Off-axis Electron Cyclotron Heating Research Articles

Investigation of core transport changes in DIII-D discharges with off-axis T e profile peaks

DIII-D discharges that transition to H-mode solely with off-axis electron cyclotron heating (ECH) often exhibit strong off-axis peaking of electron temperature profiles at the heating location. Electron heat transport properties near these off-axis temperature peaks have been studied using modulated ECH. The Fourier analyzed electron temperature data have been used to infer electron thermal diffusivity. Comparisons with numerical solutions of the time-dependent electron thermal equation find that the data are consistent with a narrow region with electron diffusivity χ e an order of magnitude lower than the average value across the plasma, suggesting an electron internal transport barrier (ITB) near the ECH heating location. Detailed profile analysis and equilibrium reconstructions suggest that the formation of these ITBs are correlated with off-axis values of the safety factor q being near 1. Furthermore, the ECH driven H-mode discharges demonstrate more rapid electron heating rate near the ECH deposition location than L-mode discharges with higher auxiliary ECH heating power. Additional modeling attributes this difference to the modification of electron heat transport in the core at the L-H transition, which also sustains the off-axis electron temperature peaks.

Predictive modeling of operation scenarios with negative central magnetic shear and large-radius ITBs on EAST

Negative central magnetic shear or magnetic shear reversal is an attractive aspect for advanced operation scenarios in tokamaks as it suppresses turbulence and facilitates the formation of internal transport barriers (ITBs). The advanced operation scenario with negative magnetic shear, which is associated with high confinement quality, a large bootstrap fraction, ITBs, and so on, is one of the future goals of the experiments on EAST. In this work, modeling efforts have been made to find the operation regimes on EAST with negative central shear and large-radius ITBs. The modeling results indicate that negative central shear and large-radius ITBs can be achieved when the electron density is 〈ne〉/nG > 0.7 and the off-axis deposited electron cyclotron heating (ECH) power is 2 MW. Additionally, scenarios with lower ECH power (∼1.5 MW) have been studied since at present, the maximum ECH power that can be provided on EAST is 1.5 MW. With a lower off-axis ECH power of 1.5 MW, scenarios with negative central shear and large-radius ITBs but with a higher electron density of 〈ne〉/nG > 0.8 are obtained. The same as the first case, if the electron density is reduced to 〈ne〉/nG = 0.8 in this case, large-radius ITBs disappeared in the predicted scenario. This reveals that high electron density and large off-axis deposited ECH power are important for scenarios that have q-profiles with negative central shear and large-radius ITBs since lower hybrid current drive (LHCD) tends to peak near the axis at lower electron density, and hence, it helps to increase the temperature gradient near the axis, which will promote bootstrap in this region, leading to a centrally peaked total current density profile. Higher off-axis deposited ECH power helps to obtain scenarios with strong negative central shear and large-radius ITBs at a lower density. Scenarios predicted in this work will guide future experiments on EAST.

Direct observation of the non-locality of non-diffusive counter-gradient electron thermal transport during the formation of hollow electron-temperature profiles in the Large Helical Device

A heating source with off-axis electron cyclotron heating (ECH) alone produced a plasma with a quasi-steady-state hollow electron-temperature profile in the Large Helical Device. The clear formation of this quasi-steady-state hollow electron-temperature profile can be explained by adding the outward heat convection term to the diffusion term, as a simple model to describe the electron heat flux, using the energy conservation equation. In addition, we directly observed the non-locality of the non-diffusive (convective) contribution in transient electron thermal transport in the condition that power-modulated on-axis ECH was applied to the plasma sustained by off-axis ECH. The experimentally evaluated flux-gradient relation shows two different positive values of the electron heat flux at zero temperature gradient by going back and forth between positive and negative temperature gradient regions in the transport hysteresis phenomenon.

Mitigation of NBI-driven Alfvén eigenmodes by electron cyclotron heating in the TJ-II stellarator

Alfvén eigenmode (AE) activity driven by NBI-produced fast ions is observed in TJ-II plasmas. A two-step response of the measured AEs to electron cyclotron heating (ECH) power is seen. In a first step, the continuous character of the unstable AEs changes to a chirping character of the marginally unstable AEs when moderate values of ECH power are applied to the NBI-only-heated plasma. In a second step, a significant reduction of the AE amplitude is observed when the ECH power is doubled. This stabilizing effect has been experimentally confirmed both on a shot-by-shot basis and along a single discharge by means of ECH modulation. The observed stabilizing effect is stronger with on-axis ECH than with off-axis ECH power injection.

