Showing papers in "Nuclear Fusion in 2003"

Electron and photon production from relativistic laser–plasma interactions

Abstract: The interaction of short and intense laser pulses with plasmas is a very efficient source of relativistic electrons with tunable properties. In low-density plasmas, we observed bunches of electrons up to 200 MeV, accelerated in the wakefield of the laser pulse. Less energetic electrons (tens of megaelectronvolt) have been obtained, albeit with a higher efficiency, during the interaction with a pre-exploded foil or a solid target. When these relativistic electrons slow down in a thick tungsten target, they emit very energetic Bremsstrahlung photons which have been diagnosed directly with photoconductors, and indirectly through photonuclear activation measurements. Dose, photoactivation, and photofission measurements are reported. These results are in reasonable agreement, over three orders of magnitude, with a model built on laser–plasma interaction and electron transport numerical simulations.

Achievement of high fusion triple product, steady-state sustainment and real-time NTM stabilization in high-βp ELMy H-mode discharges in JT-60U

Abstract: This paper reports results on the progress in steady-state high-βp ELMy H-mode discharges in JT-60U. A fusion triple product, nD(0)τETi(0), of 3.1 × 1020 m−3 s keV under full non-inductive current drive has been achieved at Ip = 1.8 MA, which extends the record value of the fusion triple product under full non-inductive current drive by 50%. A high-beta plasma with βN ~ 2.7 has been sustained for 7.4 s (~60τE), with the duration determined only by the facility limits, such as the capability of the poloidal field coils and the upper limit on the duration of injection of neutral beams. Destabilization of neoclassical tearing modes (NTMs) has been avoided with good reproducibility by tailoring the current and pressure profiles. On the other hand, a real-time NTM stabilization system has been developed where detection of the centre of the magnetic island and optimization of the injection angle of the electron cyclotron wave are done in real time. By applying this system, a 3/2 NTM has been completely stabilized in a high-beta region (βp ~ 1.2, βN ~ 1.5), and the beta value and confinement enhancement factor have been improved by the stabilization.

Global gyrokinetic simulation of ion temperature gradient driven turbulence in plasmas using a canonical Maxwellian distribution

Abstract: A new gyrokinetic toroidal particle code has been developed to study the ion temperature gradient (ITG) driven turbulence in reactor relevant tokamak parameters. We use a new method based on a canonical Maxwellian distribution FCM(P,e,μ), which is defined by three constants of motion in the axisymmetric toroidal system—the canonical angular momentum P, the energy e, and the magnetic moment μ. A quasi-ballooning representation enables linear and nonlinear high-m,n global calculations to be carried out, with a good numerical convergence. Conservation properties are improved by using optimized particle loading. From comprehensive linear global analyses over a wide range of unstable toroidal mode numbers (n = 0–100) in large tokamak parameters (a/ρti = 320–460), it is found that the reversed shear configuration produces an effective stabilizing effect on the ITG mode in the q min region through global effects. In the nonlinear simulation, it is found that the new method based on FCM can simulate a zonal flow damping correctly; and spurious zonal flow oscillations, which are observed in a conventional method based on a local Maxwellian distribution FLM(ψ,e,μ), do not appear in the nonlinear regime.

Internal transport barrier triggering by rational magnetic flux surfaces in tokamaks

Abstract: The formation of internal transport barriers (ITBs) has been experimentally associated with the presence of rational q surfaces in both JET and ASDEX Upgrade. The triggering mechanisms are related to the occurrence of magneto-hydrodynamic (MHD) instabilities such as mode coupling and fishbone activity. These events could locally modify the poloidal velocity and increase transiently the shearing rate to values comparable with the linear growth rate of ion temperature gradient modes. For JET reversed magnetic shear scenarios, ITB emergence occurs preferentially when the minimum q reaches an integral value. In this case, transport effects localized in the vicinity of zero magnetic shear and close to rational q values may be at the origin of ITB formation. The role of rational q surfaces in ITB triggering stresses the importance of q profile control for an advanced tokamak scenario and could assist in substantially lowering the access power to these scenarios in next step facilities.

