[Show abstract][Hide abstract] ABSTRACT: The amplitude of locked instabilities, likely magnetic islands, seen as precursors to disruptions has
been studied using data from the JET, ASDEX Upgrade and COMPASS tokamaks. It was found
that the thermal quench, that often initiates the disruption, is triggered when the amplitude has
reached a distinct level. This information can be used to determine thresholds for simple disruption
prediction schemes. The measured amplitude in part depends on the distance of the perturbation to
the measurement coils. Hence the threshold for the measured amplitude depends on the mode
location (i.e. the rational q-surface) and thus indirectly on parameters such as the edge safety factor,
q95, and the internal inductance, li(3), that determine the shape of the q-profile. These dependencies
can be used to set the disruption thresholds more precisely. For the ITER baseline scenario, with
typically q95=3.2, li(3)=0.9 and taking into account the position of the measurement coils on ITER ,
the maximum allowable measured locked mode amplitude normalized to engineering parameters
was estimated to be a·BML(rc)/Ip = 0.92 m mT/MA, or directly as a fraction edge poloidal magnetic
field: BML(rc)/Bq(a) = 5·10-3. But these values decrease for operation at higher q95 or lower li(3). The
analysis found furthermore that the above empirical criterion to trigger a thermal quench is more
consistent with a criterion derived with the concept of a critical island size, i.e. the thermal quench
seemed to be triggered at a distinct island width.
[Show abstract][Hide abstract] ABSTRACT: In Hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is
found that n>1 tearing activity can significantly enhance the rate of on-axis
peaking of tungsten impurities, which in turn significantly degrades discharge
performance. Core n=1 instabilities can be beneficial in removing tungsten
impurities from the plasma core (e.g. sawteeth or fishbones), but can
conversely also degrade core confinement (particularly in combination with
simultaneous n=3 activity). The nature of MHD instabilities in JET Hybrid
discharges, with both its previous Carbon wall and subsequent JET-ILW, is
surveyed statistically and the character of the instabilities is examined.
Possible qualitative models for how the n>1 islands can enhance on-axis
tungsten transport accumulation processes are presented.
[Show abstract][Hide abstract] ABSTRACT: Asymmetrical disruptions may occur during ITER operation and they may be accompanied by large sideways forces and rotation of the asymmetry. This is of particular concern because resonance of the rotating asymmetry with the natural frequencies of the vacuum vessel (and other in-vessel components) could lead to large dynamic amplification of the forces. A significant fraction of non-mitigated JET disruptions have toroidally asymmetric currents that flow partially inside the plasma and partially inside the surrounding vacuum vessel ('wall'). The toroidal asymmetries (otherwise known as the appearance of 3D structures) are clearly visible in the plasma current (Ip) and the first plasma current moments. For the first time we present here the asymmetries in toroidal flux measured by the diamagnetic loops and also propose a physical interpretation. The presented data covers the period of JET operation with a C-wall (JET-C from 2005 until late 2009) and with an ITER-like wall (JET-ILW from 2011 until late 2014), during which pick-up coil and saddle loop data at four toroidally orthogonal locations were routinely recorded. The observed rotations of the Ip asymmetries are in the range from -5 turns to +10 turns (a negative value is counted to the negative plasma current). Initial observations on COMPASS of asymmetric disruptions are presented, which are in line with JET data. The whole of the JET-ILW disruption database and the limited number of COMPASS disruptions examined confirm that the development of the toroidal asymmetry precedes the drop to unity of q95. It is shown that massive gas injection (MGI), which is routinely used to mitigate disruptions, significantly reduces the Ip asymmetries in JET. However, MGI produces fast plasma current quench and consequently high vessel eddy currents, which expose the machine to additional stresses. The effect of the large gas quantity used during the injection is of particular concern as well.
