ELMy H-mode Plasmas Research Articles

Neutron profiles and fuel ratio nT/nD measurements in JET ELMy H-mode plasmas with tritium puff

Two-dimensional (2D) spatial profile and the temporal evolution of the 14 and 2.5 MeV neutron emissivities from D–D and D–T fusion reactions were studied using the measurements of the upgraded neutron profile monitor during the last trace tritium experiments in JET. The JET neutron profile monitor provides unique capability for 2.5 and 14 MeV neutrons line-integrated measurements simultaneously. A systematic comparison of D–D and D–T neutron emissivity was performed. The tritium concentration or fuel ratio (nT/nD) was analysed for a set of 34 ELMy-H mode discharges with tritium puff. Tritium concentration is deduced with a method based on the ratio of D–T 14 MeV and D–D 2.5 MeV neutron emissivities in order to exploit the maximum information available from neutron data. With the help of a tomography algorithm recently developed at JET, 2D spatial profiles of the tritium concentration in the plasma were obtained. These profiles can be used to perform transport studies. Tritium core confinement is clearly seen to increase with plasma density for the set of discharges studied. Differences in the shape of these profiles are also found between low and high density plasmas. Shortly after tritium puffing, 2D spatial profiles of the tritium concentration exhibit typical hollow profiles and in some cases transient poloidal asymmetric features have been observed in 2D images.

Pedestal temperature models based on first and second stability limits of ballooning modes

Models for the prediction of electron pedestal temperatures at the edge of type I ELMy H-mode plasmas are developed. These models are based on theory motivated concepts for pedestal width and pressure gradient. The pedestal pressure gradient is assumed to be limited by high n ballooning mode instabilities, where both the first and second stability limits are considered. The effect of the bootstrap current, which reduces the magnetic shear in the steep pressure gradient region at the edge of the H-mode plasma, can result in access to the second stability of ballooning mode. In these pedestal models, the magnetic shear and safety factor are calculated at one pedestal width away from separatrix. The predictions of these models are compared with the experimental electron pedestal temperatures for type I ELMy H-mode discharges obtained from the latest public version (version 3.2) in the International Tokamak Physics Activity Edge (ITPA) Pedestal Database.

Long Pulse Operation of High Performance Plasmas in JT-60U

Recent experimental progress in JT-60U advanced tokamak research is presented:sustainment of the normalized beta (βN)∼ 3 in a normal magnetic shear plasma, the bootstrap current fraction (fBS) ∼ 45% in a weak shear plasma and ∼ 75% in a reversed magnetic shear plasma in a nearly fully non-inductive current drive condition for longer than the current relaxation time. Achievement of high-density, high-radiation fraction together with high-confinement in advanced plasmas is demonstrated. Achievements and findings in long pulse operations after system modification are presented as well. A 65 s discharge of Ip = 0.7 MA was successfully obtained. As a result, high-βN of 2.3 was successfully sustained for a very long period of 22.3 s. In addition, a 30 s standard ELMy H-mode plasma of Ip up to 1.4 MA was also obtained. Effectiveness of divertor pumping to control particle recycling and the electron density under the saturated wall retention was demonstrated. These achievements and issues in development are discussed.

Study of Particle Behavior for Steady-State Operation in JT-60U

This paper summarizes recent studies of particle balance in long pulse discharges, long-term erosion/deposition and hydrogen retention for carbon divertor tiles, and particle behavior in SOL and divertor plasmas related to the two foregoing topics in JT-60U. Global saturation of the particle inventory appeared in the later phase of an ELMy H-mode plasma after several long pulse discharges. As for carbon divertor tiles, erosion was dominant in the outer divertor, and deposition was dominant in the inner divertor. Hydrogen isotope concentration (H+D)/C in the deposition layer was estimated to be ∼0.032. The SOL plasma flow towards the inner divertor was observed at the low-field-side midplane, and influence of the Er × B drift flow from the outer divertor to the inner divertor via the private region on the divertor particle flux was evaluated to be large in the attached divertor plasmas. The preferential deposition in the inner divertor can be a consequence of these flows. For direct observation of the hydrogen molecular behavior in the divertor plasmas, hydrogen molecular line emission was measured and the fall in the emission intensity with distance from the divertor plates was reproduced by calculation with a neutral transport code and a collisional radiative modeling code.

