A new 10 channel Thomson scattering (TS) system was installed on the ASDEX Upgrade tokamak to measure radial profiles of electron density and temperature at the plasma edge with high radial resolution. Together with the already existing TS system, which is now used for the core plasma, electron density and temperature profiles extending from the edge to the core are now obtained in a single discharge. The TS systems are relatively calibrated by an optical parametric oscillator.

On Wendelstein 7-X, electron density and electron temperature profiles are planned to be measured by Thomson scattering. The specific system employs fiber bundles for the scattered light which is detected by means of a five-channel polychromator system. The detection system is assembled and its components are characterized. A calibration technique employing a super-continuum light source is assessed by comparison with a calibration technique using an optical parametric oscillator laser system. The characterization is preparing further diagnostic optimization studies employing a virtual instrument.

In order to assess the contribution of edge localized modes (ELMs) to plasma–wall interaction in future fusion experiments like ITER, a sound experimental database for model validation and extrapolation, and, to be prepared for the unfavourable case, the development of tools for ELM mitigation are required. On ASDEX Upgrade a large amount of experimental information has been accumulated from various diagnostics on the structure and dynamics of natural as well as pellet induced ELMs, and on related wall effects. In this paper a survey of type-I ELM results is given first and recent progress is then described in detail. In between ELMs, strong mode activity is observed in a wide mode number and frequency range, specifically large amplitude (∼20%) low frequency (several kilohertz) fluctuations. The initial dynamic ELM phase is dominated by the rapid growth of helical, low mode number structures rotating in the pedestal E × B direction, while the subsequent saturation and profile erosion phase is more complex and scenario dependent. Bursts of filaments ejected from the hot edge into the scrape-off layer are correlated with primary pedestal mode rotation. After partial edge profile collapse, a quiescent recovery phase is obtained despite substantial residual edge gradients. Pellet induced ELMs behave similarly to spontaneous ones, at least for the smallest pellets available so far.

Fast, high resolution multichannel Thomson scattering is used to quantitatively determine plasma perturbations induced by type-I edge-localized modes (ELMs) in the low-field side edge of ASDEX Upgrade H-mode plasmas. 2D snapshots of temperature and density, deduced from the laser light scattered in a vertically elongated, poloidal array of 5x10 scattering volumes, are obtained in the hot, steep edge gradient zone, which is difficult to access by other diagnostics. Local maxima and minima with large amplitude are identified during ELMs and even in the precursor phase, both in density and temperature. Interpreting these structures as footprints of approximately field aligned helical modes in accordance with previous experimental and theoretical work, toroidal mode numbers between 8 and 20 are obtained, roughly consistent with corresponding scrape-off layer and divertor measurements.

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.

Limited available pellet velocities have so far restricted the refuelling performance of efficient launch schemes from the tokamak magnetic high field side (HFS). Although pellet injection during H mode has resulted in more peaked density profiles and enhanced performance with respect to gas puff refuelling, prompt particle and energy losses caused by pellet induced ELM bursts have still limited the extension of the operational area. Now, the preliminary version of a new optimized pellet injection set-up at ASDEX Upgrade allows for significantly higher injection speeds when launching pellets from the magnetic HFS. Intact pellets with velocities up to vP = 560 m/s were successfully injected instead of the vP = 240 m/s available with the previous set-up. The velocity increase results in a deeper pellet penetration and seems to follow the vP1/3 scaling derived from a multimachine study using conventional pellet launch from the torus outside. The inward shift of the pellet particle deposition profile with respect to the penetration depths turned out to be approximately the same for both launch velocities. The respectively achieved deeper particle deposition inside the plasma column reduced the particle and energy loss rates during the immediate post-pellet phase. Thus, further enhancement of tokamak operation in the high density regime seems feasible by means of high speed pellets launched from the torus inner side.

The effect of plasma shape variation (in particular, variation of the upper and lower triangularity δ) on edge localized modes (ELMs) and H-mode pedestal properties in ASDEX Upgrade is reported here. Strongly shaped plasmas (high δ) show an increased edge pressure gradient and, without external gas puff, type-I ELMs generally have larger losses and lower frequency than in plasmas with low δ. With external gas puff, ELM losses at high δ are reduced and in the same range as found for low δ. The average ELM power loss is a constant fraction of the total loss power, independent of triangularity. The width of the steep electron temperature and pressure gradient zone remains essentially constant at low δ while at high δ it shows a variation inconsistent with a poloidal gyroradius scaling. The edge pressure gradient and the pedestal pressure scales strongly with plasma current and triangularity. In type-I ELM H-modes, the pedestal pressure is directly related to the global stored plasma energy, independent of plasma shape.

