode growth rate γ determined with GS2 and ωE × B with GLF23. The ion transport is enhanced due to the decrease of the ITG threshold with increasing Te/Ti. Comparison of the dependence of on Te/Ti and ∇vϕ between experiments and modelling indicates that the deterioration of confinement cannot be explained by the changes in only Te/Ti or ∇vϕ, but by the combined effects of both parameters. The changes in Te/Ti act directly on the ITG threshold, while the ones in ∇vϕ modify the ωE × B shearing rate leading to changes in the effective threshold.

Effects of ripple loss of fast ions on toroidal rotation in JT-60U

The driving mechanism of toroidal rotation and the momentum transport are studied on JT-60U in relation to the toroidal rotation in the direction antiparallel to the plasma current, i.e. counter (CTR) direction, with near-perpendicular neutral beam (PERP-NB) injection. Fast ion losses due to the toroidal field ripple induce CTR rotation in the peripheral region, and the magnitude of CTR rotation with PERP-NBs reduces by installing the ferritic steel tiles as a consequence of the reduction in the ripple losses. It is also found that toroidal rotation velocity profiles in the core region can be explained by momentum transport considering diffusivity and convective velocity estimated from transient momentum transport analysis in L-mode plasmas.

Rotation in a reversed field pinch with active feedback stabilization of resistive wall modes

Active feedback stabilization of multiple resistive wall modes (RWMs) has been successfully proven in the EXTRAP T2R reversed field pinch. One of the features of plasma discharges operated with active feedback stabilization, in addition to the prolongation of the plasma discharge, is the sustainment of the plasma rotation. Sustained rotation is observed both for the internally resonant tearing modes (TMs) and the intrinsic impurity oxygen ions. Good quantitative agreement between the toroidal rotation velocities of both is found: the toroidal rotation is characterized by an acceleration phase followed, after one wall time, by a deceleration phase that is slower than in standard discharges. The TMs and the impurity ions rotate in the same poloidal direction with also similar velocities. Poloidal and toroidal velocities have comparable amplitudes and a simple model of their radial profile reproduces the main features of the helical angular phase velocity. RWMs feedback does not qualitatively change the TMs behaviour and typical phenomena such as the dynamo and the ‘slinky’ are still observed. The improved sustainment of the plasma and TMs rotation occurs also when feedback only acts on internally non-resonant RWMs. This may be due to an indirect positive effect, through non-linear coupling between TMs and RWMs, of feedback on the TMs or to a reduced plasma-wall interaction affecting the plasma flow rotation. Electromagnetic torque calculations show that with active feedback stabilization the TMs amplitude remains well below the locking threshold condition for a thick shell. Finally, it is suggested that active feedback stabilization of RWMs and current profile control techniques can be employed simultaneously thus improving both the plasma duration and its confinement properties.

A Numerical Procedure to Simulate Cord-Integrated Passive Spectroscopy Measurements in TJ-II Plasmas

A local model capable of simulating the cord-integrated emission of spectral lines in the TJ-II stellarator has been developed for inferring local parameters. The procedure was implemented on a numerical code, which starting from given analytical profiles of local emissivity, ion temperature, and toroidal rotation calculates the cord-integrated emission spectra along a selected line of sight. Additionally, the procedure is capable of simulating the toroidal and poloidal velocity contributions for a selected spectral line taking into account TJ-II magnetic topology. Results show good agreement between measurements and numerical simulations for the integrated intensity and ion temperature, and a consistent integrated toroidal rotation velocity, which depends on the emissivity profile.

A neoclassical calculation of toroidal rotation profiles and comparison with DIII-D measurements

Momentum and particle balance and neoclassical viscosity were applied to calculate the radial profile of toroidal rotation velocity in several DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharges in a variety of energy confinement regimes (low-mode, low-mode with internal transport barrier, high-mode, and high-mode with quiescentd double barrier). Calculated toroidal rotation velocities generally were found to (over) predict measured values to well within a factor of 2.

