H-mode Barrier Research Articles

Investigation of the role of fishbone activity in the formation of internal transport barrier in HL-2A plasma

A tokamak scenario with q(0) close to 1 has been achieved on HL-2A tokamak, which is an internal transport barrier (ITB) at low central shear or a steady-state ITB combined with an H-mode edge barrier. In this scenario, the formation of an ITB with a steep ion temperature profile is observed to be closely linked to the q = 1 magnetic surface and magnetohydrodynamic (MHD) activities around it, such as long-lived mode (LLM) or fishbone activities. Experimental evidence and simulation analysis suggest that the fishbone activities can induce a poloidal flow, which is beneficial for the suppression of turbulence in the plasma core region. Furthermore, an application of central electron cyclotron resonance heating (ECRH) to such beam-heated weak shear ITB discharges leads to a substantial effect on central MHD stability, converting the LLM into fishbone activity and hence enhancing the strength of the ITB with a much stronger gradient. Moreover, ITBs in combination with an H-mode barrier were achieved for 10 confinement times with βN = 2.0.

The role of zonal flows in reactive fluid closures

We will give an overview of results obtained by our reactive fluid model. It is characterised as a fluid model where all moments with sources in the experiment are kept. Furthermore, full account is taken for the highest moments appearing in unexpanded denominators also including full toroidicity. It has been demonstrated that the strength of zonal flows is dramatically larger in reactive fluid closures than in those which involve dissipation. This gives a direct connection between the fluid closure and the level of excitation of turbulence. This is because zonal flows are needed to absorb the inverse cascade in quasi 2D turbulence. This also explains the similarity in structure of the transport coefficients in our model with a reactive closure in the energy equation and models which have a reactive closure because of zero ion temperature such as the Hasegawa–Wakatani model. Our exact reactive closure unifies several well-known features of tokamak experiments such as the L–H transition, internal transport barriers and the nonlinear Dimits upshift of the critical gradient for onset of transport. It also gives transport of the same level as that in nonlinear gyrokinetic codes. Since these include the kinetic resonance this confirms the validity of the thermodynamic properties of our model. Furthermore, we can show that while a strongly nonlinear model is needed in kinetic theory a quasilinear model is sufficient in the fluid description. Thus our quasilinear fluid model will be adequate for treating all relevant problems in bulk transport. This is finally confirmed by the reproduction by the model of the experimental power scaling of the confinement time τE ∼ P−2/3. This confirms the validity of our reactive fluid model. This also gives credibility to our ITER simulations including the H-mode barrier. A new result is here, that alpha heating strongly reduces the slope of the H-mode barrier. This should significantly reduce the effects of ELM’s.

Multi-fluid transport code modeling of time-dependent recycling in ELMy H-mode

Simulations of a high-confinement-mode (H-mode) tokamak discharge with infrequent giant type-I ELMs are performed by the multi-fluid, multi-species, two-dimensional transport code UEDGE-MB, which incorporates the Macro-Blob approach for intermittent non-diffusive transport due to filamentary coherent structures observed during the Edge Localized Modes (ELMs) and simple time-dependent multi-parametric models for cross-field plasma transport coefficients and working gas inventory in material surfaces. Temporal evolutions of pedestal plasma profiles, divertor recycling, and wall inventory in a sequence of ELMs are studied and compared to the experimental time-dependent data. Short- and long-time-scale variations of the pedestal and divertor plasmas where the ELM is described as a sequence of macro-blobs are discussed. It is shown that the ELM recovery includes the phase of relatively dense and cold post-ELM divertor plasma evolving on a several ms scale, which is set by the transport properties of H-mode barrier. The global gas balance in the discharge is also analyzed. The calculated rates of working gas deposition during each ELM and wall outgassing between ELMs are compared to the ELM particle losses from the pedestal and neutral-beam-injection fueling rate, correspondingly. A sensitivity study of the pedestal and divertor plasmas to model assumptions for gas deposition and release on material surfaces is presented. The performed simulations show that the dynamics of pedestal particle inventory is dominated by the transient intense gas deposition into the wall during each ELM followed by continuous gas release between ELMs at roughly a constant rate.