[Show abstract][Hide abstract] ABSTRACT: The Mega Ampère Spherical Tokamak (MAST) programme is strongly focused on addressing key physics issues in preparation for operation of ITER as well as providing solutions for DEMO design choices. In this regard, MAST has provided key results in understanding and optimizing H-mode confinement, operating with smaller edge localized modes (ELMs), predicting and handling plasma exhaust and tailoring auxiliary current drive. In all cases, the high-resolution diagnostic capability on MAST is complemented by sophisticated numerical modelling to facilitate a deeper understanding. Mitigation of ELMs with resonant magnetic perturbations (RMPs) with toroidal mode number n RMP = 2, 3, 4, 6 has been demonstrated: at high and low collisionality; for the first ELM following the transition to high confinement operation; during the current ramp-up; and with rotating n RMP = 3 RMPs. n RMP = 4, 6 fields cause less rotation braking whilst the power to access H-mode is less with n RMP = 4 than n RMP = 3, 6. Refuelling with gas or pellets gives plasmas with mitigated ELMs and reduced peak heat flux at the same time as achieving good confinement. A synergy exists between pellet fuelling and RMPs, since mitigated ELMs remove fewer particles. Inter-ELM instabilities observed with Doppler backscattering are consistent with gyrokinetic simulations of micro-tearing modes in the pedestal. Meanwhile, ELM precursors have been strikingly observed with beam emission spectroscopy (BES) measurements. A scan in beta at the L–H transition shows that pedestal height scales strongly with core pressure. Gyro-Bohm normalized turbulent ion heat flux (as estimated from the BES data) is observed to decrease with increasing tilt of the turbulent eddies. Fast ion redistribution by energetic particle modes depends on density, and access to a quiescent domain with ‘classical’ fast ion transport is found above a critical density. Highly efficient electron Bernstein wave current drive (1 A W −1 ) has been achieved in solenoid-free start-up. A new proton detector has characterized escaping fusion products. Langmuir probes and a high-speed camera suggest filaments play a role in particle transport in the private flux region whilst coherence imaging has measured scrape-off layer (SOL) flows. BOUT++ simulations show that fluxes due to filaments are strongly dependent on resistivity and magnetic geometry of the SOL, with higher radial fluxes at higher resistivity. Finally, MAST Upgrade is due to begin operation in 2016 to support ITER preparation and importantly to operate with a Super-X divertor to test extended leg concepts for particle and power exhaust.
[Show abstract][Hide abstract] ABSTRACT: The effects of flow shear on the stability of a (2,1) tearing mode are examined using numerical and analytic studies on a number of model systems. For a cylindrical reduced magnetohydrodynamic (MHD) model, linear computations using the CUTIE code show that sheared axial flows have a destabilizing effect, while sheared poloidal flows tend to reduce the growth rate of the mode. These effects are independent of the direction of the flow. For helical flows the sign of the shear in the flow matters. This symmetry breaking is also seen in the nonlinear regime where the island saturation level is found to depend on the sign of the flows. In the absence of flow, the CUTIE simulations show that the linear mode is more stable in a two fluid as compared to a single fluid model. However, in the presence of sheared axial flows a negative sheared flow is more destabilizing while a positive sheared flow is more stabilizing, compared to the single fluid model. In contrast to the cylindrical model, simulations in a toroidal model, using the MHD code NEAR, always show a stabilizing effect in the presence of a sheared toroidal flow. This is understood analytically in terms of a flow induced 'Shafranov' like shift in the profiles of the equilibrium current that results in a stabilizing change in Δ' and the saturated island size.
[Show abstract][Hide abstract] ABSTRACT: The behaviour of tungsten in the core of hybrid scenario plasmas in JET with the ITER-like wall is analysed and modelled with a combination of neoclassical and gyrokinetic codes. In these discharges, good confinement conditions can be maintained only for the first 2–3 s of the high power phase. Later W accumulation is regularly observed, often accompanied by the onset of magneto-hydrodynamical activity, in particular neoclassical tearing modes (NTMs), both of which have detrimental effects on the global energy confinement. The dynamics of the accumulation process is examined, taking into consideration the concurrent evolution of the background plasma profiles, and the possible onset of NTMs. Two time slices of a representative discharge, before and during the accumulation process, are analysed with two independent methods, in order to reconstruct the W density distribution over the poloidal cross-section. The same time slices are modelled, computing both neoclassical and turbulent transport components and consistently including the impact of centrifugal effects, which can be significant in these plasmas, and strongly enhance W neoclassical transport. The modelling closely reproduces the observations and identifies inward neoclassical convection due to the density peaking of the bulk plasma in the central region as the main cause of the accumulation. The change in W neoclassical convection is directly produced by the transient behaviour of the main plasma density profile, which is hollow in the central region in the initial part of the high power phase of the discharge, but which develops a significant density peaking very close to the magnetic axis in the later phase. The analysis of a large set of discharges provides clear indications that this effect is generic in this scenario. The unfavourable impact of the onset of NTMs on the W behaviour, observed in several discharges, is suggested to be a consequence of a detrimental combination of the effects of neoclassical transport and of the appearance of an island.
