Long-lasting core coherent Modes in TJ-II stellarator plasmas

Intermittent toroidal asymmetries in the plasma edge emissivity have been detected in the TJ-II stellarator. During the toroidal radiation asymmetry phase (TRAP), sharp up and down jumps in electron cyclotron emission and total radiation are observed. The time resolved soft x-ray spectra also show changes in the suprathermal electron population. Given that the asymmetries reveal a variation in the wall desorption near the gyrotron port, TRAP is interpreted as a sudden increase of electron cyclotron resonance heating (ECRH) driven convective flux of ripple-trapped suprathermal electrons. This effect occurs in most of the magnetic configurations explored to date and is strongly affected by the presence of low order rational surfaces. A first assessment of the implications of TRAPs for confinement in ECRH plasmas of the TJ-II device is presented.

Plasma Physics Reports, 2008

Results are presented from experimental studies of variations in the plasma parameters during the excitation of a multiaxis magnetic configuration by the induction current (up to 17 kA) in the basic magnetic configuration of the L-2M stellarator in the regime of ECR heating at a microwave power of ~200 kW (~1 MW m-3) and an average plasma density of (1-2) × 10 19 m-3. The current direction was chosen to reduce the net rotational transform (the so-called "negative" current). The current was high enough for the rotational transform to change its sign inside the plasma column. Computer simulations of the L-2M magnetic structure showed that the surface with a zero rotational transform is topologically unstable and gives rise to magnetic islands, i.e., to a multiaxis magnetic configuration. Magnetic measurements showed that, at negative currents above 10 kA, intense bursts of MHD oscillations with a clearly defined toroidal mode number n = 0 were observed in the frequency range of several kilohertz. Unfortunately, the experimental data are insufficient to draw the final conclusion on the transverse structure of these oscillations. The radial temperature profiles along the stellarator major radius in the equatorial plane were studied. It is found that the electron temperature decreases by a factor of 1.3 in the plasma core (r / a ≤ 0.6) and that the temperature jump is retained near the boundary. A change in turbulent fluctuations of the plasma density during the excitation of a negative current was studied using wave scattering diagnostics. It is found that the probability density function of the increments of fluctuations in the plasma core differs from a Gaussian distribution. The measured distribution is heavy-tailed and broadens in the presence of the current. It is found that the spectrum of turbulent fluctuations and their Doppler shift near the plasma boundary are nonuniform in the radial direction. This may be attributed to the shear of the poloidal velocity. The experimental results indicate that the formation of regions with a zero rotational transform in the plasma core somewhat intensifies plasma transport.

Physics of Plasmas, 1996

A bst rad A band of high frequency modes in the range 50-150 ldb with intermediate toroidal mode numbers 40140 are commonly observed in the core of supershot plasmas on "FIR. Two distinct varieties of MHD modes are identified conesponding to a flute-like mode predominantly appearing around the q = 1 surface and an outward ballooning mode for 4 > 1. The flute-like modes have nearly equal amplitude on the high field and low field side of the magnetic axis and are mostly observed in moderate performance supershot plasmas with z , < 2zL, while the ballooning-like modes have enhanced amplitude on the low field side of the magnetic axis and tend to appear in higher performance supershot plasmas with z , > 22,, where z , is the equivalent L-mode confinement time. The modes propagate in the ion diamagnetic drift direction and are highly localized with radial widths Ar-5-10 cm , fluctuation levels E / n, f, / T, < 0.01, and radial displacements 5,-0.1 cm. Unlike the toroidally localized high-n activity observed just prior to major and minor disruptions on TFI'R [ 11, these modes are typically much weaker more benign, and may be indicative of kinetic ballooning modes destabilized by resonant circulating neutral beam ions. fluctuation levels fi / n, fe / T, c 1%. Two distinct varieties are identified corresponding to a flute-like mode predominantly appearing around the q = 1 radius and an outward ballooning mode appearing near the peak of the pressure gradient in the region 1 < 1.5. The flute-like modes are predominantly observed in moderate performance supershots plasmas with z , c 2zL, while the ballooning modes are mainly observed in higher performance discharges with z , > 2zL. Although these two modes appear with similar frequencies and mode numbers, they are rarely observed in the same discharge. Unlike the NOTICE Page(s) size did not permit electronic reproduction. Information may be purchased by the general public from the

Nuclear Fusion, 2000

ELM-like activity has been recently observed in TJ-II, in plasmas with stored energy above 1 kJ. The plasma is observed to develop bursts of magnetic activity (seen in Mirnov coil signals) which are followed by a large and distinct spike in the Hα signal. An increase in electrostatic and magnetic fluctuations at the plasma edge and a cold pulse towards the plasma centre are also characteristics of these events. In addition, the electron temperature profile locally flattens at the plasma radius where the temperature is in the range 100-200 eV. This flattening can be explained in terms of enhanced electron heat conductivity. Between ELM-like events the electromagnetic turbulence at the edge decreases and the Te profiles recover their former shapes. This activity is probably triggered by a resonant m = 2, n = 3 mode.