Stabilization of neoclassical tearing mode by ECCD and its evolution simulation on JT-60U tokamak**This manuscript is based on the paper EX/7-4 presented at the 20th IAEA Fusion Energy Conference, Vilamoura, Portugal.

Stabilization of an m = 3/n = 2 neoclassical tearing mode (NTM) by a local electron cyclotron current drive (ECCD) has been studied in high βp ELMy H-mode plasmas in the JT-60U tokamak. In the stabilization experiment, the electron cyclotron (EC) injection timing is scanned from the ‘before mode onset’ phase (preemptive ECCD) to the mode saturation phase (late ECCD). It has been demonstrated that the ECCD is more effective when it is applied before the mode onset and the critical timing for effective ECCD is related to the mode excitation phase. The EC power to prevent the 3/2 NTM excitation by preemptive ECCD is lower by about 20% compared with complete stabilization by late ECCD. Even when the 3/2 NTM is excited, the stabilization effect is stronger than for late ECCD. For a high performance plasma of the normalized beta, βN = 3.0, the H-factor H89PL improves from 1.9 to 2.1 when the 3/2 NTM is suppressed. A numerical study has been made based on the modified Rutherford equation coupled with a 1.5D transport code and an EC code. The simulation reproduces well the time evolution of the magnetic island both at the growing and stabilizing phases. The simulation results show that it is important to adjust not only the EC current position but also its width to stabilize the NTM effectively. The reduction of EC power for NTM prevention in experiments for preemptive ECCD can be explained by the increase of the minimum seed island size for NTM excitation.

Main design features and challenges of the ITER-like ICRF antenna for JET

The ITER-like ICRF antenna for JET aims at validating novel antenna design principles in conditions as relevant as possible to ITER by coupling about 7.2 MW to an ELMy H-mode plasma on JET in the frequency range from 30 to 55 MHz. The power density, about 8 MW/m 2, for a given maximum voltage in the circuit is maximized using poloidally short straps and the resilience to fast varying RF loads by matching pairs of straps by a so-called conjugate-T. The paper reports the main characteristics of the final design as well as the main challenges encountered during the design and testing phase of the key components.

Overview of JT-60U progress towards steady-state advanced tokamak

Recent experimental results from steady-state advanced tokamak (AT) research on JT-60U are presented with emphasis on time scales longer in comparison with the characteristic time scales in plasmas. To achieve this, modification of the controls for the operation, heating and diagnostics systems have been carried out. As a result, ∼60 s current flat top and a ∼30 s H-mode are obtained. The long pulse modification has opened a door into a new domain for JT-60U. High normalized beta (βN) of 2.3 is maintained for 22.3 s and 2.5 for 16.5 s in a high-βp H-mode plasma. A standard ELMy H-mode plasma has also been extended and changes in the wall recycling on the longer time scale have been unveiled. The development and investigation of plasmas relevant to AT operation have been continued in long discharges as well as in discharges where higher NB power is available (⩽10 s). Higher βN (∼3) is maintained for 6.2 s in a high-βp H-mode plasma. High bootstrap current fraction (fBS) of ∼75% is sustained for 7.4 s in a reversed shear plasma. Neo-classical tearing mode (NTM) suppression by localized ECCD is found to be more effective with ECRF injection preceding the mode saturation. The mode is suppressed with less power compared to the injection after the mode saturates. The domain of the NTM suppression experiments is extended to the high-βN regime, and the effectiveness of m/n = 3/2 mode suppression by ECCD is demonstrated at βN ∼ 2.5–3. Genuine tokamak plasma start up without a central solenoid is demonstrated. In a current hole region, it is shown that no scheme drives current in any direction. Detailed measurement of energetic ions in both space and energy showed dynamic change in the energetic ion profile due to collective instabilities. The impact of toroidal plasma rotation on ELM behaviour is clarified in the grassy ELM and QH domains. Retention of hydrogen isotopes in the divertor tiles is analysed.

ELM triggering conditions for the integrated modeling of H-mode plasmas

Recent advances in the integrated modeling of ELMy H-mode plasmas are presented. A model for the H-mode pedestal and for the triggering of ELMs predicts the height, width, and shape of the H-mode pedestal and the frequency and width of ELMs. Formation of the pedestal and the L-H transition is the direct result of ExB flow shear suppression of anomalous transport. The periodic ELM crashes are triggered by either the ballooning or peeling MHD instabilities. The BALOO, DCON, and ELITE ideal MHD stability codes are used to derive a new parametric expression for the peeling-ballooning threshold. The new dependence for the peeling-ballooning threshold is implemented in the ASTRA transport code. Results of integrated modeling of DIII-D like discharges are presented and compared with experimental observations. The results from the ideal MHD stability codes are compared with results from the resistive MHD stability code NIMROD.

Assessment of the poloidal distribution of core plasma fueling and impurity sources in DIII-D

Measurements and modeling of the 2D poloidal Dα intensity distribution in DIII-D low density L-mode and medium density ELMy H-mode plasmas indicate that the core plasma is predominately fueled near the divertor X-point region. The neutral deuterium and ion carbon emission were measured in the divertor and inner main chamber scrape-off layer (SOL) using a plasma imaging technique, covering 85% of the poloidal cross-section. Typically, the peak emission in the inner main SOL at the tokamak midplane was three orders of magnitude lower than in the divertor. For discharges with the ion B×∇B drift direction toward the lower divertor the UEDGE/DEGAS codes predict strong core plasma fueling from the significantly higher density and lower temperature plasma calculated in the inner divertor leg. The concomitant carbon ion flow reversal in the inner divertor leg enhances the leakage of carbon from the divertor into the main SOL, and hence into the core.

Divertor regimes in NSTX

The identification of divertor operating regimes is of particular importance for heat and particle control optimization in high performance plasmas of a spherical torus, because of the magnetic geometry effects and compactness of the divertor region. Recent measurements of radiated power, heat, and particle fluxes in lower single null and double null NSTX plasmas with 0.8–6MW NBI heating suggest that the inner divertor is detached at n¯e⩽2–3×1019m-3 whereas the outer divertor is always attached. This resilient state exists in most L- and H-mode plasmas. The inner divertor transiently re-attaches in ELMy H-mode plasmas when heat pulses from type I or type III ELMs reach the divertor.

SOL and pedestal density profile evolution during DIII-D ELMy and ELM-suppressed H-mode operation

A fast profile reflectometer has been used to study the detailed characteristics of scrape-off-layer (SOL) and pedestal density profile evolution in DIII-D ELMy H-mode plasmas with high temporal (⩾10μs) and spatial (∼4mm) resolution. Results show a significant enhancement in radial particle transport to the outer vessel wall during Type-I ELMs. The radial particle transport time (∼140μs) becomes comparable to the parallel transport time, as a result of a high radial velocity (∼620m/s) of SOL profile expansion. The dynamics can vary from ELM-to-ELM. Scalings with pedestal density have been obtained, indicating that ELM is less affected as the pedestal density increases. Finally, during ELM-suppressed H-mode operation with a stochastic magnetic boundary, the radial extent of observed SOL profile modifications do not extend to the vessel wall, and profiles are steeper, substantially different from the SOL changes associated with Type-I ELMs.

Pedestal Characteristics of H-Mode Plasmas in JT-60U and ASDEX Upgrade

The role of the pedestal structure in ELMy H-mode plasmas for the core energy confinement and for the ELM energy losses were investigated in two tokamak-type devices, JT-60U and ASDEX Upgrade. The confinement degradation seen at higher densities is attributed to the reduction of the pedestal temperature limited by the ELM activities and the stiffness of the temperature profiles. In high triangularity or impurity seeded H-modes, in which higher energy confinement is generally achieved, a higher pedestal temperature is obtained by improving the edge MHD stability or the density profile peaking, respectively. The upper bound of the ELM energy loss is characterized by the pedestal energy. The ELM energy loss can become smaller at fixed pedestal energy when the pedestal collisionality is raised. Raising the pedestal collisionality also enhances the inter-ELM perpendicular transport loss and reduces the ELM loss power fraction. In ASDEX Upgrade, the continuous pellet injection is shown to effectively mitigate ELM losses while preserving the core confinement quality. In JT-60U, the operation regime of grassy ELM is investigated and shown to achieve highly integrated performance of high energy confinement with small ELMs in the low collisionality regime.

Study of plasma wall interactions in the long-pulse NB-heated discharges of JT-60U towards steady-state operation

Long time scale variation of plasma-wall interactions and its impact on particle balance, main plasma performance and particle behavior have been investigated in ELMy H-mode plasmas by extending the discharge pulse and the neutral beam heating pulse to 65s and 30s, respectively. The wall pumping rate starts to decrease in the latter phase by repeating the long-pulse discharges with 60% of Greenwald density sustained by gas-puffing. After several discharges, the wall inventory is saturated in the latter phase and, consequently, the density increases with neutral beam fuelling only. The edge pressure in the main plasma is reduced and ELMs are close to the type III regime under conditions of wall saturation. The intensities of C II emission near the X-point and CD band emission in the inner divertor start to increase before the wall saturates and continue to increase after the wall is saturated.

High-beta steady-state research and future directions on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade and the Japan Atomic Energy Research Institute Fusion Torus-2 Modified

In the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U), a high-βp ELMy H-mode (high-poloidal-beta high-confinement-mode with edge localized mode) plasma was sustained with βN∼2.7 for 7.4 s. Real-time neoclassical tearing mode (NTM) stabilization system was established and effective NTM suppression by early electron cyclotron (EC) wave injection was demonstrated. High fusion triple product of ni(0)τETi(0)=3.1×1020 keV⋅s⋅m−3 was achieved using the negative-ion based neutral beam current drive with βN∼2.5 and the bootstrap current fraction fBS∼50%. In a hot electron regime, a high electron cyclotron current drive efficiency of 4.2×1018 A/W/m2 was achieved at Te∼21 keV. Innovative current start-up scenario produced a current hole plasma with a very high fBS∼90%. No accumulation of helium and carbon impurities was observed for internal transport barrier (ITB) plasmas. While argon impurity was accumulated, EC injection effectively exhausted it across ITB. In a regime of ELM disappearance, a clear correlation between the ELM frequency and the toroidal velocity at pedestal was observed. In the Japan Atomic Energy Research Institute Fusion Torus-2 Modified (JFT-2M), high beta plasmas were produced with full ferritic inside wall up to βN=3.3, where high recycling steady H-mode discharges were developed up to βNH89P∼6 at ne/nGW∼0.7–1.0 with ITB. JT-60U started long pulse experiment in late 2003 and JFT-2M will conduct wall stabilization experiment in early 2004. The modification of JT-60 to a fully superconducting coil tokamak is regarded as the national centralized tokamak facility program to accomplish the high beta steady-state research in a collisionless regime.

The beta scaling of energy confinement in ELMy H-modes in JET

The disagreement between the weak dependence of the energy confinement time on normalized pressure, β, observed in dedicated scans and the strongly negative dependence in the confinement scaling laws used for the design of next step tokamaks and future reactors, remains an outstanding problem. As such, scans of β have been undertaken in single null, low triangularity (δ ≈ 0.2) ELMy H-mode plasmas in JET with the MarkIIGB-SRP divertor. The scans varied β by a factor of 2.8 (normalized β from 0.72 to 2.04) and covered a range of magnetic fields (1.5–2.3 T), plasma currents (1.5–2.75 MA) and safety factors (q95 = 2.8 and 3.3). A weak β dependence was observed both globally (B0τE varied less than 9% across any one scan) and locally. A scan within Type I ELMy H-modes suggests that this weaker dependence is not due to ELM regimes. A statistical analysis indicates that these results are consistent with log–linear regressions performed on a wide JET database of ELMy H-modes, if correlations in this database are considered.

Dimensionless pedestal identity experiments in JT-60U and JET in ELMy H-mode plasmas

This paper reports the first results of collaborative experiments in JT-60U and JET to compare ELMy H-mode characteristics in the two machines. The JT-60U/JET identity experiments were aimed explicitly at matching edge pedestal parameters in plasmas of the same geometry. The similar size of the two devices allows plasma configurations and conditions to be established for which the match of the dimensionless parameters (ρ*, ν*, β and q95) and of the equilibrium geometry is carried out in plasmas that are also almost identical in dimensional terms.The results of the power and density scans at fixed (and matched) Ip − BT show that, in a common range of ρ*, the pedestal electron collisionality, , and normalized pressure, βp,ped do not match in the two machines, against expectations. In particular, the range of βp,ped of JT-60U ELMy H-modes with Type I ELMs is accessible to JET plasmas only with a Type III ELM edge, and in JT-60U is consistently higher by ≈2 than in JET, for the same ρ*. A comparison of ne and Te pedestal profiles indicates that ∇Te is comparable in the two devices, while JET ne profiles are steeper than JT-60U. The global energy confinement enhancement factor (H98y − 2) is higher in JET than in JT-60U, and part of this difference is attributed to the reduced pedestal pressure in JT-60U compared with JET.The two experiments are analysed in detail to try to identify ‘hidden’ physics that may be the cause of the failure to achieve the pedestal identity as predicted. Although a full explanation for the different behaviour of the edge pedestal in the two devices has not yet been found, the possible influence of aspect ratio effects on MHD stability, differences in the plasma current profile and ripple loss (and rotation) effects are discussed.

ELMy H-modes in JET helium-4 plasmas

ELMy H-modes in helium-4 plasmas provide valuable information on ELMy H-mode physics as well as a possible early low activation operational phase for next-step tokamaks, such as ITER. With this in mind, a series of helium-4 H-mode experiments were performed on JET with pure helium-4 NBI auxiliary heating (up to 12 MW). A set of ELMy H-mode plasmas were produced, in both the Type I ELM regime and a second regime, which showed characteristics similar to the deuterium Type III regime, but with a reverse ELM frequency dependence on power. Sawteeth were also observed, and had similar behaviour to those seen in deuterium. Compared with deuterium plasmas, Type I ELMy H-mode confinement is seen to be 28 ± 6% poorer in helium-4 plasmas and the L–H power threshold 42 ± 10% larger. This is the opposite of the behaviour predicted by experimental isotope mass scalings from hydrogenic plasmas. Comparison with a wider hydrogenic database, enables the effects of isotopic charge and mass to be studied independently.

Towards the realization on JET of an integrated H-mode scenario for ITER

ELMy H-mode experiments at JET in 2000/mid-2002 have focused on discharges with normalized parameters for plasma density, energy confinement and beta similar to those of the ITER QDT = 10 reference regime (n/nGW ∼ 0.85, H98(y,2) ∼ 1, βN ∼ 1.8). ELMy H-mode plasmas have been realized reaching or even exceeding those parameters in steady-state conditions (up to ∼ 5 s or 12 τE) in a reproducible way and only limited by the duration of the additional heating phase. These results have been obtained (a) in highly triangular plasmas, by increasing the average triangularity δ towards the ITER reference value (δ ∼ 0.5), and (b) in plasmas at low triangularity (δ ∼ 0.2) by seeding of Ar and placing the X-point of the plasma on the top of the septum. Pellet injection from the high field side is a third method yielding high density and high confinement, albeit not yet under steady-state conditions. In highly triangular plasmas the influence of input power, plasma triangularity and impurity seeding with noble gases has been studied. Density profile peaking at high densities has been obtained in (a) impurity seeded low triangularity discharges, (b) ELMy H-modes with low levels of input power and (c) discharges fuelled with pellet injection from the high field side. New ELM behaviour has been observed in high triangularity discharges at high density, opening a possible route to ELM heat load mitigation, which can be further amplified by Ar impurity seeding. Current extrapolations of the ELM heat load to ITER show possibly a window for Type I ELM operation. Confinement scaling studies indicate an increase in confinement with triangularity and density peaking, and a decrease in confinement with the Greenwald number. In addition, experiments in H isotope and He indicate τE ∝ M0.19Z−0.59. The threshold power for the L–H transition in He plasmas shows the same parametric dependence as in D plasmas, but with a 50% higher absolute value.

Accessibility and properties of ELMy H-mode and ITB plasmas in TCV

This paper presents experimental results on the accessibility and the properties of plasmas with improved confinement in TCV. First, the H-mode threshold power is measured in Ohmic plasmas. Above an Ohmic threshold density, the threshold power increases with the density. A lower threshold density is found when additional electron cyclotron heating (ECH) is applied. At these low densities, the threshold power increases dramatically with decreasing density. Only a small fraction of the wide operational domain leading to the Ohmic H-mode is found to lead to a stationary regime with edge localized modes (ELMs). The ELMs have an irregular frequency, but in TCV they can be triggered by an external magnetic perturbation that induces a rapid vertical movement of the plasma. With this perturbation, the ELM frequency can be increased. The ELM triggering mechanism is provided by the vertical movement of the plasma away from the X-point of a single null configuration. This movement induces a positive current at the plasma edge, and we deduce that the ELMs are being controlled by this modification of the plasma edge current.Electron internal transport barriers (eITBs) are produced deep in the plasma during the stationary phase of TCV discharges. Different scenarios of ECH or electron cyclotron current drive (ECCD) at different radial locations have been used to obtain eITBs with and without inductively driven current. The eITBs are characterized by steep electron temperature gradients, high confinement improvement and a large fraction of bootstrap current. In plasmas with fully non-inductively driven current the size and the strength of the eITB are controlled by the location of the power deposition and by the co- or counter-direction of the central ECCD. Finally, a small inductive perturbation of an otherwise non-inductively driven plasma current profile progressively shrinks the eITB, confirming the link between current profiles and eITBs.

Integrated predictive modelling of the effect of neutral gas puffing in ELMy H-mode plasmas

The effect of neutral gas puffing in ELMy H-mode plasmas is studied using integrated predictive transport modelling, which is needed because of the strong link between the plasma core, the edge transport barrier and the scrape-off layer. Self-consistent results are obtained by coupling the one-dimensional core transport code JETTO with the two-dimensional edge code EDGE2D. In addition, magnetohydrodynamic (MHD) stability analysis is performed on the output of the transport simulations. The results of the MHD stability analysis is used to adjust MHD stability limits in the transport modelling, so that there is a feedback loop between the transport codes and the MHD stability codes. It is shown that strong gas puffing causes a sequence of causalities involving edge density, collisionality, bootstrap current, total edge current and magnetic shear, eventually triggering a transition from second to first ideal (or resistive) n = ∞ ballooning stability. Qualitatively, the transition from second to first ballooning stability resembles the experimentally observed transition from type I to type III ELMy H-mode with the accompanying increase in ELM frequency and deterioration of plasma confinement.