Coherent Magnetic Fluctuations Research Articles

MHD-blob correlations in NSTX

This paper describes a study of the cross-correlations between edge fluctuations as seen in the gas puff imaging (GPI) diagnostic and low frequency coherent magnetic fluctuations (MHD) in H-mode plasmas in NSTX. The main new result was that large blobs in the scrape-off-layer were significantly correlated with MHD activity in the 3–6 kHz range in 21 of the 223 shots examined. There were also many other shots in which fluctuations in the GPI signal level and its peak radius Rpeak were correlated with MHD activity but without any significant correlation of the MHD with large blobs. The structure and motion of the MHD are compared with those of the correlated blobs, and some possible theoretical mechanisms for the MHD-blob correlation are discussed.

Observation of MHD Instabilities Driven by Energetic Electrons in the Large Helical Device**supported by the Grant-in-Aid for Encouragement of Scientists from the Japan Society for the Promotion of Science (No. 20656150). This work was also partly supported by the JSPS-NRF-NSFC A3 Foresight Program in the field of Plasma Physics (NSFC: No. 11261140328 and NRF: No. 2012K2A2A6000443)

Coherent magnetic fluctuations in an acoustic range of frequency have been regularly observed in low-density (ne < 0.2×1019 m−3) plasmas with strong second harmonic electron cyclotron resonance heating (ECRH) on the Large Helical Device. Hard X-ray measurements indicated that energetic electrons are generated in these ECRH discharges. The magnetic fluctuations are suppressed in higher density discharges where energetic electrons are not present. The ECRH power modulation experiment indicated that the observed magnetohydrodynamic (MHD) mode has an acoustic nature rather than an Alfvénic nature.

Characterization of coherent magnetic fluctuations in JFT-2M high recycling steady high-confinement mode plasmas

The most important feature of the “high recycling steady” (HRS) high-confinement mode (H-mode) regime in the JFT-2M tokamak [H. Ninomiya et al., Fusion Science and Technology (2002), Vol. 42, p. 7] is the stationary pedestal condition in the absence of the large edge localized modes (ELMs). Accompanying the HRS H-mode transition, the coherent magnetic fluctuations in the frequency range of the order of 10–100kHz with significant variation are seen on the magnetic probes at the vessel wall. Above all, two types of edge magnetohydrodynamics (MHD) activities, which have associated toroidal mode number of n=1 and n∼7, respectively, seem to be more important for the HRS H-mode plasmas. To investigate their interaction, bispectral analysis is applied for the magnetic probe data, exhibiting the phase-coupled oscillations between two types of edge MHD activities. Parameter similarities and differences with small ELMs and stationary ELM-free H-mode regimes in other devices are also discussed.

Pedestal characteristics in JFT-2M HRS H-mode plasma

Recent experiments in the JFT-2M tokamak have concentrated on studies of the access conditions for H-mode operation without any large ELMs, namely the ‘high recycling steady’ (HRS) H-mode regime, in terms of the pedestal parameters. The HRS regime is more likely to occur at higher edge density and lower edge temperature, while the ELMy H-mode having large ELMs appears at lower edge density and higher edge temperature. It is found that the ELMy/HRS operational boundary occurs at the normalized electron collisionality of in the plasma edge region, depending slightly on the safety factor at 95% flux surface q95. A key feature of the HRS H-mode is the presence of coherent magnetic fluctuations in the frequency range of the order of 10–100 kHz. It is suggested that the edge MHD activities may keep the edge pressure below a certain level needed to induce a large ELM.

High recycling steady H-mode regime in the JFT-2M tokamak

A new operating regime, ‘high recycling steady’ (HRS) H-mode regime, has been discovered in the JFT-2M tokamak. It is easily reproduced with neutral beam heating of any co-, balanced-, or counter-injection under wall fuelling from the boronized first wall. The HRS is characterized by good energy confinement (H89P ∼ 1.6) at a high density (ne/nGW ∼ 0.7; nGW is the Greenwald density), low radiation loss power fraction (, typically), and the complete disappearance of large ELMs. Accompanying the HRS H-mode transition, the coherent magnetic fluctuations in the frequency range of the order of 10–100 kHz with significant variation are seen on magnetic probes at the vessel wall. In addition, the HRS H-mode edge condition is compatible with an improved core confinement. We have demonstrated that an internal transport barrier can be produced under a HRS H-mode edge condition, achieving βNH89P ∼ 6 at ne/nGW ∼ 0.7–1.0, transiently.

High frequency MHD activity related to type I ELMs in ASDEX Upgrade

High frequency coherent magnetic fluctuations related to the type I edge localized mode (ELM) cycle have been found in ASDEX Upgrade. Typical frequencies are in the range 300–500 kHz, and the poloidal mode number, m, measured at the outboard midplane, is estimated to be 10–15. These MHD modes are present in many low density (low collisionality) discharges, while they have not been found in high density ones yet. New findings on already known low frequency type I ELM precursors confirm that in ASDEX Upgrade type I ELM mode precursors are normally phase locked to the wall in discharges with NBI heating parallel to and in the direction of the plasma current (co-injection) and can be seen with a non-zero rotation frequency only in shots with anti-parallel NBI (counter-injection). Additional information is given by ECE data. The existence of low and high frequency MHD events suggests an interpretation in the framework of the peeling–ballooning model. The high frequency magnetic activity seems to be related strictly to the increase in (and eventual saturation of) edge electron temperature, while low frequency modes are related directly to the ELM crash itself.