[Show abstract][Hide abstract] ABSTRACT: A key feature of disruptions during vertical displacement events, discovered in JET in 1996, is the toroidal variation in the measured plasma current I p , i.e. the plasma current asymmetries, lasting for almost the entire current quench. The unique magnetic diagnostics at JET (full set of poloidal coils and saddle loops recorded either from two toroidally opposite or from four toroidally orthogonal locations) allow for a comprehensive analysis of asymmetrical disruptions with a large scale database. This paper presents an analysis of 4854 disruptions over an 18 year period that includes both the JET carbon (C) wall and the ITER-like (IL) wall (a mixed beryllium/tungsten first wall). In spite of the I p quench time significantly increasing for the IL-wall compared to C-wall disruptions, the observed toroidal asymmetry time integral (∼ sideways force impulse), did not increase for IL-wall disruptions. The I p asymmetry has a dominantly n = 1 structure. Its motion in the toroidal direction has a sporadic behaviour, in general. The distributions of the number of rotation periods are found to be very similar for both C- and IL-wall disruptions, and multi-turn rotation was sometimes observed. The I p asymmetry amplitude has no degradation with rotation frequency for either the C- or IL-wall disruption. Therefore dynamic amplification remains a potentially serious issue for ITER due to possible mechanical resonance of the machine components with the rotating asymmetry.
[Show abstract][Hide abstract] ABSTRACT: A disruption of a tokamak discharge is a sudden loss of confinement, or thermal quench, in turn resulting
in a quench of the plasma current. The fast release of thermal and magnetic energy could result in very
large thermal and electromagnetic loads on the surrounding structures, such plasma facing components
or the vessel, especially in large devices such as JET and ITER. Understandably, considerable research
efforts are dedicated to develop both timely detectors of these events and mitigating actions.
Magneto-hydrodynamic (MHD) instabilities are often seen as precursors to disruptions. The
growth of large, overlapping, magnetic islands is thought to be behind the destruction of the flux
surface structure that provides the plasma confinement, triggering the thermal quench [1-4]. The
detection of these modes is used to predict disruptions. Usually the analysis of these instabilities
focuses on how early and at what level they can first be detected . This paper will investigate a
different but related question; is there a specific maximum perturbation level that triggers a thermal
quench? This study provides experimental insight in the processes that may trigger tokamak
disruptions. The perturbation amplitudes that trigger thermal quenches in JET and ASDEX Upgrade
are compared and the results form a strong physics basis to determine protection thresholds to be
used at future devices, such as ITER.
[Show abstract][Hide abstract] ABSTRACT: Calculations of tearing mode stability in tokamaks split conveniently
into one in an external region, where marginally stable ideal
magnetohydrodynamics (MHD) is applicable, and one in a resonant layer
around the rational surface where sophisticated kinetic physics is
needed. These two regions are coupled by the stability parameter
Δ‧. Axisymmetric pressure and current perturbations
localized around the rational surface significantly alter
Δ‧. Equations governing the changes in the external solution
and Δ‧ are derived for arbitrary perturbations in
axisymmetric toroidal geometry. These equations can be used in two ways:
(i) the Δ‧ can be calculated for a physically occurring
perturbation to the pressure or current; (ii) alternatively we can use
these equations to calculate Δ‧ for profiles with a pressure
gradient at the rational surface in terms of the value when the
perturbation removes this gradient. It is the second application we
focus on here since resistive magnetohydrodynamics (MHD) codes do not
contain the appropriate layer physics and therefore cannot predict
stability for realistic hot plasma directly. They can, however, be used
to calculate Δ‧. Existing methods (Ham et al 2012 Plasma
Phys. Control. Fusion 54 025009)
for extracting Δ‧ from resistive codes are unsatisfactory
when there is a finite pressure gradient at the rational surface and
favourable average curvature because of the Glasser stabilizing effect
(Glasser et al 1975 Phys. Fluids 18 875). To overcome this
difficulty we introduce a specific artificial pressure flattening
function that allows the earlier approach to be used. The technique is
first tested numerically in cylindrical geometry with an artificial
favourable curvature. Its application to toroidal geometry is then
demonstrated using the toroidal tokamak tearing mode stability code T7
(Fitzpatrick et al 1993 Nucl. Fusion 33 1533) which
employs an approximate analytic equilibrium. The prospects for applying
this approach to resistive MHD codes such as MARS-F (Liu et al 2000
Phys. Plasmas 7 3681)
which utilize a fully toroidal equilibrium are discussed.