ELM frequency control by continuous small pellet injection in ASDEX upgrade

Abstract: Injection of cryogenic deuterium pellets has been successfully applied in ASDEX Upgrade for external edge localized mode (ELM) frequency control in type-I ELMy H-mode discharge scenarios. A pellet velocity of 560 m s−1 and a size of about 6 × 1019 D-atoms was selected for technical reasons, although even lower masses were found sufficient to trigger ELMs. A moderate repetition rate close to 20 Hz was chosen to avoid over-fuelling of the core plasma. Pellet sequences of up to 4 s duration were injected into discharges close to the L–H threshold, intrinsically developing large compound ELMs at a rate of 3 Hz. With pellet injection, these large ELMs were completely replaced by smaller type-I ELMs at the much higher pellet frequency, accompanied by a slight increase of density and even of stored energy. This external ELM control could be repeatedly switched on and off by just interrupting the pellet train. ELMs were triggered in less than 200 µs after pellet arrival at the plasma edge, at which time only a fraction of the pellet has been ablated, forming a rather localized, three-dimensional plasmoid, which drives the edge unstable well before the deposited mass is spread toroidally. The pellet controlled case has also been compared with a discharge at a somewhat lower density, but with otherwise rather similar data, developing spontaneous 20 Hz type-I ELMs. Despite the different trigger mechanisms, the general ELM features turn out to be qualitatively similar, possibly because of the similarity of the two cases in terms of ELM relevant parameters. The scaling with background plasma, heating power, pellet launch parameters, etc over a larger range still remains to be investigated.

Scaling laws for edge plasma parameters in ITER from two-dimensional edge modelling

Abstract: Results of a detailed study of the parameter space of the ITER divertor with the B2-Eirene code are presented. Relations between plasma parameters at the separatrix, the interface between the core and edge plasma, are parametrized to provide a set of boundary conditions for the core models. The reference ITER divertor geometry is compared with the straight target option, and the possibility of controlling the edge density by shifting the plasma equilibrium in ITER is explored.

Simulations of steady-state scenarios for Tore Supra using the CRONOS code

Abstract: Scenarios of steady-state, fully non-inductive current in Tore Supra are predicted using a package of simulation codes (CRONOS). The plasma equilibrium and transport are consistently calculated with the deposition of power. The achievement of high injected energy discharges up to 1 GJ is shown. Two main scenarios are considered: a low density regime with 90% non-inductive current driven by lower hybrid waves-lower hybrid current drive (LHCD)-and a high density regime combining LHCD and ion cyclotron resonance heating with a bootstrap current fraction up to 25%. The predictive simulations of existing discharges are also reported.

Effects of localized electron heating and current drive on the sawtooth period

Abstract: Localized electron heating and current drive, like those produced by electron cyclotron heating (ECH) systems, are powerful tools for controlling the sawtooth period. They allow the direct modification of the plasma parameters which determine the sawtooth stability. In this paper we report a set of new experimental results obtained in the Tokamak a Configuration Variable (TCV) and a set of related simulations obtained applying a sawtooth period model in a transport code. The TCV device, equipped with a very flexible and powerful ECH system, is specifically suited for this kind of study. In previous works, the experimental behaviour observed in TCV and JET was found consistent with a sawtooth period model first proposed to predict the sawtooth period in burning plasmas. In this paper, new experimental results have motivated a set of simulations which allow the identification of the effects of localized heating and current drive separately. In particular, two heating locations exist at opposite sides of the q = 1 surface which allow most efficiently sawtooth stabilization and destabilization. Moreover, the modelling shows that counter- and co-current drive alone, without the presence of heating, have opposite effects on the sawtooth period at symmetrical locations as compared with the position of the q = 1 surface. The main features of the experimental behaviour can be explained as due to the modification of the local plasma parameters involved in the linear resistive stability threshold of the internal kink, in particular the dynamics of the magnetic shear at the q = 1 surface. However it is shown that the most effective locations to modify the sawtooth period are not exactly at q = 1.

Recent advances in the LHD experiment

Abstract: In the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (> 10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island (n/m = 1/1) is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.

Overview of quasi-single helicity experiments in reversed field pinches

Abstract: We report the results of an experimental and theoretical international project dedicated to the study of quasi-single helicity (QSH) reversed field pinch (RFP) plasmas. The project has involved several RFP devices and numerical codes. It appears that QSH spectra are a robust feature common to all the experiments. Our results expand and reinforce the evidence that the formation of self-organized states with one dominant helical mode (Ohmic SH state) is an approach complementary to that of active control of magnetic turbulence to improve confinement in a steady state RFP.