Response of toroidal rotation velocity to electron cyclotron wave injection in JT-60U

The spontaneous toroidal rotation velocity under the no/low direct toroidal momentum input, particularly from the electron cyclotron (EC) wave injection has been investigated in JT-60U plasmas. It is found that the response of toroidal rotation velocity to the central EC injection is towards the co-current direction in L-mode plasma. The region of the change in the toroidal rotation velocity is wider than the EC deposition profile and similar to that in electron temperature. The observed co-rotation velocity in the combined heating of EC and lower hybrid wave increases with the increase in the stored energy and a large positive radial electric field is formed in the strong co-rotating plasma. Furthermore the short pulse off-axis EC injection experiment shows that the perturbation of the toroidal rotation velocity towards the co-direction propagates to the centre with increase in its amplitude, suggesting an inward pinch in momentum transport.

Observations of the collapse and recovery of the temperature pedestal using diagnostics with the fast temporal resolution in JT-60U

We investigated the relative change in ion temperature (ΔTi/Ti), electron temperature (ΔTe/Te), pedestal stored energy (ΔWELM/Wped), and toroidal rotation velocity (Vt) due to type-I ELMs using diagnostics with fast temporal resolution in JT-60U. The increase in Vt was found in the direction of the plasma current right after the ELM. The recovery of Vt was found to be faster than that of Wdia, Te and Ti. It is also found that ΔTi/Ti can be larger than ΔTe/Te.

Neoclassical momentum transport in an impure rotating tokamak plasma

It is widely believed that transport barriers in tokamak plasmas are caused by radial electric-field shear, which is governed by angular momentum transport. Turbulence is suppressed in the barrier, and ion thermal transport is comparable to the neoclassical prediction, but experimentally angular momentum transport has remained anomalous. With this motivation, the collisional transport matrix is calculated for a low collisionality plasma with collisional impurity ions. The bulk plasma toroidal rotation velocity is taken to be subsonic, but heavy impurities undergo poloidal redistribution due to the centrifugal force. The impurities give rise to off-diagonal terms in the transport matrix, which cause the plasma to rotate spontaneously. At conventional aspect ratio, poloidal impurity redistribution increases the angular momentum flux by a factor up to ε−3∕2 over previous predictions, making it comparable to the “banana” regime heat flux. The flux is primarily driven by radial pressure and temperature gradients.

Advances in the physics of steady-state plasmas by long pulse experiments on Tore Supra

Long pulse, multi-MW operation on Tore Supra, with active control of the main plasma parameters and new diagnostics, has allowed the investigation of several important issues relevant to the physics of steady-state plasmas, sometimes leading to the discovery of new physics effects. Results obtained in the most recent experimental campaigns are reported. Discharges lasting several resistive times, with combined LHCD and minority ICRH at high power level (>10 MW) in plasmas close to the Greenwald limit, at Te ∼ Ti, were realized. In these discharges, toroidal rotation velocities of the order of 50 km s−1 induced by ICRH were measured by charge exchange spectroscopy. Doppler backscattering was used to complete these measurements and to provide turbulence spectra. MHD phenomena characteristic of steady-state plasmas, such as double tearing modes, were studied using a new 32-channel ECE radiometer and correlation ECE. After the discovery of stationary regimes characterized by non-linearly coupled temperature and current oscillations, a new regime was observed, characterized by spontaneous transitions between two cycles of electron temperature oscillations: low-amplitude (ΔTe/Te ∼ 0.1) and giant (ΔTe/Te > 0.5).

Experimental studies of toroidal momentum transport in ASDEX Upgrade

Characteristics of toroidal plasma rotation have been experimentally investigated using charge exchange recombination spectroscopy in the ASDEX Upgrade tokamak. Ion cyclotron resonance frequency (ICRF) heating is found to cause a reduction of the toroidal rotation velocity, Vϕ, driven by neutral beam injection (NBI) in the co- and counter-current directions. The reduction of plasma rotation is attributed to an increasing momentum diffusivity connected with the confinement degradation by the additional ICRF power flux, and not to an ICRF induced toroidal force related to radial non-ambipolar transport of resonant particles. Toroidal momentum transport is found to be anomalous in various plasma regimes including standard and improved H-modes and ion-internal transport barrier (ITB) plasmas. In the inner half of those plasmas, except for high density H-modes with on-axis NBI only, the momentum diffusivity, χϕ, is found to be similar to the ion and electron heat diffusivities, χi and χe. In the outer half region, χϕ becomes smaller than χi, while χϕ is still comparable with χe except for ITB plasmas. It is found that the normalized gradient length of the toroidal rotation velocity, is smaller than that of the ion temperature, , in H-modes and ITB plasmas. The magnitude of in an ITB region exceeds that in H-modes, as seen for the Ti profile. In H-modes, the Ti profile is stiff (), while the Vϕ profile is not stiff, with ranging from 0 to 7. The Vϕ profile tends to become flat at high densities with on-axis NBI only. Additional ICRF heating can lead to a small decrease in both and , while it sometimes causes a flattening of the Vϕ profile in the inner region. It is shown that the neoclassical correction of Vϕ does not affect strongly the results obtained with the measured Vϕ.