MHD instabilities and their effects on plasma confinement in Large Helical Device plasmas

Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device. Spontaneous L–H transition is often observed in high beta plasmas close to 2% at low toroidal fields (Bt ⩽ 0.75 T). The stored energy starts to rise rapidly just after the transition accompanying the clear rise in the electron density but quickly saturates due to the growth of the m = 2/n = 3 mode (m and n: poloidal and toroidal mode numbers), the rational surface of which is located in the edge barrier region, and edge localized mode (ELM) like activities having fairly small amplitude but high repetition frequency. Even in low beta plasmas without L–H transitions, ELM-like activities are sometimes induced in high performance plasmas with a steep edge pressure gradient and transiently reduce the stored energy up to 10%. Energetic ion driven MHD modes such as Alfvén eigenmodes (AEs) are studied in a very wide range of characteristic parameters (the averaged beta of energetic ions, ⟨βb∥⟩, and the ratio of energetic ion velocity to the Alfvén velocity, Vb∥/VA), of which range includes all tokamak data. In addition to the observation of toroidicity induced AEs (TAEs), coherent magnetic fluctuations of helicity induced AEs (HAEs) have been detected for the first time in NBI heated plasmas. The transition of a core-localized TAE to a global AE (GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon observed in a reversed shear tokamak. At low magnetic fields, bursting TAEs transiently induce a significant loss of energetic ions, up to 40% of injected beams, but on the other hand play an important role in triggering the formation of transport barriers in the core and edge regions.

Investigation of fast particle driven modes on Alcator C-Mod

High-frequency coherent magnetic fluctuations have been observed with hydrogen minority ion cyclotron resonant frequency (ICRF) heating in Alcator C-Mod in low- or reversed-shear current rise conditions. The mode frequencies (150-450 kHz) are in the range expected for toroidal Alfvén eigenmodes (TAEs) with central q values greater than unity. These modes have low n3 and appear to rotate in the ion diamagnetic drift direction. Under these relatively low-density current rise conditions with low or reversed magnetic shear, TAEs can be destabilized by a predicted fast hydrogen ion tail of just over 100 keV. Calculations with the CASTOR code indicate that a broad eigenmode can exist with a calculated frequency at the measured mode frequency, which falls in a clear TAE gap across the profile. The frequency of the current rise modes can increase or decrease rapidly from 150 to 400 kHz in less than 15 ms, which cannot be explained by linear theory. In some cases, only a single mode is excited with a linearly increasing frequency, in contrast to the multiple modes with decreasing frequency usually observed on other devices.

ELMs and fast edge fluctuations in Alcator C-Mod

Just above the H-mode threshold power at low density, small amplitude high repetition rate (1-5 kHz) type III edge-localized modes (ELMs) are observed. At higher input power, the H-modes become ELM-free. At high density and high input power (, MW), the plasma often enters a new H-mode regime now called the enhanced H-mode, which is a steady-state H-mode often without distinct ELMs but with high levels of emission. Enhanced H-modes are accompanied by persistent high-frequency broadband (50-400 kHz) density and magnetic fluctuations. Above about , the enhanced H-modes may also exhibit bursting or continuous low m, n coherent magnetic fluctuations with frequencies from 60-90 kHz. In the H-mode, steep edge gradients are observed with a nine-channel electron cyclotron emission (ECE) polychromator and a 38-channel, high-resolution, edge soft-x-ray array. The soft-x-ray pedestal width remains around 3 to 4 mm in the ELM-free H-mode and between type III ELMs. The pedestal collapses within during an ELM. Calculations indicate that the plasma in the H-mode is often near ideal ballooning first stability limits, but no type I ELMs have yet been identified. Edge micro-tearing modes destabilized by a steep temperature gradient are also considered as a possible explanation of the observed fluctuations during the enhanced H-mode.

Measurements of the high confinement mode pedestal region on Alcator C-Mod

Measurements of the steep transport barrier at the edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The parameters at the top of this barrier are in the range Te=150–750 eV and ne=0.5−3.3×1020 m−3, depending on the confinement regime. Type III edge localized modes (ELMs) have an upper temperature limit. Te pedestal profiles show a barrier width ΔT≃8 mm. Soft x-ray emissivity profiles are narrower, with Δ=2–4 mm. Edge currents are calculated to alter the ideal stability boundary favorably, leading to ideally stable pedestal profiles. High frequency, broadband, edge density fluctuations are sometimes observed in H-mode (high-confinement mode) and are associated with enhanced particle transport. Coherent magnetic fluctuations localized near the pedestal are also seen.

Determination of the structure of magnetic islands on TFTR (invited)

The Mirnov coil array, the horizontal soft x-ray camera, and the ECE (electron cyclotron emission) grating polychromator are used to unfold the structure of m≥2 coherent magnetic fluctuations (assumed to be tearing modes) observed in TFTR. The modes are found predominantly in the enhanced confinement, or supershot, regime, and when present seriously degrade confinement. The poloidal and toroidal mode numbers (m and n) are determined with the Mirnov coil array. The radial structure is found by calculating the theoretical radial eigenmode (based on the calculated current-density profile) and scaling the resulting linear eigenmode with the measured fluctuation amplitude at the plasma edge. The presence of rotating magnetic islands is detected as fluctuations on both the chord-averaged soft-x-ray emissivity measurements and the fast grating polychromator electron temperature measurements. The ECE and soft-x-ray systems are used to identify coherent modes as islands and confirm estimates of the islands’ location inferred from the Mirnov coil data and calculated current-density profiles.