In the last 30 days

• Open/close Effect of resonant magnetic perturbations on particle confinement

  Q. Yu and S. Günter 2009 Nucl. Fusion 49 062001 Tag this article

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  The effect of externally applied resonant magnetic perturbations (RMPs) on plasma particle transport is investigated based on two fluid equations. It is found that, depending on the frequency and direction of plasma rotation, the RMP of a moderate amplitude can either increase or decrease the plasma density gradient around the corresponding rational surface. The local density profile flattens only for a sufficiently large RMP.

• Open/close Chapter 2: Plasma confinement and transport

  E.J. Doyle (Chair Transport Physics) et al 2007 Nucl. Fusion 47 S18 Tag this article

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  The understanding and predictive capability of transport physics and plasma confinement is reviewed from the perspective of achieving reactor-scale burning plasmas in the ITER tokamak, for both core and edge plasma regions. Very considerable progress has been made in understanding, controlling and predicting tokamak transport across a wide variety of plasma conditions and regimes since the publication of the ITER Physics Basis (IPB) document ( 1999 Nucl. Fusion 39 2137–2664). Major areas of progress considered here follow. (1) Substantial improvement in the physics content, capability and reliability of transport simulation and modelling codes, leading to much increased theory/experiment interaction as these codes are increasingly used to interpret and predict experiment. (2) Remarkable progress has been made in developing and understanding regimes of improved core confinement. Internal transport barriers and other forms of reduced core transport are now routinely obtained in all the leading tokamak devices worldwide. (3) The importance of controlling the H-mode edge pedestal is now generally recognized. Substantial progress has been made in extending high confinement H-mode operation to the Greenwald density, the demonstration of Type I ELM mitigation and control techniques and systematic explanation of Type I ELM stability. Theory-based predictive capability has also shown progress by integrating the plasma and neutral transport with MHD stability. (4) Transport projections to ITER are now made using three complementary approaches: empirical or global scaling, theory-based transport modelling and dimensionless parameter scaling (previously, empirical scaling was the dominant approach). For the ITER base case or the reference scenario of conventional ELMy H-mode operation, all three techniques predict that ITER will have sufficient confinement to meet its design target of Q = 10 operation, within similar uncertainties.

• Open/close Preliminary results of ELMy H-mode experiments on the HL-2A tokamak

  X.R. Duan et al 2010 Nucl. Fusion 50 095011 Tag this article

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  Typical ELMy H-mode discharges have been achieved on the HL-2A tokamak with combined auxiliary heating of NBI and ECRH. The minimum power required is about 1.1 MW at a density of 1.6 × 10 ¹⁹ m ⁻³ and increases with a decrease in density, almost independent of the launching order of the ECRH and NBI heating. The energy loss by each edge localized mode (ELM) burst is estimated to be lower than 3% of the total stored energy. At a frequency of typically 400 Hz, the energy confinement time is only marginally reduced by the ELMs. The supersonic molecular beam injection fuelling is found to be beneficial for triggering an L–H transition due to less induced recycling and higher fuelling efficiency. The dwell time of the L–H transition is 20–200 ms, and tends to decrease as the power increases. The delay time of the H–L transition is 10–30 ms for most discharges and is comparable to the energy confinement time. The ELMs with a period of 1–3 ms are sustained for more than ten times the energy confinement time with enhanced confinement factor H ₈₉ > 1.5, which tends to decrease with the total heating power. The confinement time in the H-mode discharges increases with plasma current approximately linearly.

• Open/close Observations of core toroidal rotation reversals in Alcator C-Mod ohmic L-mode plasmas

  J.E. Rice et al 2011 Nucl. Fusion 51 083005 Tag this article

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  Direction reversals of intrinsic toroidal rotation have been observed in Alcator C-Mod ohmic L-mode plasmas following modest electron density or toroidal magnetic field ramps. The reversal process occurs in the plasma interior, inside of the q = 3/2 surface. For low density plasmas, the rotation is in the co-current direction, and can reverse to the counter-current direction following an increase in the electron density above a certain threshold. Reversals from the co- to counter-current direction are correlated with a sharp decrease in density fluctuations with k _R ≥ 2 cm ⁻¹ and with frequencies above 70 kHz. The density at which the rotation reverses increases linearly with plasma current, and decreases with increasing magnetic field. There is a strong correlation between the reversal density and the density at which the global ohmic L-mode energy confinement changes from the linear to the saturated regime.

• Open/close Study on H-mode access at low density with lower hybrid current drive and lithium-wall coatings on the EAST superconducting tokamak

  G.S. Xu et al 2011 Nucl. Fusion 51 072001 Tag this article

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  The first high-confinement mode (H-mode) with type-III edge localized modes at an H factor of H _{IPB98( y,2)} ~ 1 has been obtained with about 1 MW lower hybrid wave power on the EAST superconducting tokamak. The first H-mode plasma appeared after wall conditioning by lithium (Li) evaporation before plasma breakdown and the real-time injection of fine Li powder into the plasma edge. The threshold power for H-mode access follows the international tokamak scaling even in the low density range and a threshold in density has been identified. With increasing accumulation of deposited Li the H-mode duration was gradually extended up to 3.6 s corresponding to ~30 confinement times, limited only by currently attainable durations of the plasma current flat top. Finally, it was observed that neutral density near the lower X-point was progressively reduced by a factor of 4 with increasing Li accumulation, which is considered the main mechanism for the H-mode power threshold reduction by the Li wall coatings.