Introduction. The measurement of toroidal rotation has become an important goal in fusion plasmas for several reasons. First, it enters into the basic equation for deriving the radial electric field when using spectroscopic techniques. Second, there exists evidence that rotation, stability and confinement are not independent. Third, moment confinement is an interesting topic, in particular when external sources of momentum, such as neutral beam injection are involved.

Nuclear Fusion, 2011

High frequency modes (150-300 kHz) are found in several magnetic configurations of TJ-II plasmas heated by Neutral Beam Injection (NBI). The clear dependence of mode frequency on plasma density and mass species suggests them to be Alfvén Eigenmodes. The appearance of these modes is linked to the presence of low order rational surfaces close to the rotational transform profile. They can exhibit steady or chirping behaviour depending on the plasma profiles. NBI plasmas with broader temperature profiles show frequency chirping meanwhile it is not-or rarelyobserved for relatively peaked ones. The Alfvénic activity has been characterized in detail with magnetic coils for the standard configuration. Cross analyses with HIBP and reflectometer signals have yielded spatial resolution and radial profiles of the perturbation. Correlation of magnetic coil signals with the ones from diagnostics sensitive to edge ion losses, as Langmuir probes and a Fast Ion Loss Detector, has been observed in some cases and characterized taking advantage of the chirping nature of the observed Alfvénic activity.

Plasma Physics and Controlled Fusion, 2006

The first toroidal rotation measurements in TCV ohmic L-mode plasmas with no external momentum injection are presented. The toroidal velocity profile of the fully stripped carbon species is measured by active Charge eXchange Recombination Spectroscopy with a temporal resolution of typically 90 ms and a spatial resolution of 2.5 cm, about 1/10 of the plasma radius. The observed carbon velocity is of the order of the deuterium diamagnetic drift velocity and up to 1/5 of the deuterium thermal velocity. It is directed opposite to plasma current in the electron diamagnetic toroidal drift direction. The profile reverses when reversing the plasma current. The angular velocity profile is flat, or hollow, inside the sawtooth inversion radius and decreases quasi linearly towards the plasma edge. By vertically shifting the plasma magnetic axis within the TCV vessel the plasma edge velocity profile was measured with high resolution. Such experiments confirm that, close to the limiter, the stationary rotation velocity is close to zero or somewhat positive (co-current directed). This suggests that the angular momentum is not driven from the plasma edge. The maximum carbon velocity scales as v φ,Max [km s −1 ] = −12.5T i /I p [eV/kA] for a significant range of densities and values of the edge safety factor. Comparison with neoclassical predictions show that the TCV plasma rotation is mainly driven by radial electric fields, with a negligible contribution from the toroidal electric fields. The neoclassical theory of small toroidal rotation quantitatively and qualitatively disagrees with the experimental observation. An alternative empirical equation for the angular momentum flux, able to reproduce the measured stationary profile outside the inversion radius, is proposed.

Nuclear Fusion, 2011

This paper reports the results of recent experiments performed on the JET tokamak on Alfvén Eigenmodes (AEs) with toroidal mode number (n) in the range n=3-15. The stability properties and the use of these medium-n AEs for diagnostic purposes is investigated experimentally using a new set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate and amplitude for each individual toroidal mode number. First, we report on the development of a new algorithm for mode detection and discrimination using the Sparse Signal Representation theory. The speed and accuracy of this algorithm has made it possible to use it in our plant control software, allowing real-time tracking of individual modes during the evolution of the plasma background on a 1ms time scale. Second, we report the first quantitative analysis of the measurements of the damping rate for stable n=3 and n=7 Toroidal AEs as function of the plasma elongation. The damping rate for these modes increases for increasing elongation, as previously found in JET for n=0-2 AEs. A theoretical analysis of these JET data has been performed with the LEMan, CASTOR and TAEFL codes. The LEMan and TAEFL results are in good agreement with the measurements for all magnetic configurations where there is only a minor up/down asymmetry in the plasma poloidal cross-section. The CASTOR results indicate that continuum damping is not the only mechanism affecting the stability of these medium-n AEs. The diagnostic potential of these modes has being confirmed during the recent gas change-over experiment, where independent measurements of the effective plasma isotope ratio A EFF have been provided in addition to the more routinely employed spectroscopic and gas-balance ones. These data shows a slight difference in the measurement of A EFF when using n<5 and n>7 modes, suggesting a radial dependence in the effective plasma isotope ratio. * Appendix of F.Romanelli, 23 rd IAEA Fusion Energy Conference, paper OV1/3, this conference.