The global confinement studies in ASDEX Upgrade indicate that in the high density regimes required for ITER, confinement degradation occurs in the H mode (from f(H) approximate to 2.0 down to 1.2), but not in the L mode (f(H) approximate to 1). In scenarios including edge radiation, density peaking counterbalances the degradation of the K mode (up to f(H) approximate to 1.6) and improves the L mode to values close to the H mode. Transport studies associate an inward drift to this effect. Improved L modes are also observed just below the H mode threshold when the latter is high. The thermal electron diffusivity inferred from ECRH modulation is close to that of power balance, whereas results from sawteeth are generally larger, the latter seeming essentially determined by the sawtooth amplitude. Dimensionless similar rho(*) scans in the L mode yield Bohm scaling, whereas H mode scans exhibit a gyro-Bohm behaviour The K mode is sensitive to the neutral gas pressure and therefore the required profile matching at the plasma edge is difficult to achieve.

Divertor plasmas with strong external gas puffing in ASDEX Upgrade have shown very efficient impurity retention, increasing with the divertor neutral gas density. The experiments presented here use feedback-controlled gas puffs in discharges with different pumping speed to keep the divertor neutral gas flux density the same. This allows for the first time a decoupling of the divertor neutral gas flux density and the external gas flow. The resulting plasmas are almost identical and show identical impurity retention, clearly demonstrating the importance of the divertor neutral gas density over the externally induced flow.

Controlled modification of the radial profiles of the plasma current, electron temperature and density was achieved in the ASDEX divertor tokamak with lower hybrid waves. Variation of the lower hybrid power and current deposition profiles was accomplished by tailoring the launched wave spectra or by compound antenna phasing. The current and temperature profiles could be largely decoupled. MHD modes were strongly influenced by the current profile modifications. Sawteeth and m=1 modes were suppressed up to the density limit of lower hybrid current drive on ASDEX (ne=5*1019 m-3). The electron temperature profile peaked in this case and the global confinement was improved

Current drive and heating with lower hybrid (LH) waves were combined with neutral beam injection (NBI) heating on ASDEX up to total powers of 4.2 MW. The same LH current drive efficiency as in Ohmic target plasmas was obtained. For the energy confinement time, similar parametric dependences on heating power, plasma current and q(a) value as in plateau-like scaling were found for combined operation of NBI and LH current drive in the L-mode. H-modes were triggered routinely by injection of LH waves during NBI. Peaked current density profiles give better global confinement than broad profiles in combined NBI and LH current drive.

A detailed analysis is presented of experimental data on the energy content in the fast electron population created by lower hybrid waves in the ASDEX tokamak and it is shown that in most circumstances direct losses due to diffusion of these electrons can be neglected. The value of the fast electron diffusion coefficient is inferred from the time behaviour of hard X-ray emission during power modulation experiments. It turns out that fast electrons are better confined than thermal electrons. This experimental result is compared with a theoretical prediction based on transport driven by electrostatic or magnetic turbulence

Transport code calculations were made for experiments with the combined operation of lower hybrid current drive and heating and of neutral beam injection heating on ASDEX. Peaking or flattening of the electron temperature profile are mainly explained by modifications of the MHD induced electron heat transport. They originate from current profile changes due to lower hybrid and neutral beam current drive and to contributions from the bootstrap current. Ion heat transport cannot be described by one single model for all heating scenarios. The ion heat conductivity is reduced during lower hybrid heated phases with respect to Ohmic and neutral beam heating

Impurity puffing experiments have been performed in ultra-clean ASDEX discharges (via wall boronization) in order to explore the role of impurities for the confinement in high-density Ohmic discharges. Successful changes in confinement are achieved by neon puffing where edge radiation is increased such that the power load onto the divertor plates takes its minimum value. This low value correlates with a reduced separatrix pressure and hence may trigger a peaking of the density profile leading to favourable transport behaviour in the bulk plasma. Therefore, the access to improved Ohmic confinement can now be understood in terms of impurity radiation and recycling fluxes.

