ELM Behaviour Research Articles

The global build-up to intrinsic ELM bursts and comparison with pellet triggered ELMs seen in JET

We focus on JET plasmas in which ELMs are triggered by pellets in the presence of ELMs which occur naturally. We perform direct time domain analysis of signals from fast radial field coils and toroidal full flux azimuthal loops. These toroidally integrating signals provide simultaneous high time resolution measurements of global plasma dynamics and its coupling to the control system. We examine the time dynamics of these signals in plasmas where pellet injection is used to trigger ELMs in the presence of naturally occurring ELMs. Pellets whose size and speed are intended to provide maximum local perturbation for ELM triggering are launched at pre-programmed times, without correlation to the occurrence times of intrinsic ELMs. Pellet rates were sufficiently low to prevent sustained changes of the underlying plasma conditions and natural ELM behaviour. We find a global signature of the build-up to natural ELMs in the temporal analytic phase of both the full flux loops and fast radial field coil signals. Before a natural ELM, the signal phases align to the same value on a 2–5 ms timescale. This global build up to a natural ELM occurs whilst the amplitude of the full flux loop and fast radial field coil signals are at their background value: it precedes the response seen in these signals to the onset of ELMing. In contrast these signals do not clearly phase align before the ELM for ELMs which are the first to occur following pellet injection. This provides a direct test that can distinguish when an ELM is triggered by a pellet as opposed to occurring naturally. It further supports the idea [–] of a global build up phase that precedes natural ELMs; pellets can trigger ELMs even when the signal phase is at a value when a natural ELM is unlikely to occur.

ELM behaviour and linear MHD stability of edge ECRH heated ASDEX Upgrade plasmas

In order to test the peeling–ballooning ELM model, ECRH heating was applied to the edge of ASDEX Upgrade type-I ELMy H-mode plasmas to alter the pedestal pressure and current density profiles. The discharges were analysed with respect to ideal MHD stability. While the ELM frequency increased and the pedestal gradients relaxed with edge ECRH, the MHD stability boundary did not change. The results indicate that the peeling–ballooning model is insufficient to fully explain the triggering of ELM instabilities in the presence of edge ECRH heating.

Pedestal study across a deuterium fuelling scan for high δ ELMy H-mode plasmas on JET with the carbon wall

We present the results from a new fuelling scan database consisting of 14 high triangularity (δ ∼ 0.41), type I ELMy H-mode JET plasmas. As the fuelling level is increased from low, (ΓD ∼ 0.2 × 1022 el s−1, ne,ped/nGW = 0.7), to high dosing (ΓD ∼ 2.6 × 1022 el s−1, ne,ped/nGW = 1.0) the variation in ELM behaviour is consistent with a transition from ‘pure type I’ to ‘mixed type I/II’ ELMs (Saibene et al 2002 Plasma Phys. Control. Fusion 44 1769). However, the pulses in this new database are better diagnosed in comparison to previous studies and most notable have pedestal measurements provided by the JET high resolution Thomson scattering (HRTS) system. We continue by presenting, for the first time, the role of pedestal structure, as quantified by a least squares mtanh fit to the HRTS profiles, on the performance across the fuelling scan. A key result is that the pedestal width narrows and peak pressure gradient increases during the ELM cycle for low fuelling plasmas, whereas at high fuelling the pedestal width and peak pressure gradient saturates towards the latter half of the ELM cycle. An ideal MHD stability analysis shows that both low and high fuelling plasmas move from stable to unstable approaching the ideal ballooning limit of the finite peeling–ballooning stability boundary. Comparison to EPED predictions show on average good agreement with experimental measurements for both pedestal height and width however when presented as a function of pedestal density, experiment and model show opposing trends. The measured pre-ELM pressure pedestal height increases by ∼20% whereas EPED predicts a decrease of 25% from low to high fuelling. Similarly the measured pressure pedestal width widens by ∼55%, in poloidal flux space, whereas EPED predicts a decrease of 20% from low to high fuelling. We give two possible explanations for the disagreement. First, it may be that EPED under predicts the critical density, which marks the transition from kink-peeling to ballooning-limited plasmas. Second, the stronger broadening of the experimental pedestal width than predicted by EPED is an indication that other transport related processes contribute to defining the pedestal width such as enhanced inter-ELM transport as observed at high fuelling, for mixed type I/II ELMy pulses.