ELM flushing and impurity transport in the H-mode edge barrier in ASDEX Upgrade

Density profiles of He2+, C6+, Ne10+ and Ar16+ are measured at ASDEX Upgrade with high temporal (1.9ms) and radial resolution (>3mm) using CXRS. This allows the transport coefficients at the edge transport barrier to be analyzed in between type-I ELMs during H-mode. All measurements resemble the neoclassical transport coefficients, which means that the diffusion coefficient ranges from 0.15 to 0.35m2/s and the maximum of the inward convection velocity is Z-dependent increasing from 15m/s (He2+) to 90m/s (Ar16+). These results are combined into a simplified transport-erosion model for W which calculates the neoclassical transport coefficients self-consistently while taking several impurity species into account. The model also calculates erosion fluxes at the wall, prompt re-deposition of W and loss terms due to parallel flows in the scrape-off layer. While several free parameters exist in the model, it demonstrates that a quantitative consistency between the measured erosion fluxes of W and the observed W concentration inside the plasma is obtainable.

Nonlinear condensation modes

AbstractIn the far edge, in L-mode plasmas and the outer parts of the H-mode barrier, tokamak plasmas may be so collisional that parallel electron motion can be neglected or included as a perturbation. Thus we have a regime that is basically two dimensional. We will here consider the outer part of the H-mode barrier where transport is small. Recently a condensation mode was found for this regime. We have extended the theory to the nonlinear regime.

Compatibility of ITER scenarios with full tungsten wall in ASDEX Upgrade

The transition of ASDEX Upgrade (AUG) from a graphite device to a full tungsten device is demonstrated with a reduction by an order of magnitude in both the carbon deposition and deuterium retention. The tungsten source is dominated by sputtering from intrinsic light impurities, and the tungsten influxes from the outboard limiters are the main source for the plasma. In H-mode discharges, central heating (neutral beams, ECRH) is used to increase turbulent outward transport avoiding tungsten accumulation. ICRH can only be used after boronization as its application otherwise results in large W influxes due to light impurities accelerated by electrical fields at the ICRH antennas. ELMs are important in reducing the inward transport of tungsten in the H-mode edge barrier and are controlled by gas puffing. Even without boronization, stationary, ITER baseline H-modes (confinement enhancement factor from ITER 98(y, 2) scaling H98 ∼ 1, normalized beta βN ∼ 2), with W concentrations below 3 × 10−5 were routinely achieved up to 1.2 MA plasma current.The compatibility of high performance improved H-modes with unboronized W wall was demonstrated, achieving H98 = 1.1 and βN up to 2.6 at modest triangularities δ ⩽ 0.3 as required for advanced scenarios in ITER. With boronization the light impurities and the radiated power fraction especially in the divertor were reduced and the divertor plasma was actively cooled by N2 seeding. N2 seeding does not only protect the divertor tiles but also considerably improves the performance of improved H-mode discharges. The energy confinement increased to H98-factors of 1.25 (βN ∼ 2.7) and thereby exceeded the best values in a carbon-dominated AUG machine under similar conditions. Recent investigations show that this improvement is due to higher temperatures rather than to peaking of the electron density profile.Further ITER discharge scenario tests include the demonstration of ECRF assisted low voltage plasma start-up and current rise to q95 = 3 at toroidal electric fields below 0.3 V m−1, to achieve a ITER compatible range of plasma internal inductance of 0.71–0.97. The results reported here strongly support tungsten as a first wall material solution.

Transport barrier fluctuations governed by SOL turbulence spreading

Turbulence spreading, namely turbulent transport extending into a stable region is reported both for the flat density profiles in the far SOL and into a modeled H-mode barrier. It is shown that due to turbulence penetration, the pedestal width fluctuates and that its effective width is a factor 2 smaller than the linear predicted width. Turbulence overshooting throughout the pedestal leads to a non-vanishing turbulent transport within the barrier and provides a coupling of core and SOL turbulence despite the transport barrier.