• Open/close Modelling of plasma response to resonant magnetic perturbation fields in MAST and ITER

  Yueqiang Liu et al 2011 Nucl. Fusion 51 083002 Tag this article

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  The resonant magnetic perturbation (RMP) fields, including the plasma response, are computed within a linear, full toroidal, single-fluid resistive magnetohydrodynamic (MHD) model, and under realistic plasma conditions for MAST and ITER. The response field is found to be considerably reduced, compared with the vacuum field produced by the magnetic perturbation coils. This field reduction relies strongly on the screening effect from the toroidal plasma rotation. Computations also quantify three-dimensional (3D) distortions of the plasma surface, caused by RMP fields. A correlation is found between the computed mode structures, the plasma surface displacement and the observed density pump-out effect in MAST experiments. Generally, the density pump-out tends to occur when the surface displacement peaks near the X-points.

• Open/close Plasma response to externally applied resonant magnetic perturbations

  Q. Yu and S. Günter 2011 Nucl. Fusion 51 073030 Tag this article

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  Based on recent finding that applied resonant magnetic perturbation (RMP) can either increase or decrease the local electron density gradient around the resonant surface (Yu and Günter 2009 Nucl. Fusion 49 062001), the plasma response to the RMP of a single helicity is studied numerically by further taking into account the electron energy transport. It is found that the changes in the local electron density and temperature by RMPs are coupled. If the local parallel heat diffusivity and/or the island width are sufficiently large, the local electron temperature profile flattens, which enhances the change in the local electron density gradient. Depending on the plasma parameters, either the plasma rotation frequency or the electron diamagnetic frequency (electron pressure gradient) can be significantly changed by RMPs, and these two changes affect each other. With a reduced set of equations that does not include the electron energy transport equation, the particle transport in stochastic magnetic fields is found to be similar to that across a single magnetic island.

• Open/close JET snake magnetohydrodynamic equilibria

  W.A. Cooper et al 2011 Nucl. Fusion 51 072002 Tag this article

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  Magnetohydrodynamic (MHD) equilibrium states with a three-dimensional helical core that display the characteristics of a saturated ideal internal kink mode are computed to model snake structures that have been observed in the JET tokamak (Weller et al 1987 Phys. Rev. Lett. 59 2303). The equilibrium states are calculated with a peaked pressure profile and a weak to moderate reversed core magnetic shear with a minimum safety factor q _min near unity in the neighbourhood of the mid-radius of the plasma. Snake equilibrium states are computed in the range 0.94 < q _min < 1.03. This range aligns with linearly unstable ideal MHD internal kink solutions of the purely axisymmetric branch of the equilibrium states. The energy difference between the bifurcated axisymmetric and helical snake equilibrium solutions is minimal. One very important novelty is that the helical structures are computed with an equilibrium code developed for three-dimensional (3D) stellarator applications in a tokamak context and cannot be obtained with standard Grad–Shafranov equation solvers.

• Open/close Consequences of profile shearing on toroidal momentum transport

  Y. Camenen et al 2011 Nucl. Fusion 51 073039 Tag this article

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  Turbulent transport of toroidal momentum is investigated in global linear gyrokinetic simulations. The poloidal tilt of the global mode structure arising from the radial variation of the equilibrium (profile shearing) is shown to induce non-diagonal non-pinch momentum transport (residual stress). Local simulations performed at finite radial wave vector show that the effect is mainly due to the antisymmetric radial component of the magnetic drift. The residual stress resulting from profile shearing enhances co-current rotation for ion temperature gradient turbulence and counter-current rotation for trapped electron mode turbulence.

• Open/close ITER predictions using the GYRO verified and experimentally validated trapped gyro-Landau fluid transport model

  J.E. Kinsey et al 2011 Nucl. Fusion 51 083001 Tag this article

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  The trapped gyro-Landau fluid (TGLF) transport model computes the quasilinear particle and energy driftwave fluxes in tokamaks with shaped geometry, finite aspect ratio and collisions. The TGLF particle and energy fluxes have been successfully verified against a large database of collisionless nonlinear gyrokinetic simulations using the GYRO code. Using a new collision model in TGLF, we find remarkable agreement between the TGLF quasilinear fluxes and 64 new GYRO nonlinear simulations with electron–ion collisions. In validating TGLF against DIII-D and JET H-mode and hybrid discharges we find the temperature and density profiles are in excellent agreement with the measured profiles. ITER projections using TGLF show that the fusion gains are somewhat more pessimistic than the previous GLF23 results primarily due to finite aspect ratio effects included only in TGLF. The synergistic effects of density peaking, finite β and E × B shear due to finite toroidal rotation lead to significant increases in fusion power above a reduced physics ITER base case. The TGLF results for ITER are confirmed using nonlinear GYRO simulations in place of TGLF to predict the temperature profiles within the TGYRO transport code. These results represent a snapshot of the ongoing effort to improve the TGLF model, validate it against experimental data, and make predictions for ITER.