Pedestal width and ELM size identity studies in JET and DIII-D; implications for ITER

The dependence of the H-mode edge transport barrier width on normalized ion gyroradius (ρ* = ρ/a) in discharges with type I ELMs was examined in experiments combining data for the JET and DIII-D tokamaks. The plasma configuration as well as the local normalized pressure (β), collisionality (ν*), Mach number and the ratio of ion and electron temperature at the pedestal top were kept constant, while ρ* was varied by a factor of four. The width of the steep gradient region of the electron temperature (Te) and density (ne) pedestals normalized to machine size showed no or only a weak trend with ρ*. A ρ1/2 or ρ1 dependence of the pedestal width, given by some theoretical predictions, is not supported by the current experiments. This is encouraging for the pedestal scaling towards ITER as it operates at lower ρ* than existing devices. Some differences in pedestal structure and ELM behaviour were, however, found between the devices; in the DIII-D discharges, the ne and Te pedestal were aligned at high ρ* but the ne pedestal shifted outwards in radius relative to Te as ρ* decreases, while on JET the profiles remained aligned while ρ* was scanned by a factor of two. The energy loss at an ELM normalized to the pedestal energy increased from 10% to 40% as ρ* increased by a factor of two in the DIII-D discharges but no such variation was observed in the case of JET. The measured pedestal pressures and widths were found to be consistent with the predictions from modelling based on peeling–ballooning stability theory, and are used to make projections towards ITER

Pedestal and ELM response to impurity seeding in JET advanced scenario plasmas

Advanced scenario plasmas must often be run at low densities and high power, leading to hot edge temperatures and consequent power handling issues at plasma–surface interaction zones. Experiments at JET are addressing this issue by exploring the use of extrinsic impurity seeding and D2 puffing to reduce heat fluxes.The experiments presented in this paper continue the line of advanced tokamak (AT) scenario studies at high triangularity in JET by concentrating on the characterization of the edge pedestal and the ELM behaviour with deuterium and/or light impurity fuelling (neon, nitrogen). Both injection of extrinsic impurities and D2 puffing are shown to have a significant impact on the edge pedestal in typical JET AT conditions. The ELM energy loss, ΔWELM/Wdia, can be reduced to below 3% and the maximum ELM penetration depth can be limited to r/a > 0.7, thus enhancing the possibility for sustainable internal transport barriers at large plasma radius. These conditions can be achieved in two separate domains, either at a radiated power fraction (Frad) of 30% or at a fraction of >50%. At the lower Frad the ELMs are type I and a high pedestal pressure is maintained, but the occasional large ELM may still occur. At Frad > 50% the pedestal pressure is degraded by 30–50%, but the ELMs are degraded to type III. The intermediate regime at Frad ∼ 40% is unattractive for ITB scenarios because large type I ELMs occur intermittently during the predominantly type III ELM phases (compound type I/III). Frad = 30% can be obtained with D2 fuelling alone, whereas neon or nitrogen seeding is needed to achieve Frad > 50%. Only a limited number of tests have been carried out with nitrogen seeding, with the preliminary conclusion that the plasma edge behaviour is similar to that with neon seeding once the radiated fraction is matched.

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

The properties of ASDEX Upgrade's type-II ELM regime, which combines good confinement with benign temporal variation of the power exhaust, make it an attractive candidate for ITER baseline scenario operation. As yet it is not understood what the physics origin of this regime is. In previous type-II ELM studies (Stober J et al 2001 Nucl. Fusion 41 1123, Stober J et al 2005 Nucl. Fusion 45 1213) a whole range of fluctuations have been reported. In this paper the properties of these fluctuations are inspected in more detail, and their relevance for the regime investigated. For the high frequency (∼150–220 kHz) magnetic fluctuation activity, from its absence in certain type-II ELM scenarios and behaviour during regime transitions it is concluded that it is not an essential ingredient of type-II ELM regimes on ASDEX Upgrade. A stronger contender to explain the type-II ELM regime appears to be given by small repetitive electromagnetic bursts, and low frequency (∼5–25 kHz) magnetic precursor activity associated with these bursts. It is demonstrated that these bursts give rise to small but frequent heat loads of the right order to influence ELM cycles. These are best detected in the vicinity of the inner strike point in the lower divertor through infrared thermography and Langmuir probes. Their properties further suggest that the bursts are neither small type-I ELMs nor type-III ELMs. However, it remains to be shown that the losses associated with these bursts are high enough to replace the type-I ELM losses. Other enhanced density fluctuation activity at low frequency (10–30 kHz) is detected by reflectometry near the plasma boundary. The information available about this mode is more limited, but so far a causal implication of this mode in the type-II ELM regime cannot be excluded. Overall it can be concluded that, of the three potential dissipation mechanisms, the low frequency electromagnetic burst behaviour seems to be the most promising candidate to account for type-II ELMs and their energy losses.