The scaling of the energy confinement time with plasma density and current has been investigated for ohmically heated tokamak discharges in ASDEX. The linear dependence tau-E approximately nBAR(e) is maintained in the high density improved Ohmic confinement (IOC) regime with peaked density profiles. The peaking of the radial density profile can be brought about by reducing the net power flow through the plasma surface, thereby leading to a reduction of the edge density. Tailoring of the radiation profile with the addition of low-Z impurities, for example neon, gives access to the IOC regime under conditions where otherwise the degraded saturated Ohmic confinement (SOC) behaviour prevails. The energy confinement time tau-E increases with current and decreases with heating power also in Ohmic discharges, as is shown by a statistical analysis. However, with the intrinsic coupling between power and current, the two relationships cancel and tau-E becomes independent of P(OH) and I(p). The two most prominent features of Ohmic confinement can therefore be explained on the basis of simple physical models.

A database of high density (0.3 < n(e)BAR[10(20) m-3] < 0.8), low q(a)(1.9 < q(a) < 3.4), Ohmic discharges from the ASDEX experiment is analysed statistically. Bulk parameter scalings and parameterized temperature and density profile shapes are presented. The total plasma kinetic energy, assuming T(i) = T(e), scales as n(e)0.54 +/- 0.01BAR I(p)0.90 +/- 0.04 and is almost independent of the toroidal magnetic field. The electron temperature profile peaking factor scales as T0(3/2)/ = 0.94 (+/- 0.04) q(a)1.07 +/- 0.04, in close agreement with the assumption of classical resistive equilibrium. In the inner half of the plasma, the inverse fall-off length for both temperature and density has a strong dependence on q(a), with the temperature dependence being more pronounced. Outside the half-radius, the q(a) dependence disappears, but the density profile broadens near the edge with increasing plasma current. A second database of moderate density, moderate q(a) discharges (0.2 < n(e)BAR/[10(20) m-3] < 0.4, 2.4 < q(a) < 4.2), is presented for comparison.

To compare different Ohmic confinement regimes in ASDEX, the edge conditions are analyzed in detail. The results show that the improved Ohmic confinement coincides with a drop of the separatrix density. This drop allows the density profile to peak and seems to be the trigger of a change in the transport. A universal scaling between the electron temperature and the electron density at the separatrix prevails for all Ohmic scenarios. In addition, the total particle flux across the separatrix is evaluated and found to be strongly correlated to the separatrix density. Thus, the convective energy loss contributes less to the total energy losses when the confinement is improved. Since the correlations between the edge parameters do not change in different Ohmic confinement regimes of ASDEX, the edge physics appears to remain the same. Improved Ohmic confinement is characterized by an optimum separatrix density which provides a sufficiently high edge temperature together with low particle fluxes. These optimum conditions yield the maximum particle confinement.

The paper summarizes the experiments performed with ion cyclotron resonance heating (ICRH) on ASDEX, from November 1984 until March 1986; the most interesting results are reported and discussed in detail. Heating and confinement studies using the hydrogen second harmonic scheme and the hydrogen minority scheme (PIC < 2.6 MW, tIC < 1.5 s) show a typical L-mode behaviour, i.e. a power dependent confinement degradation, which is rather similar to that found with neutral beam injection (NBI) heating. ICRH is accompanied by a slightly improved particle and energy confinement compared with that of NBI; this is also true for a combined ICRH + NBI scheme, up to Ptot ≈ 4.5 MW, absorbed in the plasma. Particular efforts have been devoted to investigations of the second harmonic regime in H/D plasmas with nH/ne ≈ 0.1 - 1, with a view to heating mixtures in reactor relevant plasmas. The achievement of H-mode transitions with ICRH alone in the hydrogen minority scheme at an absorbed RF power of about 1.1 MW supports the assumption of common confinement properties in auxiliary heated tokamaks, since they appear to be widely independent of the additional heating method. ICRH specific impurity problems, such as the strong release of iron from the vessel walls, have been overcome by applying extensive in situ wall carbonization. The mechanisms responsible for impurity generation have partly been identified and analysed; however, the problem still remains to be solved. Impurities preferentially released from the ICRH antenna do not pose problems.

Both in Ohmically and beam-heated L-mode discharges of ASDEX, the electron-temperature (Te) profile shape can be varied over a wide range by the choice of the safety factor qa. The power-deposition profile, on the contrary, has no effect on the Te profile shape. In current-free W-VII-A stellarator plasmas, no such invariance property is found. An independent constraint seems to fix the current distribution j(r) of the tokamak, which defines the conditions of electron heat transport.Received 4 February 1986DOI:https://doi.org/10.1103/PhysRevLett.56.2187©1986 American Physical Society