Calibration of the charge exchange recombination spectroscopy diagnostic for core poloidal rotation velocity measurements on JET

This article describes recent improvements in the measurement of C6+ impurity ion poloidal rotation velocities in the core plasma of JET using charge exchange recombination spectroscopy. Two independent techniques are used to provide an accurate line calibration. The first method uses a Perkin–Elmer type 303–306 samarium hollow cathode discharge lamp, with a Sm I line at 528.291 nm close to the C VI line at 529.1 nm. The second method uses the Be II at 527.06 nm and C III at 530.47 nm in the plasma spectrum as two marker lines on either side of the C VI line. Since the viewing chords have both a toroidal and poloidal component, it is important to determine the contribution of the toroidal rotation velocity component separately. The toroidal rotation velocity in the plasma core is measured with an independent charge exchange recombination spectroscopy diagnostic, looking tangentially at the plasma core. The contribution of this velocity along the lines of sight of the poloidal rotation diagnostic has been determined experimentally in L-mode plasmas keeping the poloidal component constant (K. Crombé et al., Proc. 30th EPS Conference, St. Petersburg, Russia, 7–11 July 2003, p. 1.55). The results from these experiments are compared with calculations of the toroidal contribution that take into account the original design parameters of the diagnostic and magnetic geometry of individual shots.

High resolution transmission grating spectrometer for edge toroidal rotation measurements of tokamak plasmas

We present a high throughput (f/3) visible (3500–7000 Å) Doppler spectrometer for toroidal rotation velocity measurements of the Alcator C-Mod tokamak plasma. The spectrometer has a temporal response of 1 ms and a rotation velocity sensitivity of ∼105 cm/s. This diagnostic will have a tangential view and map out the plasma rotation at several locations along the outer half of the minor radius (r/a&amp;gt;0.5). The plasma rotation will be determined from the Doppler shifted wavelengths of Dα and magnetic and electric dipole transitions of highly ionized impurities in the plasma. The fast time resolution and high spectral resolving power are possible due to a 6 in. diam circular transmission grating that is capable of λ/Δλ∼15 500 at 5769 Å in conjunction with a 50 μm slit.

Comparison of toroidal rotation velocities of different impurity ions in the DIII-D tokamak

Measurements of the relative toroidal rotation velocities of low Z impurity ions have been made in DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] plasmas that have a strong core pressure gradient and reduced anomalous transport. They show that the differences in toroidal rotation velocities agree with predictions of neoclassical transport theory. The toroidal rotation velocities are measured via charge exchange recombination (CER) spectroscopy of the impurity species. Taking into account the non-negligible effect of energy dependent charge exchange cross sections, quantitative agreement is found between the measured and predicted difference in toroidal rotation velocities, including the strong Z dependence. The predicted difference between the measured impurity rotation velocity and the main ion rotation velocity can be substantial and therefore care should be taken in using quantitative results from impurity rotation measurements and applying them to the bulk plasma.

A neoclassical model for toroidal rotation and the radial electric field in the edge pedestal

A model for the calculation of toroidal rotation velocities and the radial electric field in the edge pedestal of tokamaks is described. The model is based on particle and momentum balance and the use of the neoclassical gyroviscous expression for the toroidal viscous force. Predicted toroidal rotation velocities in the edge pedestal are found to agree with measured values to within about a factor of 2 or less, for a range of DIII-D [Luxon, Nucl. Fusion 42, 614 (2002)] edge pedestal conditions.