The H-mode pedestal, ELMs and TF ripple effects in JT-60U/JET dimensionless identity experiments

This paper summarizes results of dimensionless identity experiments in JT-60U and JET, aimed at the comparison of the H-mode pedestal and ELM behaviour in the two devices. Given their similar size, dimensionless matched plasmas are also similar in their dimensional parameters (in particular, the plasma minor radius a is the same in JET and JT-60U). Power and density scans were carried out at two values of Ip, providing a q scan (q95 = 3.1 and 5.1) with fixed (and matched) toroidal field. Contrary to initial expectations, a dimensionless match between the two devices was quite difficult to achieve. In general, pped in JT-60U is lower than in JET and, at low q, the pedestal pressure of JT-60U with a Type I ELMy edge is matched in JET only in the Type III ELM regime. At q95 = 5.1, a dimensionless match in ρ*, ν* and βp,ped is obtained with Type I ELMs, but only with low power JET H-modes. These results motivated a closer investigation of experimental conditions in the two devices, to identify possible ‘hidden’ physics that prevents obtaining a good match of pedestal values over a large range of plasmas parameters. Ripple-induced ion losses of the medium bore plasma used in JT-60U for the similarity experiments are identified as the main difference with JET. The magnitude of the JT-60U ripple losses is sufficient to induce counter-toroidal rotation in co-injected plasma. The influence of ripple losses was demonstrated at q95 = 5.1: reducing ripple losses by ≈2 (from 4.3 to 1.9 MW) by replacing positive with negative neutral beam injection at approximately constant Pin resulted in an increased pped in JT-60U, providing a good match to full power JET H-modes. At the same time, the counter-toroidal rotation decreased. Physics mechanisms relating ripple losses to pedestal performance are not yet identified, and the possible role of velocity shear in the pedestal stability, as well as the possible influence of ripple on thermal ion transport are briefly discussed. Toroidal rotation of the ITER reference inductive Q = 10 H-mode is predicted to be rather low, of the order of ∼1/10 of the frequency of typical JET H-modes. Nonetheless, fast ion ripple losses in that scenario are also predicted to be negligible (≲1%), and therefore plasma toroidal rotation slow-down or ripple-induced counter-rotation should not affect pedestal parameters and stability in ITER. Finally, the possible effect of ripple on thermal transport may deserve more attention in future experiments and modelling, since the ripple magnitude of ITER is intermediate between that of JET and JT-60U.

JET new diagnostic capability on the route to ITER

The JET scientific programme is directed towards the development of ITER relevant scenarios. In support of this, significant effort has been made to develop diagnostics to better characterise the power deposition on the plasma facing components, to investigate in more detail the radiation losses particularly in the divertor region and to better detect Magneto Hydrodynamic Modes (MHD) instabilities and their effects on fast ion confinement. A new wide-angle infrared camera provides for the first time the opportunity to perform infrared thermography in the JET main chamber, even during fast events like ELMs and disruptions. A completely new bolometric system, with better spatial resolution particularly in the divertor, is now used to investigate the total radiation losses and their influence on the ELM behaviour. A new set of microwave waveguides has improved by 20 dB the signal to noise ratio of the JET X-mode reflectometers, that are now routinely used to detect MHD instabilities and in particular to localise the location of Alfvén Eigenmodes. This improved diagnostic capability to monitor MHD instabilities is complemented by two new diagnostics to detect lost fast particles. Both the new scintillator probe and a poloidal array of Faraday cups have already shown clear correlations between MHD activity and ion losses at the edge.

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.

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.

