Strong Electron Cyclotron Heating Research Articles

‘BAAE’ instabilities observed without fast ion drive

The instability that was previously identified (Gorelenkov 2009 Phys. Plasmas 16 056107) as a fast-ion driven beta-induced Alfvén-acoustic eigenmode (BAAE) in DIII-D was misidentified. In a dedicated experiment, low frequency modes (LFMs) with characteristic ‘Christmas light’ patterns of brief instability linked to the safety factor evolution occur in plasmas with electron temperature T e ≳ 2.1 keV but modest beta. To isolate the importance of different driving gradients on these modes, the electron cyclotron heating (ECH) power and 80 keV, sub-Alfvénic neutral beams are altered for 50–100 ms durations in reproducible discharges. Although beta-induced Alfvén eigenmodes and reversed-shear Alfvén eigenmodes stabilize when beam injection ceases (as expected for a fast-ion driven instability), the LFMs that were called BAAEs persist. Data mining reveals that characteristic LFM instabilities can occur in discharges with no beam heating but strong ECH. A large database of over 1000 discharges shows that LFMs are only unstable in plasmas with hot electrons but modest overall beta. The experimental LFMs have low frequencies (comparable to diamagnetic drift frequencies) in the plasma frame, occur near the minimum of the safety factor q min, and appear when q min is close to rational values. Theoretical analysis suggests that the LFMs are a low frequency reactive instability of predominately Alfvénic polarization.

Role of density gradient driven trapped electron mode turbulence in the H-mode inner core with electron heating

A series of DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] low torque quiescent H-mode experiments show that density gradient driven trapped electron mode (DGTEM) turbulence dominates the inner core of H-mode plasmas during strong electron cyclotron heating (ECH). Adding 3.4 MW ECH doubles Te/Ti from 0.5 to 1.0, which halves the linear DGTEM critical density gradient, locally reducing density peaking, while transport in all channels displays extreme stiffness in the density gradient. This suggests that fusion α-heating may degrade inner core confinement in H-mode plasmas with moderate density peaking and low collisionality, with equal electron and ion temperatures, key conditions expected in burning plasmas. Gyrokinetic simulations using GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] (and GENE [Jenko et al., Phys. Plasmas 7, 1904 (2000)]) closely match not only particle, energy, and momentum fluxes but also density fluctuation spectra from Doppler backscattering (DBS), with and without ECH. Inner core DBS density fluctuations display discrete frequencies with adjacent toroidal mode numbers, which we identify as DGTEMs. GS2 [Dorland et al., Phys. Rev. Lett. 85, 5579 (2000)] predictions show the DGTEM can be suppressed, to avoid degradation with electron heating, by broadening the current density profile to attain q0>qmin>1.

Application of ECH to the Study of Transport in ITER Baseline Scenario-like Discharges in DIII-D

Recent DIII-D experiments in the ITER Baseline Scenario (IBS) have shown strong increases in fluctuations and correlated reduction of confinement associated with entering the electron-heating-dominated regime with strong electron cyclotron heating (ECH). The addition of 3.2 MW of 110 GHz EC power deposited at � ~0.42 to IBS discharges with ~3 MW of neutral beam injection causes large increases in low-k and medium-k turbulent density fluctuations observed with Doppler backscatter (DBS), beam emission spectroscopy (BES) and phase-contrast imaging (PCI) diagnostics, correlated with decreases in the energy, particle, and momentum confinement times. Power balance calculations show the electron heat diffusivitye increases significantly in the mid-radius region 0.4<� <0.8, which is roughly the same region where the DBS and BES diagnostics show the increases in turbulent density fluctuations. Confinement of angular momentum is also reduced during ECH. Studies with the TGYRO transport solver show that the model of turbulent transport embodied in the TGLF code quantitatively reproduces the measured transport in both the neutral beam (NB)-only and in the NB plus EC cases. A simple model of the decrease in toroidal rotation with EC power is set forth, which exhibits a bifurcation in the rotational state of the discharge.

Progress in the 10-MW 140-GHz ECH System for the Stellarator W7-X

During the last years, electron cyclotron heating (ECH) was proven to be one of the most attractive heating schemes for stellarators because it provides net-current-free plasma startup and heating. Both the stellarator Wendelstein 7-X (W7-X), which is under construction at the Max-Planck-Institut fu umlr Plasmaphysik, Greifswald, Germany, and the International Thermonuclear Experimental Reactor (ITER) tokamak, which will be built in Cadarache, France, will be equipped with a strong ECH and current-drive system. Both systems are comparable in frequency and have continuous-wave capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for ITER). The commissioning of the ECH plant for W7-X is well underway; the status of the project and the first integrated full-power test results from two modules are reported and may provide valuable input for the ITER plant. The ten gyrotrons at W7-X will be arranged in two subgroups symmetrically to a central beam duct in the ECH hall. The RF wave of each subgroup will be combined and transmitted by a purely optical multibeam-waveguide (MBWG) transmission line from the gyrotrons to the torus. The combination of five 1 MW gyrotron beams to one beam line with a power of 5 MW reduces the complexity of the system considerably. The single- and MBWG mirrors and the polarizers have been manufactured. Cold tests of a full-size uncooled prototype line delivered an efficiency exceeding 90%. The microwave power will be launched to the plasma through ten chemical-vapor-deposited-diamond barrier windows and in-vessel quasi-optical plug-in launchers, allowing each 1-MW RF beam to be steered independently. The polarization, as well as the poloidal and toroidal launch angles, will be adjusted individually to provide optimum conditions for different heating and current-drive scenarios.

Overview of TCV results**This paper was submitted as part of the special issue with overview and summary reports based on the 2006 Fusion Energy Conference contributions (Chengdu, China, 16–21 October 2006). To view the special issue go to stacks.iop.org/NF/47/i=10.

This overview highlights the progress accomplished on tokamak à configuration variable (TCV) during the past two years, along five research avenues: particle, energy and momentum transport, edge physics, H-mode physics under strong electron heating, electron cyclotron (EC) heating and electron cyclotron current drive physics, scenarios with internal transport barriers and large non-inductive current fractions. Peaked density profiles are measured in the absence of a Ware pinch or a core particle source. Decreasing the plasma triangularity leads to a significant reduction in χe. Measurements of the plasma toroidal rotation in the absence of external torque are inconsistent with diffusion of toroidal momentum from the edge. Scrape-off layer fluctuation measurements and the relevant modelling using a fluid turbulence code indicate radial interchange motion of plasma filaments as the cause of cross-field transport. Third harmonic EC heating (1.5 MW) applied to ELMy H-modes leads to βN ∼ 2, large ELMs and peaked density profiles, significant ion heating (Ti ∼ 1 keV, with Ti/Te ∼ 0.4) and to quasi-stationary ELM-free H-modes lasting for many energy confinement times. Supra-thermal electrons produced during strong EC heating and following sawtooth crashes are shown to undergo rapid cross-field transport. Electron Bernstein wave heating is demonstrated in the O–X–B conversion scheme. Electron internal transport barriers are generated in a variety of scenarios, leading to significant confinement improvement and bootstrap current fractions in excess of 70%.

Molybdenum transport in high density FTU plasmas with strong radio frequency electron heating

Effects of radio frequency heating on particle confinement have been observed in various tokamaks, where it was found that impurities can be effectively pumped out of the plasma core when sufficient central ion cyclotron (ICH) and/or electron cyclotron heating (ECH) power is applied. It has been proposed that radio frequency heating prevents impurity accumulation by modifying the gradients of temperature and current and hence the turbulence regime responsible for particle anomalous flow. In this paper we present a study of the behavior of molybdenum injected in full lower-hybrid current drive plasmas in the Frascati Tokamak Upgrade with strong ECH. Three similar discharges with increasing level of ECH power are discussed in terms of plasma and power deposition profiles. Strong accumulation of the impurity is observed during off axis heating, while for central ECH deposition the concentration of molybdenum in the core of the discharges decreases at the highest level of ECH power (PECH = 1.2 MW). Microturbulence stability analysis has been carried out for the three plasmas and the relative importance of the off diagonal terms in the transport matrix for the pinch velocity of molybdenum has been studied with reference to FTU results on electron transport and in the framework of recent theoretical results.

Investigation of Electron Distribution Functions in the Plug Region of the GAMMA 10 Tandem Mirror

Generalized scaling laws for the formation of plasma confining potentials are investigated to find the physics essentials common to representative tandem-mirror operational modes in GAMMA 10. These modes are characterized in terms of (i) a high-potential mode and (ii) a hot-ion mode. The potential-formation scalings in these modes are consolidated and generalized on the basis of the consistency with finding of the wider validity of Cohen’s strong electron-cyclotron heating (ECH) theory covering over both modes. A plateau-shaped electron distribution function is observed when a plug electron-confining potential is formed in the hot ion mode of GAMMA 10, as predicted in terms of the strong ECH theory.

Extended consolidation of scaling laws of potential formation and effects covering the representative Tandem mirror operations in GAMMA 10

Scaling laws of potential formation and associated effects along with their physical interpretations are consolidated on the basis of experimental verification using the GAMMA 10 tandem mirror. A proposal of extended consolidation and generalization of the two major theories—(i) Cohen's strong electron cyclotron heating (ECH) theory for the formation physics of plasma confining potentials and (ii) the generalized Pastukhov theory for the effectiveness of the produced potentials on plasma confinement is made through the use of the energy balance equation. This proposal is then followed by verification using experimental data from two representative operational modes of GAMMA 10, characterized in terms of (i) a high-potential mode having plasma confining potentials of the order of kilovolts and (ii) a hot ion mode yielding fusion neutrons with bulk ion temperatures of 10–20 keV. The importance of the validity of the proposed physics-based scaling is highlighted by the possibility of extended capability inherent in Pastukhov's prediction of requiring an ion confining potential of ∼30 kV for a fusion Q value of unity on the basis of an application of Cohen's potential formation method. In addition to the above potential physics scaling, an externally controllable parameter scaling of the potential formation increasing with either plug or barrier ECH powers is summarized. The combination of (i) the physics-based scaling of the proposed consolidation of potential formation and effects with (ii) the externally controllable practical ECH power scaling provides a new direction for future tandem mirror studies.

Spatial resolved high-energy particle diagnostic system using time-of-flight neutral particle analyzer in large helical device

The time-of-flight-type neutral particle analyzer has an ability of horizontal scanning from 40 to 100° of the pitch angle. The information from the spatially resolved energy spectrum gives not only the ion temperature but also the information of the particle confinement and the electric field in plasmas. We have been studying the energy distributions at various magnetic configurations in the neutral beam injection (NBI) plasma. The spatially resolved energy spectra can be observed during long discharges of the NBI plasma by continuous scanning of the neutral particle analyzer. The shape of spectra is almost similar from 44° to 53°. However, the spectra from 55° are strongly varied. They reflect the injection pitch angle of the beam. The pitch angle scanning experiment during the long discharge of NBI plasma has also been made under the reversal of the magnetic field direction. NBI2 becomes counter injected with the reversal. We can easily observe the difference between co- and counter injections of NBI. During the electron cyclotron heating in the low-density plasma for the formation of the internal thermal barrier, large neutral particle increase or decease can be observed. The degree of the increase/decrease depends on the energy and the density. The reason for the variation of the particle flux is that the orbit of the trapped particle changes due to the electric field formed by the strong electron cyclotron heating.

Summarized Scaling Laws of Plasma Confining Potential Formation and Effects in the Gamma 10 Tandem Mirror

Generalization and consolidation of scaling laws of potential formation and associated effects are investigated in the GAMMA 10 tandem mirror. The scaling covers over representative tandem-mirror operational modes, characterized in terms of (i) a high-potential mode having kV-order plasma-confining potentials, and (ii) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures. A novel proposal of extended consolidation and generalization of the two major theories of (i) Cohen's strong electron cyclotron heating (ECH) theory for the formation physics of plasma confining potentials, and (ii) the generalized Pastukhov theory for the effectiveness of the produced potentials on plasma confinement has been made through the use of the energy-balance equation. This proposal is then followed by the verification on the basis of experimental data from the above two representative modes in GAMMA 10. The importance of the validity of this proposed consolidation is highlighted by a possibility of extended capability inherent in Pastukhov's prediction of requiring ion-confining potential (ɸc) of 30 kV for a fusion Q value of unity through an application of Cohen's potential formation method. In addition to the above potential physics scaling, an externally controllable parameter scaling including ECH powers for potential formation is investigated: The construction of the ɸc formation scaling with both plug and barrier ECH is carried out. Data on ɸc. [kV] (or equivalently ɸh with np/nc from the strong ECH theory) as a function of externally controllable plug and barrier ECH powers, (PPECH [kW] and PBECH [kW], respectively), and nc [1018 m−3] are summarized as follows:ɸc= 1.0×10−4(1+5.0×10−3 ×PBECH1.04±0.02) PPECH1.73±0.02 × [c (np/nc)2/3-1]exp [-(0.33±0.05) nc]. Here, c=9-11 and 7-9 for the hot-ion and high-potential modes, respectively. The present paper covers the following updated results: (i) A verification of our novel proposal for potential formation and effects is carried out so as to consolidate two major (Pastukhov's and Cohen's) theories by the use of the “third mode” with the central ECH. The validity of the theory provides a future scenario for combining present representative modes so as to upgrade to hot-ion plasmas with high potentials. (ii) A novel efficient scaling of ɸc formation with both plug and barrier ECH is summarized. The combination of the physics scaling of (i) with the externally controllable practical power scaling of (ii) provides a scalable way for the future tandem-mirror researches.

Dynamics of Tearing Modes during Strong Electron Cyclotron Heating on the FTU Tokamak

The localized electron cyclotron resonance heating power that can suppress sawteeth reconnection often drives m = 2 tearing modes in a tokamak operating at constant current. The dynamics of mode onset and coupled mode evolution is described in detail and compared with a nonlinear theoretical model that identifies the effects of mode coupling, finite inertia of the rotating islands, and wall braking.

Observation of a plateau electron distribution function due to electron cyclotron heating for an efficient plug potential formation.

A plateau-shaped electron distribution function is observed when an electrostatic electron-trapping potential is formed by the electron cyclotron heating (ECH) in the plug region of the GAMMA 10 tandem mirror. Also, a remarkable thermal isolation effect due to a kV-range thermal barrier is observed along with the difference between distribution functions in thermally separated regions. These new findings as well as the relation between ion-confining potentials and thermal-barrier potentials in the kV range consistently support the validity of Cohen's strong ECH theory.

Microwave absorption at the fundamental electron cyclotron resonance and potential formation in a tandem mirror

Strong one-pass absorption of high-power microwaves is observed at the fundamental electron cyclotron resonance in an axisymmetric end mirror of the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985); Phys. Fluids 29, 2781 (1986)]. The radial profile of microwave power deposition is directly measured without the influence of wall reflection. It coincides with that of the axial flow of warm electrons driven by the fundamental heating as well as with that of the plasma potential at the resonance. This indicates that strong electron cyclotron heating largely affects the potential formation through driving an axial flow of warm electrons.

Potentials in thermal barriers with strong electron-cyclotron heating

Expressions for the potential difference Φpb between the plug and barrier regions of a tandem-mirror thermal barrier cell are derived in the limit of strong electron-cyclotron heating. It is found that Φpb varies as the 2/3 power of the ratio of the density of electrostatically trapped plug electrons to the density of thermal electrons at the barrier. Electric-field requirements for validity of the solutions are presented.