Full-text · Article · Dec 2013 · Plasma Physics and Controlled Fusion
[Show abstract][Hide abstract] ABSTRACT: New diagnostic, modelling and plant capability on the Mega Ampère Spherical Tokamak (MAST) have delivered important results in key areas for ITER/DEMO and the upcoming MAST Upgrade, a step towards future ST devices on the path to fusion currently under procurement. Micro-stability analysis of the pedestal highlights the potential roles of micro-tearing modes and kinetic ballooning modes for the pedestal formation. Mitigation of edge localized modes (ELM) using resonant magnetic perturbation has been demonstrated for toroidal mode numbers n = 3, 4, 6 with an ELM frequency increase by up to a factor of 9, compatible with pellet fuelling. The peak heat flux of mitigated and natural ELMs follows the same linear trend with ELM energy loss and the first ELM-resolved Ti measurements in the divertor region are shown. Measurements of flow shear and turbulence dynamics during L–H transitions show filaments erupting from the plasma edge whilst the full flow shear is still present. Off-axis neutral beam injection helps to strongly reduce the redistribution of fast-ions due to fishbone modes when compared to on-axis injection. Low-k ion-scale turbulence has been measured in L-mode and compared to global gyro-kinetic simulations. A statistical analysis of principal turbulence time scales shows them to be of comparable magnitude and reasonably correlated with turbulence decorrelation time. Te inside the island of a neoclassical tearing mode allow the analysis of the island evolution without assuming specific models for the heat flux. Other results include the discrepancy of the current profile evolution during the current ramp-up with solutions of the poloidal field diffusion equation, studies of the anomalous Doppler resonance compressional Alfvén eigenmodes, disruption mitigation studies and modelling of the new divertor design for MAST Upgrade. The novel 3D electron Bernstein synthetic imaging shows promising first data sensitive to the edge current profile and flows.
[Show abstract][Hide abstract] ABSTRACT: Disruptions are a critical issue for ITER because of the high thermal and magnetic energies that are released on short timescales, which results in extreme forces and heat loads. The choice of material of the plasma-facing components (PFCs) can have significant impact on the loads that arise during a disruption. With the ITER-like wall (ILW) in JET made of beryllium in the main chamber and tungsten in the divertor, the main finding is a low fraction of radiation. This has dropped significantly with the ILW from 50–100% of the total energy being dissipated during disruptions in CFC wall plasmas, to less than 50% on average and down to just 10% for vertical displacement events (VDEs). All other changes in disruption properties and loads are consequences of this low radiation: long current quenches (CQs), high vessel forces caused by halo currents and toroidal current asymmetries as well as severe heat loads. Temperatures close to the melting limit have been locally observed on upper first wall structures during deliberate VDE and even at plasma currents as low as 1.5 MA and thermal energy of about 1.5 MJ only. A high radiation fraction can be regained by massive injection of a mixture of 10% Ar with 90% D2. This accelerates the CQ thus reducing the halo current and sideways impulse. The temperature of PFCs stays below 400 °C. MGI is now a mandatory tool to mitigate disruptions in closed-loop operation for currents at and above 2.5 MA in JET.
[Show abstract][Hide abstract] ABSTRACT: JET has been recently refurbished with an ITER-like Be first wall and W divertor (ILW), to study
plasma wall interaction processes and integrated scenario development for ITER. With the change
of the divertor material, and the related presence of radiating W ions in the plasma, several changes
in the pre-existing MHD phenomenology have been observed. The experimental signature of the new MHD behaviour will be characterized in this work using high bandwidth pick up coils, fast ECE signals and fast Soft X-ray signals
[Show abstract][Hide abstract] ABSTRACT: The new full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions. The main difference is a significantly lower fraction (by up to a factor of 5) of energy radiated during the disruption process, yielding higher plasma temperatures after the thermal quench and thus longer current quench times. Thus, a larger fraction of the total energy was conducted to the wall resulting in larger heat loads. Active mitigation by means of massive gas injection became a necessity to avoid beryllium melting already at moderate levels of thermal and magnetic energy (i.e. already at plasma currents of 2 MA). A slower current quench, however, reduced the risk of runaway generation. Another beneficial effect of the ILW is that disruptions have a negligible impact on the formation and performance of the subsequent discharge.