Seeding of impurities in JET H-mode discharges to mitigate the impact of ELMs

ELMy H-mode is one of the foreseen operational scenarios for ITER-FEAT. Good confinement and density is usually associated with Type I ELMs, which, however, will have a severe impact on the divertor target plates in ITER. At the JET-tokamak, impurities have been seeded into a variety of discharge configurations, by which the radiation level can be risen by a factor of 2. This delays the appearance of ELMs and reduces the power load onto the divertor target plates. In this paper, the influence of the impurities on the ELM behaviour and the plasma edge properties will be discussed. Several divertor diagnostics, such as electrical probes and infrared camera indicate a reduction of the heat power load at the inner divertor target tiles and a shrinking of the impact zone at the outer divertor.

H-mode plasmas in the MAST spherical tokamak

Operating scenarios in the Mega-Ampère Spherical Tokamak (MAST) have been recently developed to access routinely H-mode plasmas over a wide range of plasma conditions. Gas refuelling at the inboard mid-plane has proved invaluable in facilitating L- to H-mode transitions. Already, a comprehensive range of H-mode behaviour has been observed providing similar and contrasting comparisons with conventional tokamaks. For example, similarities are found in H-mode edge and global evolutions, increases in confinement and ELM behaviour. In contrast, H-mode threshold powers are about an order of magnitude higher in MAST than those predicted by conventional aspect ratio tokamak scalings. At the highest βθ values obtained so far, transient confinement times of ~100 ms are achieved and calculations indicate that bootstrap currents may approach 50% of the plasma current.

Inboard gas puffing and behaviour of H-mode edge parameters in COMPASS-D

It is shown that fuelling from the inboard midplane extends the operation window for H-mode in comparison with fuelling from the outboard midplane. In Ohmically heated plasmas, the minimum line-averaged density required for transition to ELM-free H-mode is approximately ~30% lower in comparison with the fuelling from the outboard midplane. The power at the transition to H-mode is similarly reduced.Temporal and spatial resolutions of thermal helium beam, Doppler diagnostics and Thomson scattering were improved to study the edge parameter evolutions during the L-H transitions and ELMs. At L-H and H-L transitions, the diagnostics demonstrate a coincident temporal and spatial evolution of gradients in the radial electric field and electron pressure. During regular ELM behaviour similar evolutions are observed.

Type II ELMy H modes on ASDEX Upgrade with good confinement at high density

H modes with good confinement and small ELMswith the characteristics of type II or grassy ELMs have been observed on ASDEX Upgrade.Such an ELM behaviour is essential to minimize erosion of the divertor tilesin any next step device. For the first time, operation with this favourableELM type could be demonstrated close to the Greenwald density.Even for such high densities, energy confinement times were close to recent H mode scalings. Highdensity even seems to be favourable, since steady state puretype II ELMy H mode phases on ASDEX Upgrade are obtained only above n̄e/n̄GW ⩾ 0.85. Additional requirements are q95 ⩾ 4.2 and an equilibrium close to a double null configurationwith an average triangularity δ = 0.40. For these small ELMs magnetic precursors are observed with a frequency of ≈ 30 kHz and dominantmode numbers of toroidally 3 and 4 and poloidally ⩾ 14.

Overview of ASDEX Upgrade results

The closed ASDEX Upgrade Divertor II, `LYRA', is capable of handling heating powers of up to 20 MW or P/R of 12 MW/m, owing to a reduction of the maximum heat flux to the target plates by more than a factor of 2 compared with the open Divertor I. This reduction is caused by high radiative losses from carbon and hydrogen inside the divertor region and is in agreement with B2-EIRENE modelling predictions. At medium densities in the H mode, the type I ELM behaviour shows no dependence on the heating method (NBI, ICRH). ASDEX Upgrade-JET dimensionless identity experiments showed compatibility of the L-H transition with core physics constraints, while in the H mode confinement, inconsistencies with the invariance principle were established. At high densities close to the Greenwald density, the MHD limited edge pressures, the influence of divertor detachment on separatrix parameters and increasing edge transport lead to limited edge densities and finally to temperatures below the critical edge temperatures for H mode. This results in a drastic increase of the H mode threshold power and an upper H mode density limit with gas puff refuelling. The H mode confinement degradation approaching this density limit is caused by the ballooning mode limited edge pressures and `stiff' temperature profiles relating core and edge temperatures. Repetitive high field side pellet injection allows for H mode operation well above the Greenwald density; moreover, higher confinement than with gas fuelling is found up to the highest densities. Neoclassical tearing modes limit the achievable β depending on the collisionality at the resonant surface. In agreement with the polarization current model, the onset β is found to be proportional to the ion gyroradius in the collisionless regime, while higher collisionalities are stabilizing. The fractional energy loss connected with saturated modes at high pressures is about 25%. A reduction of neoclassical mode amplitude and an increase of β have been demonstrated by using phased ECRH and ECCD in the O point of islands. Advanced tokamak operation with internal transport barriers for both ions and electrons has been achieved with flat shear profiles and q0 > 1 or with reversed shear and qmin > 2. With flat shear a stationary H mode scenario was maintained for 40 confinement times and several internal skin times with βN = 2 and HITERL-89P = 2.4, where fishbones keep q0 at 1. βN is limited by either neoclassical tearing modes in the case of flat shear or kink modes with reversed shear.