Active control of internal transport barrier formation due to off-axis electron-cyclotron heating in GAMMA 10 experiments

The controlled formation of an internal transport barrier (ITB) is observed in GAMMA 10 [T. Cho et al., Nucl. Fusion 45, 1650 (2005)]. The barrier is localized within a layer of a strongly sheared Er×B plasma rotation (5.5<rc⩽10cm). This high-vorticity layer is formed and maintained by off-axis electron-cyclotron heating, which generates a cylindrical layer (4<rc<7cm) with a high-energy electron population that modifies the initial Gaussian radial potential profile into a nonmonotonic one with a hump structure. The local gradients of Ti and Te are appreciably enhanced in the ITB layer, similarly to those of the ITB in tokamaks and stellarators. Reductions in the effective ion and electron thermal diffusivities are obtained in the barrier layer. A reduction of the observed low-frequency turbulence in the ITB layer and a partial decoupling of the turbulent structures localized on either side of the layer are demonstrated by two-dimensional x-ray diagnostics.

Overview of Recent Progress in the GAMMA 10 Tandem Mirror

(1) Four-time progress in ion-confining potentials Φc to 3.0 kV in comparison to Φc attained 1992-2002 is achieved in the hot-ion mode (Ti=several keV). A scaling of Φc, which favorably increases with plug electron-cyclotron heating (ECH) powers (PPECH), is obtained. (2) The advance in Φc leads to a finding of remarkable effects of radially sheared electric fields (dEr/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in Φc with increasing nc to ˜1019 m-3 with the recovery of Φc with increasing PPECH is obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential Φc, generates a strong Er shear and peaked vorticity with the direction reversal of Err × B sheared flow near the Φc peak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in Te and Ti rises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW, 20 ms) in a standard tandem-mirror operation raises Te0 from 70 to 300 eV together with Ti[perpendicular]0 from 4.5 to 6.1 keV, and Ti//0 from 0.5 to 1.2 keV with τp0=95 ms for Φc (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of τp0/τE0 is observed to be 1.7 for Φc trapped ions (consistent with Pastukhov’s theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (ηICH˜0.3; nlc=4.5×1017 m-2). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (Te=600 eV and Ti=6.6 keV) with azimuthal Er×B sheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation.

Effects of Radially Sheared Electric Field on Turbulence Reduction Analyzed with End-Loss Analysing Systems

Mirror devices, having open-ended regions, provide intrinsic important advantages in terms of the control of radial-potential or Er shear profiles on the basis of axial particle loss control for ambipolar potential formation. From the viewpoint of the mechanism investigation of radially localized transport-barrier formation, an idea of off-axis-resonant electron-cyclotron heating (ECH) is proposed and applied in the tandem mirror GAMMA 10, by the use of the aforementioned intrinsic advantage of mirror devices. The off-axis ECH produces a cylindrical layer with energetic electrons and facilitates the formation for a localized bumped ambipolar potential with a strong shear of radial electric fields Er. Remarkable effects of dEr/dr on the suppression of turbulent fluctuations with plasma confinement improvement are found by the use of signals of end-loss currents flowing from the central cell as well as the central-cell soft x ray. In association with the reduction of the fluctuations due to strong Er shear formation, increases in ion and electron temperatures are found.

Observation and Control of Transverse Energy-Transport Barrier due to the Formation of an Energetic-Electron Layer with ShearedE×BFlow

Off-axis electron-cyclotron heating in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, producing a localized bumped ambipolar potential Phi(C), forms a strong shear of radial electric fields E(r) and peaked vorticity with the direction reversal of E(r)xB sheared flow near the Phi(C) peak. Intermittent vortexlike turbulent structures near the layer are suppressed in the central cell by this actively produced transverse energy-transport barrier; this results in T(e) and T(i) rises surrounded by the layer.

Investigation of electron heat pinch in ASDEX-Upgrade

The existence of an electron heat pinch has been experimentally investigated in ASDEX-Upgrade in plasmas with strong off-axis electron cyclotron heating (ECH). Localization of 1.6 MW of ECH power at ρdep = 0.65 in plasmas with low Ohmic power (<100 kW) still resulted in peaked electron temperature (Te) profiles at densities ne0 ∼ 2.7 × 1019 m−3. Two out of four gyrotrons were modulated in order to study convective effects also on the Te transient response at different frequencies. Peculiar profiles of the modulation amplitude have been measured, very flat in the region just inside ρdep (0.3 < ρtor < 0.65) or even increasing rather than decreasing away from ρdep on the inner side. The phase profiles instead regularly indicate the propagation of the heat wave in both directions away from ρdep. Power balance analysis shows that the heat flux in the core region is very close to zero, but uncertainties do not allow a clear determination of its sign. Modelling steady-state and modulation results with an empirical model featuring a diffusive term characterized by a critical gradient length (LTe = −Te/∇Te) behaviour and a constant Gaussian shaped heat pinch profile allows good reproduction of the data when the region just inside ρdep is close to the critical threshold and oscillates around it, and a small convective term U ∼ 1–2 m s−1 is present in the same region. At higher density the Te profile becomes flatter and the amplitude profiles recover the usual shape with stronger decay inside with respect to outside ρdep. In this case no heat pinch is needed and the plasma goes below the critical threshold inside ρdep. The results are consistent with the theoretical prediction of a turbulence regulated by a threshold in R/LTe and a small turbulence generated electron heat pinch term. However initial attempts to perform first principle-based simulations using the quasi-linear electrostatic drift wave Weiland model have failed to reproduce the experimental results.