Full-text · Article · Nov 2012 · Plasma Physics and Controlled Fusion
[Show abstract][Hide abstract] ABSTRACT: A numerical study is carried out, based on a simple toroidal tokamak equilibrium, to demonstrate the radial re-distribution of the electromagnetic torque density, as a result of a rotating resistive plasma (linear) response to a static resonant magnetic perturbation field. The computed electromagnetic torque peaks at several radial locations even in the presence of a single rational surface, due to resonances between the rotating response, in the plasma frame, and both Alfvén and sound continuum waves. These peaks tend to merge together to form a rather global torque distribution, when the plasma resistivity is large. The continuum resonance induced net electromagnetic torque remains finite even in the limit of an ideal plasma.
[Show abstract][Hide abstract] ABSTRACT: Two methods for calculating tearing mode stability are described in this paper. A fast method using the recently improved T7 code (Ham et al 2012 Plasma Phys. Control. Fusion54 025009) and a new method based on the MARS-F MHD stability code (Liu et al 2000 Phys. Plasmas7 3681) which constructs the tearing mode solution from calculated basis functions in the full geometry of the problem. The effects of plasma toroidicity and cross-sectional shaping on tearing mode stability are investigated using both of the methods; the resultant stabilizing effects are in reasonable agreement over the range of parameters investigated. The parameter-space explored includes JET-like and ITER-like plasma shaping. While T7 can be used for rapid calculations and parameter scans, the MARS-F construction technique produces the more accurate value of the tearing mode stability index.
Preview · Article · Oct 2012 · Plasma Physics and Controlled Fusion
[Show abstract][Hide abstract] ABSTRACT: JET has been recently refurbished with an ITER-like Be first wall and W
divertor, to study plasma wall interaction processes for ITER. In this
work the new behaviour of the MHD instabilities will be characterized in
the hybrid scenario, which with the C-wall in JET achieved high energy
confinement, combined with good MHD stability to NTMs and ideal kinks.
The same scenario developed for the ILW has produced good confinement,
but interactions are observed between MHD phenomena and impurities
coming from the wall. The q=1 MHD activity with the JET C-wall showed a
negligible effect on plasma confinement, except NTM triggering. In some
ILW hybrid pulses at the start of the heating phase a q=1 fishbone
occurs, as with the C-wall, but it is often replaced by a continuous q=1
mode, with a significant reduction of confinement. ECE measurements also
highlight a change from pure kink fluctuations to islands centered on
q=1. NTMs have also been observed in these plasmas. Their appearance is
coincident with a flattening of electron temperature profile within the
island (the effect with the C-wall), but it is also correlated with
enhanced radiation from the plasma core and a slow decrease of central
[Show abstract][Hide abstract] ABSTRACT: Two possible ways of modifying the linear tearing mode index, by active magnetic feedback and by drift kinetic effects of deeply trapped particles, are analytically investigated. Magnetic feedback schemes, studied in this work, are found generally stabilizing for Δ′. The drift kinetic effects from both thermal particles and hot ions tend to reduce the power of the large solution from the outer region. This generally leads to a destabilization of Δ′ for the toroidal analytic equilibria considered here.
No preview · Article · Sep 2012 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: Resonant field amplification (RFA) has been systematically measured on JET, using active MHD spectroscopy to probe plasma stability at high and low beta, and compared with theoretical predictions. RFA has been measured as a plasma response to externally applied fields. At high beta, RFA has been used to identify the ideal no-wall beta limit. It was found experimentally and explained theoretically that the beta limit strongly depends on the current density and q profiles, and in particular on the qmin value, and the current density profile near the plasma edge. At low beta, RFA has been observed and analysed in detail during edge-localized mode (ELM)-free periods prior to the first ELM either after L–H transition or after long ELM-free periods during a pulse. These observations confirm that the measured increase in the RFA in some cases (e.g. at low beta) may not be connected with the no-wall beta limit associated with the RWM, but may reflect a proximity to other stability thresholds. Reduction in RFA is observed during an outer mode for the first time. The first results on n = 2 probing on JET are presented.