Role of sawtooth in avoiding impurity accumulation and maintaining good confinement in JET radiative mantle discharges

Abstract: Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in re-distributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At and high Ar injection rates, quasi-steady-states, which previously only lasted <1τE, were now maintained for the duration of the heating (Δ t ~ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.

Stationary high-performance discharges in the DIII-D tokamak

Abstract: Discharges which can satisfy the high gain goals of burning plasma experiments have been demonstrated in the DIII-D tokamak under stationary conditions at relatively low plasma current (q95>4). A figure of merit for fusion gain (βNH89/q952) has been maintained at values corresponding to Q = 10 operation in a burning plasma for >6 s or 36τE and 2τR. The key element is the relaxation of the current profile to a stationary state with qmin>1. In the absence of sawteeth and fishbones, stable operation has been achieved up to the estimated no-wall β limit. Feedback control of the energy content and particle inventory allow reproducible, stationary operation. The particle inventory is controlled by gas fuelling and active pumping; the wall plays only a small role in the particle balance. The reduced current lessens significantly the potential for structural damage in the event of a major disruption. In addition, the pulse length capability is greatly increased, which is essential for a technology testing phase of a burning plasma experiment where fluence (duty cycle) is important.

Multi-mode error field correction on the diii-d tokamak

Abstract: Error field optimization on DIII-D tokamak (Luxon J.L. 2002 Nucl. Fusion 42 814) plasma discharges has routinely been done for the last ten years with the use of the external `n = 1 coil' or the `C-coil'. The optimum level of correction coil current is determined by the ability to avoid the locked mode instability and access previously unstable parameter space at low densities. The locked mode typically has toroidal and poloidal mode numbers n = 1 and m = 2, respectively, and it is this component that initially determined the correction coil current and phase. Realization of the importance of nearby n = 1 mode components m = 1 and m = 3 has led to a revision of the error field correction algorithm. Viscous and toroidal mode coupling effects suggested the need for additional terms in the expression for the radial `penetration' field Bpen that can induce a locked mode. To incorporate these effects, the low density locked mode threshold database was expanded. A database of discharges at various toroidal fields, plasma currents, and safety factors was supplemented with data from an experiment in which the fields of the n = 1 coil and C-coil were combined, allowing the poloidal mode spectrum of the error field to be varied. Using this technique to optimize the error field reveals a new and clear low density limit determined by the onset of runaway electrons, not locked modes. A multi-variate regression analysis of this new low density locked mode database was done to determine the low density locked mode threshold scaling relationship ne∝BT−0.01q95−0.79Bpen and the coefficients of the poloidal mode components in the expression for Bpen. Improved plasma performance is achieved by optimizing Bpen by varying the applied correction coil currents.

Fusion energy with lasers, direct drive targets, and dry wall chambers

Abstract: A coordinated, focused effort is underway to develop Laser Inertial Fusion Energy. The key components are developed in concert with one another and the science and engineering issues are addressed concurrently. Recent advances include: target designs have been evaluated that show it could be possible to achieve the high gains (>100) needed for a practical fusion system.These designs feature a low-density CH foam that is wicked with solid DT and over-coated with a thin high-Z layer. These results have been verified with three independent one-dimensional codes, and are now being evaluated with two- and three-dimensional codes. Two types of lasers are under development: Krypton Fluoride (KrF) gas lasers and Diode Pumped Solid State Lasers (DPSSL). Both have recently achieved repetitive 'first light', and both have made progress in meeting the fusion energy requirements for durability, efficiency, and cost. This paper also presents the advances in development of chamber operating windows (target survival plus no wall erosion), final optics (aluminium at grazing incidence has high reflectivity and exceeds the required laser damage threshold), target fabrication (demonstration of smooth DT ice layers grown over foams, batch production of foam shells, and appropriate high-Z overcoats), and target injection (new facility for target injection and tracking studies).

Real-time control of the q-profile in JET for steady state advanced tokamak operation

Abstract: In order to simultaneously control the current and pressure profiles in high performance tokamak plasmas with internal transport barriers (ITB), a multi-variable model-based technique has been proposed. New algorithms using a truncated singular value decomposition (TSVD) of a linearized model operator and retaining the distributed nature of the system have been implemented in the JET control system. Their simplest versions have been applied to the control of the current density profile in reversed shear plasmas using three heating and current drive actuators (neutral beam injection, ion cyclotron resonant frequency heating and lower hybrid current drive). Successful control of the safety factor profile has been achieved in the quasi-steady-state, on a timescale of the order of the current redistribution time. How the TSVD algorithm will be used in the forthcoming campaigns for the simultaneous control of the current profile and of the ITB temperature gradient is discussed in some detail, but this has not yet been attempted in the present pioneering experiments.

Development of 170 and 110 GHz gyrotrons for fusion devices

Abstract: Development of 170 and 110 GHz gyrotrons and their application to electron cyclotron heating systems are presented. A parasitic oscillation that degraded the electron beam quality was suppressed, and the performance of the gyrotron improved significantly. Up to now, 0.9 MW/9.2 s, 0.5 MW/30 s, 0.3 MW/60 s, 0.2 MW/132 s, etc, have been demonstrated at 170 GHz. At 110 GHz, 1.3 MW/1.5 s, 1.2 MW/4.1 s, 1 MW/5 s were obtained. It is found that the reduction of stray radiation and the enhancement of cooling capability are important for continuous wave operation. Four 110 GHz gyrotrons are under operation in the electron cyclotron heating and current drive system of JT-60U. Power up to approximately 3 MW/2.7 s was injected into the plasma through movable mirrors in the poloidal direction and contributed to electron heating and the suppression of the neo-classical tearing modes.

Design and technology development of solid breeder blanket cooled by supercritical water in Japan

Abstract: This paper presents results of conceptual design activities and associated RD neutronics experiments and analyses; and development of the blanket module fabrication technology. Upon developing the fabrication technology for the first wall and box structure, a hot isostatic pressing bonded F82H first wall mock-up with embedded rectangular cooling channels was successfully fabricated. It withstood the high heat flux test at 2.7 MW m−2. Also, a correlation parameter of the Li2TiO3 pebble bed made by the sol–gel method was verified by measurement of the thermal conductivity of the breeder pebble bed, which is one of the most important design data.

β-Limiting MHD instabilities in improved-performance NSTX spherical torus plasmas

Abstract: Global magnetohydrodynamic (MHD) stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized β and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable β. As a result of these improvements, peak β values have reached (not simultaneously) βT = 35%, βN = 6.4, βN = 4.5, βN/li = 10, and βP = 1.4. High βP operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 s with sustained periods of βN≈6 above the ideal no-wall limit and near the with-wall limit. Details of the β-limit scalings and β-limiting instabilities in various operating regimes are described.

Resistive wall stabilization of high beta plasmas in diii-d

Abstract: Recent DIII?D experiments show that ideal kink-modes can be stabilized at high beta by a resistive wall, with sufficient plasma rotation. However, the resonant response to static magnetic field asymmetries by a marginally stable resistive wall mode can lead to strong damping of the rotation. Careful reduction of such asymmetries has allowed plasmas with beta well above the ideal MHD no-wall limit, and approaching the ideal-wall limit, to be sustained for durations exceeding 1?s. Feedback control can improve plasma stability by direct stabilization of the resistive wall mode or by reducing magnetic field asymmetry. Assisted by plasma rotation, direct feedback control of resistive wall modes with growth rates more than five times faster than the characteristic wall time has been observed. These results open a new regime of tokamak operation above the free-boundary stability limit, accessible by a combination of plasma rotation and feedback control.

Relationship between particle and heat transport in JT-60U plasmas with internal transport barrier

Abstract: The relationship between particle and heat transport in an internal transport barrier (ITB) has been systematically investigated in reversed shear (RS) and high βp mode plasmas of JT-60U. The electron effective diffusivity is well correlated with the ion thermal diffusivity in the ITB region. The ratio of particle flux to electron heat flux, calculated on the basis of the linear stability analysis, shows a similar tendency to an experiment in the RS plasma with a strong ITB. However, the calculated ratio of ion anomalous heat flux to electron heat flux is smaller than the experiment in the ITB region. Helium and carbon are not accumulated inside the ITB even with ion heat transport close to a neoclassical level, but argon is accumulated. The helium diffusivity (DHe) and the ion thermal diffusivity (χi) are 5–15 times higher than the neoclassical level in the high βp mode plasma. In the RS plasma, DHe is reduced from 6–7 times to a 1.4–2 times higher level than the neoclassical level when χi is reduced from 7–18 times to a 1.2–2.6 times higher level than the neoclassical level. The carbon and argon diffusivities estimated assuming the neoclassical inward convection velocity are 4–5 times larger than the neoclassical value, even when χi is close to the neoclassical level. Argon exhaust from the inside of the ITB is demonstrated by applying electron cyclotron heating (ECH) in the high βp mode plasma, where both electron and argon density profiles become flatter. The flattening of the argon density profile is consistent with the reduction of the neoclassical inward convection velocity due to the reduction of the bulk plasma density gradient. In the RS plasma, the density gradient is not decreased by ECH and argon is not exhausted. These results suggest the importance of density gradient control in suppressing impurity accumulation.

Anomalies in the applied magnetic fields in DIII-D and their implications for the understanding of stability experiments

Abstract: Small non-axisymmetric magnetic fields are known to cause serious loss of stability in tokamaks, leading to loss of confinement and abrupt termination of plasma current (disruptions). The best known examples are the locked mode and the resistive wall mode. Understanding of the underlying field anomalies (departures in the hardware-related fields from ideal toroidal and poloidal fields on a single axis) and the interaction of the plasma with them is crucial to tokamak development. Results of both locked mode experiments (Scoville J.T. and La Haye R.J. 2003 Nucl. Fusion 43 250) and resistive wall mode experiments (Garofalo A.M., La Haye R.J. and Scoville J.T. 2002 Nucl. Fusion 42 1335) done in DIII-D tokamak plasmas have been interpreted to indicate the presence of a significant anomalous field. New measurements of the magnetic field anomalies of the hardware systems have been made in DIII-D. The measured field anomalies due to the plasma shaping coils in DIII-D are smaller than previously reported (La Haye R.J. and Scoville J.T. 1991 Rev. Sci. Instrum. 61 2146). Additional evaluations of systematic errors have been made. New measurements of the anomalous fields of the Ohmic heating and toroidal coils have been added. Such detailed in situ measurements of the fields of a tokamak are unique. The anomalous fields from all the coils are one-third the values indicated from the stability experiments (Garofalo et al 2002, Scoville and La Haye 2003). These results indicate limitations in the understanding of the interaction of the plasma with the external field. They indicate that it may not be possible to deduce the anomalous fields in a tokamak from plasma experiments and that we may not have the basis needed to project the error field requirements of future tokamaks.

A two-term model of the confinement in Elmy H-modes using the global confinement and pedestal databases

Abstract: Two different physical models of the H-mode pedestal are tested against the joint pedestal–core database. The first is a confinement model in which the transport down the steep edge gradient is assumed to be dominated by thermal conduction. The second model is a magnetohydrodynamics (MHD) limit model in which it is assumed that the dominant loss mechanism is by the edge localized modes (ELMs), the pressure gradient being determined by a MHD stability limit. These models are then combined with models for the core and shown to give a good fit to the ELMy H-mode database. The resulting two-term scaling expressions are shown to give very similar predictions for the confinement time, in the next step machines ITER and FIRE, to that of the one-term model IPB98(y,2). The predicted stored energy in the pedestal is 28–50% of the total stored energy.

Theoretical analysis of the influence of external biasing on long range turbulent transport in the scrape-off layer

Abstract: Two-dimensional fluid simulations of scrape-off layer (SOL) turbulence with non-constrained energy content (flux driven) are characterized by profile relaxation and strong outward bursts of density. The ballistic propagation extends well beyond the e-folding length of the SOL with a Mach number of M⊥ ~ 0.04. Turbulence stabilization is achieved by biasing part of the limiter surface. The critical radial extent to achieve this stabilization is derived. This effect governs the size of the biased ring required to insulate the wall from long range bursts of matter. The same characteristic scale also governs the critical size of Langmuir probe tips. For probe tips in excess of this size, the flux tube to the probe is found to be decoupled from the background plasma.