Control of plasma profiles in DIII-D discharges

Active control of plasma profiles is an essential requirement for operating within plasma stability limits, for steady-state operation and for optimization of the plasma performance. In DIII-D, plasma profiles have been actively controlled using various actuators in the following manner: (a) real time closed loop control of the q profile evolution using electron cyclotron heating and neutral beam injection as actuators; (b) active control of the density and pressure profiles in quiescent H-mode and quiescent double barrier plasmas using electron cyclotron current drive (ECCD) and pellet injection; (c) active control of the edge profiles to suppress edge localized modes using resonant magnetic perturbation with toroidal mode number n = 3, (d) real time control of the current density profile to suppress neoclassical tearing modes using localized deposition of co-ECCD.

Investigations on the ELM cycle by local 3D perturbation experiments

An experimental study is performed to investigate the underlying physics of ELM triggering by imposing local perturbations at the plasma edge. Deuterium is injected during type-I ELMy H-mode phases by small solid pellets or as a supersonic gas jet. In both cases the repetition rate is small compared with the intrinsic ELM frequency, aiming for a small perturbation of the intrinsic ELM cycle. Active triggering of ELMs requires a density perturbation in the gradient region about 1 cm inside the separatrix (measured at horizontal mid-plane), a condition that is achieved by pellets of any size and speed but not by even the strongest available gas jets. The pellet-induced triggering of ELMs always occurs when pellets reach the H-mode barrier region. A density perturbation produced by a gas jet remains localized at the separatrix, and even if larger by a factor of at least 100, is not sufficient to trigger an ELM. No significant dependence of the ELM size on the ELM onset time was found, measured from the onset of the previous ELM.

Characterization of the H-mode edge barrier at ASDEX Upgrade

The scaling of the edge transport barrier (ETB) that sustains H-mode plasmas is crucial for the performance of next step tokamaks. At ASDEX Upgrade, the suite of edge diagnostics has been significantly improved to address this issue. High spatial resolution profiles of most of the key edge plasma parameters necessary to determine the magneto hydrodynamic (MHD) stability are now available. New high temporal resolution measurements give clear indications of the nonlinear evolution of the ELM crash. The correlation lengths of edge turbulence have been shown to be correlated with the edge radial electric field shear using a new correlation Doppler reflectometer system. The measured pressure gradient in the ETB is found to be consistent with ideal MHD stability limits, both for Type I and II ELMs. In addition, the edge electron temperature and density gradient lengths are found to be strongly correlated, leaving only the ETB width as a free parameter. In ASDEX Upgrade, the ETB width does not vary significantly over the entire H-mode edge database. Modelling of the transport of comparison discharges in hydrogen and deuterium shows that the expected mass effect on neutral penetration is largely compensated by more efficient heating of deuterium neutrals but requires a transport barrier in both the energy and particle channels in order to reproduce the measured edge temperature and density profiles.

Studies of the ‘Quiescent H-mode’ regime in ASDEX Upgrade and JET

The stationary edge localized mode (ELM)-free ‘Quiescent H-mode’ (QH-mode) regime, obtained with counter neutral beam injection, is studied in ASDEX Upgrade and Joint European Torus. QH-mode plasmas have high pedestal and core ion temperatures together with good High-confinement mode (H-mode). ELMs are replaced by continuous MHD oscillations, the ‘edge harmonic oscillation’ (EHO) and the ‘high frequency oscillation’. Stationarity of particle and impurity densities is linked to the occurrence of these MHD modes. The EHO location in the steep-gradient region and its appearance with increasing edge pressure points towards the edge pressure or pressure gradient as possible drivers for the EHO. Injection of small cryogenic pellets can raise the plasma density to about 40% of the Greenwald density limit without triggering ELMs. Orbit following calculations of the slowing-down distribution show the presence of an enhanced fast particle density in the H-mode barrier region despite the large loss currents with counter-injection. The radial electrical field in the edge barrier region, about twice as large in QH-mode as in ELMy H-mode, is large enough to reverse the precession drift direction of injected beam ions, leading to a resonance of the EHO with the drift precession frequency.