Search for a critical electron temperature gradient in DIII-D L-mode discharges

Two experiments on DIII-D have been performed with the purpose of searching for evidence of a critical electron temperature gradient or gradient scale length. Both experiments employed off-axis electron cyclotron heating to vary the local value of inverse temperature scale length while holding the total heating power and thus edge temperatures constant. The incremental or heat pulse thermal diffusivity was monitored as a means of identifying the existence of a threshold or critical value of , kcrit, where a large increase in heat pulse diffusivity may occur while going from below to above kcrit. No evidence of a critical value was found, but the existence of one cannot be ruled out by the experimental results. The results can be used to place an upper bound on kcrit. If kcrit exists, the experimental results indicate kcrit < 3.8 m−1 at ρ = 0.45 and kcrit < 2.5 m−1 at ρ = 0.29 corresponding to temperature scale lengths relative to the plasma minor radius of and 0.65, respectively. Over most of the plasma radius (ρ ⩾ 0.3) the temperature profile in the L-mode discharges studied exceeds these upper bounds on kcrit. A transport model based on a critical gradient is consistent with experimental observations but is not unique. Models other than one based on the existence of a critical gradient are also consistent with experimental observations.

Global Plasma Turbulence Simulations ofq=3Sawtoothlike Events in the RTP Tokamak

A two-fluid computer model of electromagnetic tokamak turbulence, CUTIE, is used to study the dynamic structure and turbulent transport in the Rijnhuizen Tokamak Project tokamak. A discharge with dominant, off-axis electron cyclotron heating is the main focus of the simulations which were extended over several resistive diffusion times. CUTIE reproduces the turbulent transport and MHD phenomena of the experiment. The noninductive components of the current density profile, viz., the dynamo current and the bootstrap current, are identified as key players in the turbulent transport and its suppression and in off-axis MHD events.

Off-axis electron cyclotron heating and the sandpile paradigm for transport in tokamak plasmas

Previous observations that suggest a substantial role for nondiffusive energy transport in tokamaks subjected to off-axis electron cyclotron heating (ECH) are compared to the output from a sandpile model. The observations considered include local and global aspects of temperature profile evolution in the DIII-D [for example, C. C. Petty and T. C. Luce, Nucl. Fusion 34, 121 (1994)] and RTP (Rijnhuizen Tokamak Project) [for example, M. R. de Baar, M. N. A. Beurskens, G. M. D. Hogeweij, and N. J. Lopes Cardozo, Phys. Plasmas 6, 4645 (1999)] tokamaks. The sandpile model employed is an extension, to incorporate noncentral fueling, of one used previously to address tokamak physics issues [S. C. Chapman, R. O. Dendy, and B. Hnat, Phys. Rev. Lett. 86, 2814 (2001)]. It is found that there are significant points of resemblance between the phenomenology of the noncentrally fueled sandpile and of the tokamaks with off-axis ECH. This suggests that the essential ingredient of the sandpile model, namely avalanching conditioned by a local critical gradient, may be one of the key transport effects generated by the tokamak plasma physics.

An overview of results from the TCV tokamak

The Tokamak à Configuration Variable (TCV) tokamak (R = 0.88 m, a < 0.25 m, B < 1.54 T) programme is based on flexible plasma shaping and heating for studies of confinement, transport, control and power exhaust. Recent advances in fully sustained off-axis electron cyclotron current drive (ECCD) scenarios have allowed the creation of plasmas with high bootstrap fraction, steady-state reversed central shear and an electron internal transport barrier. High elongation plasmas, κ = 2.5, are produced at low normalized current using far off-axis electron cyclotron heating and ECCD to broaden the current profile. Third harmonic heating is used to heat the plasma centre where the second harmonic is in cut-off. Both second and third harmonic heating are used to heat H-mode plasmas, at the edge and centre, respectively. The ELM frequency is decreased by the additional power. In separate experiments, the ELM frequency can be affected by locking to an external perturbation current in the internal coils of TCV. Spatially resolved current profiles are measured at the inner and outer divertor targets by Langmuir probe arrays during ELMs. The strong, reasonably balanced currents are thought to be thermoelectric in origin.

ECH physics and new operational regimes on TCV

The physics of tokamak plasmas, in which electrons are heated by electron cyclotron heating (ECH) and whose current is driven by electron cyclotron current drive (ECCD), is investigated in this paper together with applications on tokamak à configuration variable (TCV) using modifications of the pressure and current profiles to improve the operational regimes. In order to explain the experimentally determined current drive efficiency and hard x-ray and electron cyclotron emission measurements, it is shown that quasi-linear effects and radial transport of the suprathermal electrons are necessary. Plasmas with fully non-inductively driven currents were obtained with 0.9 MW of off-axis ECCD and 0.45 MW of on-axis counter ECCD. The combination of the driven current and the bootstrap current, accounting for 50% of the total current and peaking off-axis, yields a reversed safety factor profile and a wide and stable electron internal transport barrier. This barrier leads to an enhancement in the energy confinement by a factor of 4.5. ECH is also used to broaden the current profile of high elongation, low normalized-current plasmas whose vertical position would otherwise be uncontrollable on TCV, but whose MHD stability properties should allow high β values. An elongation of 2.47 at a normalized-current of 1.05 MA mT−1 is obtained with off-axis ECH absorbed at an optimized normalized radius between 0.55 and 0.7. Finally, third harmonic ECH is tested in various scenarios, all using vertical beam launching. In particular, high density Ohmic target and preheating with second harmonic ECH are presented. The fraction of third harmonic power absorbed reaches 65% and 85%, respectively.

Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à configuration variable (TCV)

Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak à configuration variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi-stationary core confinement regime, with little or no sawtooth activity, has been obtained by a combination of off-axis ECH and on-axis CNTR–ECCD.

Long-pulse improved central electron confinement in the TCV tokamak with electron cyclotron heating and current drive.

Current profile tailoring by electron cyclotron heating (ECH) and current drive (ECCD) is used to improve central electron energy confinement in the TCV tokamak. Counter-ECCD on axis alone achieves this goal in a transient manner only. A stable scenario is obtained by a two-step sequence of off-axis ECH, which stabilizes magnetohydrodynamics modes, and on-axis counter-ECCD, which generates a flat or inverted current profile. This high-confinement regime, with central temperatures up to 9 keV (at a normalized beta(N) approximately 0.6), has been sustained for the entire duration of the heating pulse, or over 200 electron energy confinement times and 5 current redistribution times.

Internal kink instability during off-axis electron cyclotron current drive in the DIII-D tokamak

Experimental evidence is reported of an internal kink instability driven by a new mechanism: barely trapped suprathermal electrons produced by off-axis electron cyclotron heating on the DIII-D tokamak. It occurs in plasmas with an evolving safety factor profile q(r) when q(min) approaches 1. This instability is most active when ECCD is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode number = m/n = 1/1. In positive magnetic shear plasmas, this mode becomes the fishbone instability. This observation can be qualitatively explained by the drift reversal of the barely trapped suprathermal electrons.

Bifurcated States of Ohmically Heated Tokamak Plasmas

Experiments in the Rijnhuizen Tokamak Project are reported which show bifurcated states of plasmas in the linear Ohmic confinement regime. One branch is the usual Ohmic state. The second, newly discovered, state features a much broader temperature profile, while the density has a very pronounced gradient near the half radius. The bifurcated states form after off-axis electron cyclotron heating is switched off. The new state contradicts theoretical models which predict improved confinement for peaked density profiles or high collisionality.

Electron Thermal Transport Barrier and Magnetohydrodynamic Activity Observed in Tokamak Plasmas with Negative Central Shear

In the Rijnhuizen Tokamak Project, plasmas with steady-state negative central shear (NCS) are made with off-axis electron cyclotron heating. Shifting the power deposition by 2 mm results in a sharp transition of confinement. The good confinement branch features a transport barrier at the off-axis minimum of the safety factor (q), where q less than or equal to 3, and two magnetohydrodynamic (MHD) instabilities, where one is localized at the off-axis minimum of q and one covers the entire NCS region. The low confinement branch has q > 3 everywhere, no transport barrier, and no MHD activity.

Steady-state hollow electron temperature profiles in the Rijnhuizen Tokamak Project.

In the Rijnhuizen Tokamak Project steady-state hollow electron temperature ( ${T}_{e}$) profiles have been sustained with strong off-axis electron cyclotron heating, creating a region of reversed magnetic shear. In this region the effective electron thermal diffusivity ( ${\ensuremath{\chi}}_{e}^{\mathrm{pb}}$) is close to neoclassical in high density plasmas. For medium density, ${\ensuremath{\chi}}_{e}^{\mathrm{pb}}$ is lower than neoclassical and may even be negative, indicating that off-diagonal elements in the transport matrix drive an electron heat flux up the ${T}_{e}$ gradient.