Progress towards steady-state operation and real-time control of internal transport barriers in JET

Abstract: In JET, advanced tokamak research mainly focuses on plasmas with internal transport barriers (ITBs) that are strongly influenced by the current density profile. A previously developed optimized shear regime with low magnetic shear in the plasma centre has been extended to deeply negative magnetic shear configurations. High fusion performance with wide ITBs has been obtained transiently with negative central magnetic shear configuration: HIPB98(y,2) ~ 1.9, βN = 2.4 at Ip = 2.5 MA. At somewhat reduced performance, electron and ion ITBs have been sustained in full current drive operation with 1 MA of bootstrap current: HIPB98(y,2) ~ 1, βN = 1.7 at Ip = 2.0 MA. The ITBs were maintained for up to 11 s for the latter case. This duration, much larger than the energy confinement time (37 times larger), is already approaching a current resistive time. New real-time measurements and feedback control algorithms have been developed and implemented in JET for successfully controlling the ITB dynamics and the current density profile in the highly non-inductive current regime.

Electron heat transport in ASDEX Upgrade: experiment and modelling

Abstract: The electron heat transport is investigated in ASDEX Upgrade using electron cyclotron heating (ECH) combining steady-state and power modulation schemes. Experiments in which the electron heat flux has been varied in the confinement region while the edge was kept constant were performed. They demonstrate that ∇ Te and ∇ Te/Te can be varied by a factor of 3 and 2, respectively. They allow a detailed determination of the transport characteristics by comparing steady-state and modulation data with modelling. The analyses clearly show the existence of a threshold (∇ Te/Te)crit above which transport increases. Both steady-state and modulation experiments agree with such a transport model. The experiments have been carried out at low density in the L-mode to ensure low electron–ion coupling and good conditions for studying electron heat transport. The experiments were carried out at two different values of plasma current and show that transport increases at low current, as well-known from global scaling laws for confinement time. In the pure off-axis cases the region inside the ECH deposition is just at the (∇ Te/Te)crit threshold, which allows it to be measured directly from the profile of ∇ Te/Te deduced from the experimental Te profile. Using this technique, it appears that the turbulence threshold agrees with that expected from the trapped electron mode driven turbulence. It has the correct absolute value and seems to have the correct radial dependence that is determined by the trapped electron fraction and by the density gradient. It almost does not vary with other plasma parameters. In contrast, the threshold calculated for electron temperature gradient modes is higher than the experimental values of ∇ Te/Te and this turbulence is therefore not expected to be excited under these experimental conditions.

Toroidal rotation in neutral beam heated discharges in DIII-D

Abstract: It is known that the toroidal angular momentum and the ion thermal energy are correlated in tokamak discharges heated by neutral beam injection Here, data from ten years of measurements on DIII-D are considered, for representative discharges from all types and all conditions The ratio of simple replacement times for momentum and energy is found to order this correlation indicating that these times are approximately equal, across the minor radius Representative discharges of several types are discussed in more detail, as well as transport analysis results for the momentum and thermal ion diffusivities

Onset of neoclassical tearing modes on JET

Abstract: A detailed study has been made of the onset and behaviour of (m = 3, n = 2) neoclassical tearing modes (NTMs) on JET, validating many of the underlying concepts in NTM physics. In particular, fitting onset thresholds in terms of the correctly motivated physics parameters produces scalings closer to theoretical expectations. The use of such locally measured parameters also removes previous inconsistencies in the data between different divertor configurations. The evolution of island size with β confirms the predicted levels of bootstrap current, and highlights the role of small island size stabilization terms, which give rise to a seeding requirement for NTMs from some other instability. This seeding requirement is confirmed, and is generally provided by sawteeth in the cases studied here. Although there are a variety of possible seeding mechanisms, for the particular cases studied here, it appears doubtful that the seeds are induced by magnetic coupling. In particular the NTM always appears well below the n = 2 sawtooth precursor harmonic frequency—although they often lock together once the NTM has grown close to saturation. Also, while plasma rotation profiles are favourable for a three wave interaction involving (4, 3) and n = 1 modes, with irregular matches to the appropriate mode frequency condition, there is no clear correlation between such matches and (3, 2) NTM growth rates. Thus, other seeding models should be considered—we have discussed two possibilities: a temporary change to classical tearing stability due to the sawtooth, and flow variation leading to a change in ion polarization current polarity, but neither of these are yet confirmed by the data. A complete resolution of this issue is essential if a predictive theory of NTMs is to be obtained, but it requires further, more detailed experimental measurements, and development of theoretical models. Finally, the nature of the (4, 3) NTM has also been explored. This mode also requires a seed perturbation to be triggered (in these cases supplied by a sawtooth), and occurs at similar local parameter values as the (3, 2) NTM, but with lower heating powers and βN. A cascade process emerges in which successively lower mode number NTMs occur as β is raised.

Tokamak-like confinement at a high beta and low toroidal field in the MST reversed field pinch

Abstract: Energy confinement comparable with tokamak quality is achieved in the Madison Symmetric Torus (MST) reversed field pinch (RFP) at a high beta and low toroidal magnetic field. Magnetic fluctuations normally present in the RFP are reduced via parallel current drive in the outer region of the plasma. In response, the electron temperature nearly triples and beta doubles. The confinement time increases ten-fold (to ~10 ms), which is comparable with L- and H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Runaway electron confinement is evidenced by a 100-fold increase in hard x-ray bremsstrahlung. Fokker–Planck modelling of the x-ray energy spectrum reveals that the high energy electron diffusion is independent of the parallel velocity, uncharacteristic of magnetic transport and more like that for electrostatic turbulence. The high core electron temperature correlates strongly with a broadband reduction of resonant modes at mid-radius where the stochasticity is normally most intense. To extend profile control and add auxiliary heating, rf current drive and neutral beam heating are in development. Low power lower-hybrid and electron Bernstein wave injection experiments are underway. Dc current sustainment via ac helicity injection (sinusoidal inductive loop voltages) is also being tested. Low power neutral beam injection shows that fast ions are well-confined, even in the presence of relatively large magnetic fluctuations.

Burning plasma projections using drift wave transport models and scalings for the h-mode pedestal

Abstract: The GLF23 and multi-mode core transport models are used along with models for the H-mode pedestal to predict the fusion performance for the International Thermonuclear Experimental Reactor, Fusion Ignition Research Experiment, and IGNITOR tokamak designs. Simulations using combinations of core and pedestal models have also been compared with experimental data for H-mode profiles in DIII-D, JET, and Alcator C-Mod. Power-independent (ballooning mode limit) and power-dependent pedestal scalings lead to very different predictions when used with the core models. Although the two drift-wave transport models reproduce the core profiles in a wide variety of tokamak discharges, they differ in their projections to burning plasma experiments for the same pedestal parameters. Differences in the core transport models in their response to the ion temperature gradient (i.e. their stiffness) and impact of the power dependence of the H-mode pedestal on fusion performance predictions are discussed.

Neural-net disruption predictor in JT-60U

Abstract: The prediction of major disruptions caused by the density limit, the plasma current ramp-down with high internal inductance li, the low density locked mode and the β-limit has been investigated in JT-60U. The concept of 'stability level', newly proposed in this paper to predict the occurrence of a major disruption, is calculated from nine input parameters every 2 ms by the neural network and the start of a major disruption is predicted when the stability level decreases to a certain level, the 'alarm level'. The neural network is trained in two steps. It is first trained with 12 disruptive and six non-disruptive shots (total of 8011 data points). Second, the target output data for 12 disruptive shots are modified and the network is trained again with additional data points generated by the operator. The 'neural-net disruption predictor' obtained has been tested for 300 disruptive shots (128 945 data points) and 1008 non-disruptive shots (982 800 data points) selected from nine years of operation (1991–1999) of JT-60U. Major disruptions except for those caused by the β-limit have been predicted with a prediction success rate of 97–98% at 10 ms prior to the disruption and higher than 90% at 30 ms prior to the disruption while the false alarm rate is 2.1% for non-disruptive shots. This prediction performance has been confirmed for 120 disruptive shots (56 163 data points), caused by the density limit, as well as 1032 non-disruptive shots (1004 611 data points) in the last four years of operation (1999–2002) of JT-60U. A careful selection of the input parameters supplied to the network and the newly developed two-step training of the network have reduced the false alarm rate resulting in a considerable improvement of the prediction success rate.