Study of quiescent H-mode plasmas in ASDEX Upgrade

In a recent campaign with counter-current neutral beam injection (NBI), the quiescent high confinement mode (QH-mode) is studied in ASDEX Upgrade. The QH-mode regime is obtained with all accessible NBI angles, with and without additional ion cyclotron heating, at various values of edge safety factor q95 and at values of Zeff down to 2.5 after fresh boronization. In contrast to the ELMy H-mode, the pedestal pressure depends significantly on radial heat flux in the QH-mode, giving rise to a reduced heating power degradation of H-mode confinement. The ‘edge harmonic oscillation’, a characteristic MHD feature of the QH-mode that replaces ELMs, occurs at rational surfaces in the H-mode barrier region, and has so far been found only with a toroidal mode number n = 1. These conditions seem to define windows in q95 for favourable QH-mode access.

Comparison of H-mode barrier width with a model of neutral penetration length

Pedestal studies in DIII-D find that the width of the region of steep gradient in the H-mode density is comparable with the neutral penetration length, as computed from a simple analytic model. This model has analytic solutions for the edge plasma and neutral density profiles, which are obtained from the coupled particle continuity equations for electrons and deuterium atoms. In its range of validity (edge temperature between 40 and 500 eV), the analytic model quantitatively predicts the observed decrease in the width as the pedestal density increases and the observed strong increase in the gradient of the density as the pedestal density increases. The model successfully predicts that L-mode and H-mode profiles with the same pedestal density have gradients that differ by less than a factor of 2. The width of the density barrier, measured from the edge of the electron temperature barrier, is the lower limit for the observed width of the temperature barrier. These results support the hypothesis that particle fuelling is an important part of the physics that determines the structure of the H-mode transport barrier.

Recent experimental studies of edge and internal transport barriers in the DIII-D tokamak

Results from recent experiments on the DIII-D tokamak have revealed many important details on transport barriers at the plasma edge and in the plasma core. These experiments include: (a) the formation of the H-mode edge barrier directly by pellet injection; (b) the formation of a quiescent H-mode edge barrier (QH-mode) which is free from edge localized modes, but which still exhibits good density and radiative power control; (c) the formation of multiple transport barriers, such as the quiescent double barrier (QDB) which combines an internal transport barrier with the quiescent H-mode edge barrier. Results from the pellet-induced H-mode experiments indicate that: (a) the edge temperature (electron or ion) does not need to attain a critical value for the formation of the H-mode barrier, (b) pellet injection leads to an increased gradient in the radial electric field, Er, at the plasma edge; (c) the experimentally determined edge parameters at barrier transition are well below the predictions of several theories on the formation of the H-mode barrier, (d) pellet injection can lower the threshold power required to form the H-mode barrier. The quiescent H-mode barrier exhibits good density control as the result of continuous magnetohydrodynamic activity at the plasma edge called the edge harmonic oscillation (EHO). The EHO enhances the edge particle transport whilst maintaining a good energy transport barrier. The ability to produce multiple barriers in the QDB regime has led to long duration, high-performance plasmas with βNH89 values of 7 for up to 10 times the confinement time. Density profile control in the plasma core of QDB plasmas has been demonstrated using on-axis electron cyclotron heating.

H-mode barrier control with external magnetic perturbations

While present plasma scenarios rely on self generated plasma conditions, active control with ergodic divertor coils provide the means to operate in H-mode with divertor and SOL plasma conditions that are compatible with the technical constraints. The stochastisation of field lines in the vicinity of the separatrix, will produce a localized enhancement of electron heat transport. This will provide a control on the edge temperature (and thus pressure) barriers. Consequently, the local current profile, which is essentially bootstrap, will be modified. H-mode edge barriers can be controlled and type I Elms avoided in this way. Such controls are required for advanced regimes. The paper will discuss this in light of the theoretical and experimental advances of the last years: Tore Supra experiments gave evidence of the fine tuning capability of the perturbation on a well defined magnetic configuration while JFT-2M experiments showed that the edge stochastisation of the X point configuration has widened the operating windows of the type III ELM regime. The possible implementations of such control in a medium size tokamak (DIII-D) and eventually on ITER will also be described.

Measurements of large poloidal variations of impurity density in the Alcator C-Mod H-mode barrier region

Simultaneous high resolution measurements of the soft x-ray emission at the top and outboard edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. Impurity density profiles are derived from these measurements. The impurity density varies considerably on a flux surface in the high confinement mode (H-mode) pedestal region. The soft x-ray and impurity density pedestals are not on the same flux surface at the top and outboard edge. The pedestal widths are consistently larger at the outboard edge in the Enhanced D-alpha (EDA) H-mode, suggesting a ballooning-like character of the EDA quasi-coherent mode. The usual separation of time scales between radial and parallel impurity transport does not hold in the H-mode pedestal region of Alcator C-Mod. Thus, one should not necessarily expect that the impurity density be constant on a flux surface in this region.

Quiescent H-mode plasmas in the DIII-D tokamak

H-mode operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge-localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the β limit and reduced core transport regions needed for advanced tokamak operation. Recent experimental results from DIII-D have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation which is ELM-free and yet has good density control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or >25 energy confinement times τE). By forming the core barrier and then stepping up the input power, we have achieved βNH89 = 7for up to 10 times the τE of 160 ms. The βNH89values of 7 substantially exceed the value of 4 routinely achieved in standard ELMing \\mbox{H-mode.} The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density control in the quiescent H-mode is possible because of the presence of an edge MHD oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.

Role of neutrals in density pedestal formation in DIII-D

An analytic model for the width of the electron density transport barrier shows qualitative and quantitative agreement with systematic data obtained in the DIII-D tokamak. The width of the barrier in H-mode decreases as the pedestal density increases. Moreover, for L-mode and H-mode density profiles with the same pedestal density, the gradients of the barriers are of similar magnitude even though the temperature and pressure gradients are much larger in H-mode. These results are consistent with the hypothesis that neutral penetration provides important physics which helps to set the width of the H-mode barrier for the electron density.

Turbulence reduction in internal transport barriers on ASDEXUpgrade

A reduction in turbulent density fluctuations is observed in internaltransport barrier (ITB) discharges in the ASDEX Upgrade tokamak with an L-modeedge and reversed magnetic shear. The turbulence reduction is localizedspatially to around the ITB gradient and the ITB foot-point region, coincidentwith strong shearing in the toroidal rotation velocity. The radial extent anddegree of reduction is related to the strength of the ITB. Strong (narrow andsteep) ITBs are well localized and display greater turbulence reduction.Weaker ITBs are less localized with less reduction. The formation of asubsequent H-mode edge barrier moves the velocity shearing region to the edgeand weakens the ITB to the point of extinction. During the ITB the turbulenceis reduced across a broad frequency range, however a residual level oflow-frequency fluctuations (f<10 kHz) remains. Comparison of linear growthand shearing rates suggest that E×B shear may be a factor in theturbulence reduction.

Quiescent double barrier high-confinement mode plasmas in the DIII-D tokamak

High-confinement (H-mode) operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the beta limit and reduced core transport regions needed for advanced tokamak operation. Experimental results from DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] this year have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation that is ELM-free and yet has good density and impurity control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (&amp;gt;3.5 s or &amp;gt;25 energy confinement times τE), yielding a quiescent double barrier regime. By slowly ramping the input power, we have achieved βNH89=7 for up to 5 times the τE of 150 ms. The βNH89 values of 7 substantially exceed the value of 4 routinely achieved in the standard ELMing H mode. The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density and impurity control in the quiescent H mode is possible because of the presence of an edge magnetohydrodynamic (MHD) oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.