The edge operational space in JET

In this paper we investigate differences of the ELM behaviour between RF and NB heated H-mode discharges in JET. The main result of this investigation is that the edge pedestal pressure with NB heating is higher than with RF heating for equal power fluxes crossing the separatrix. This can be explained by a higher population of fast ions in the edge region in the case of NB heating. Using data from NB heated steady-state H-modes we derive a scaling law for the width of the transport barrier. We find that the barrier width is proportional to the poloidal Larmor radius, which in turn is controlled by the ion population with the highest energy in the plasma edge.

H-modes on COMPASS-D with high-power ECRH

Recent high-power electron-cyclotron resonance heating (ECRH) H-mode experiments on COMPASS-D have used fundamental high-power ECRH to access high densities. Within the operating range set by the H-mode density threshold and ECRH accessibility, H-mode power thresholds are significantly higher than expected from ITER scalings, and show a contrary dependence on density. Energy confinement is somewhat lower than that suggested by the present ITER scalings, but correction for incomplete RF absorption may be needed. Complex H-mode and ELM behaviour is observed.

Critical parameters for turbulent transport in the SOL: mechanism for the L - H transition and its impact on the H-mode power threshold and density limits

An explanation for the L - H transition through the electromagnetic mechanism (suppression of drift wave turbulence by the skin effect) is offered. In dimensional space, the bifurcation is attributed to the involvement of two parameters in L - H transition physics: and (the latter scales as in physical parameters). The maximum of the diffusion coefficient, corresponding to the L - H transition, is reached when the collisional skin depth ( is the drift frequency) equals the characteristic radial displacement of the drift turbulence. The same criterion can also be presented as: - the condition for equal rates of the plasma diffusion into the magnetic field and the diffusion of the magnetic field into the plasma. The analysis yields the combination (scales as in dimensionless parameters) as a critical parameter for the L - H transition, for the case of the scaling for the wavevector of the drift turbulence. This threshold parameter should be applied near the separatrix position. At low densities, the requirement that the collisionless skin depth must be smaller than the radial displacement of the drift fluctuations in the L-mode, which is necessary for turbulence suppression, determines the threshold for the L - H transition. The proposed mechanism for the L - H transition clarifies the R dependence of the H-mode power threshold: scaling is predicted, with for in ITER. The critical parameter for the L - H transition, together with dimensionless parameters characterizing pressure gradient and resistivity, create the set of similarity parameters describing ELM behaviour. The scaling for the separatrix density, normalized to the Greenwald density limit with the machine size and toroidal field which ensures `similar' ELM behaviour, can thus be obtained. For the fixed similarity parameters, the analysis yields weak but favourable dependence of on the major radius. In recent experiments on JET and other machines, the degradation in the edge confinement associated with increased ELM frequency was found to be responsible for the density limit in high-power H-modes. Owing to the approximately dependence, an excess over the Greenwald limit, , by about 30% higher in ITER compared with JET for `similar' conditions (q, , separatrix and the ratio, wall conditions, the use of pellets etc) in ELMy H-modes is predicted. This is with the provision that a limit on the central density, related to mechanisms in the plasma core, is not encountered.

Ohmic H-modes in the TCV tokamak

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors up to 2 and line average densities up to , corresponding to Ithe Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary H-modes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of ELMs have been found to depend on configurational parameters such as the position of the `active' X-point in unbalanced double-null discharges and the plasma - wall separation in single-null discharges. These dependencies have permitted active control of ELM behaviour in TCV. A continuous spectrum of ELM amplitudes and frequencies has been observed, ranging from clearly identifiable type III ELMs to large, low-frequency ELMs which expel up to 12% of the stored energy and up to 7% of the particle content and are reminiscent of type I ELMs. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed.

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment