T. Szepesi

ELM pacing and high-density operation using pellet injection in the ASDEX Upgrade all-metal-wall tokamak

Edge-localized mode (ELM) triggering and pacing in an all-metal wall environment shows significant differences to a first-wall configuration containing carbon. Here we report on experiments performed at ASDEX Upgrade revisiting the issue with all plasma-facing surfaces now fully replaced by tungsten. This investigation was motivated by experimental findings indicating that ELM triggering becomes more intricate when the carbon is replaced by a metal wall. ELM pacing could no longer be achieved by magnetic triggering in ASDEX Upgrade under conditions that previously showed a positive response. Also, recent investigations at JET indicate that a lag time occurs in pellet ELM triggering when operating with the new ITER-like wall. The ASDEX Upgrade centrifuge-based launching system was revitalized and upgraded for this study, now allowing detailed analysis of the ELM trigger response. The appearance of a lag time for pellet ELM triggering in an all-metal wall environment was confirmed. While different lag time durations were found for several type-I ELMy H-mode scenarios, the magnitude of the pellet perturbation was found to cause no difference. Reducing the auxiliary heating power for ELM triggering clearly makes the pellet tool less efficient for ELM control purposes; however, this affords a major benefit when applied for fuelling. Plasma operation with benign ELM behaviour at core densities far beyond the Greenwald limit was demonstrated, this being fully reversible and not affecting the energy confinement.

High-density H-mode operation by pellet injection and ELM mitigation with the new active in-vessel saddle coils in ASDEX Upgrade

Recent experiments at ASDEX Upgrade demonstrate the compatibility of ELM mitigation by magnetic perturbations with efficient particle fuelling by inboard pellet injection. ELM mitigation persists in a high-density, high-collisionality regime even with the strongest applied pellet perturbations. Pellets injected into mitigation phases trigger no type-I ELM-like events unlike when launched into unmitigated type-I ELMy plasmas. Furthermore, the absence of ELMs results in improved fuelling efficiency and persistent density build-up. Pellet injection is helpful to access the ELM-mitigation regime by raising the edge density beyond the required threshold level, mostly eliminating the need for strong gas puff. Finally, strong pellet fuelling can be applied to access high densities beyond the density limit encountered with pure gas puffing. Core densities of up to 1.6 times the Greenwald density have been reached while maintaining ELM mitigation. No upper density limit for the ELM-mitigated regime has been encountered so far; limitations were set solely by technical restrictions of the pellet launcher. Reliable and reproducible operation at line-averaged densities from 0.75 up to 1.5 times the Greenwald density is demonstrated using pellets. However, in this density range there is no indication of the positive confinement dependence on density implied by the ITERH98P(y,2) scaling.

Performance and properties of the first plasmas of Wendelstein 7-X

The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction and one year commissioning. Errors in the magnet assembly were confirmend to be small. Plasma operation was started with 5 MW electron cyclotron resonance heating (ECRH) power and five inboard limiters. Core plasma values of keV, keV at line-integrated densities were achieved, exceeding the original expectations by about a factor of two. Indications for a core-electron-root were found. The energy confinement times are in line with the international stellarator scaling, despite unfavourable wall conditions, i.e. large areas of metal surfaces and particle sources from the limiter close to the plasma volume. Well controlled shorter hydrogen discharges at higher power (4 MW ECRH power for 1 s) and longer discharges at lower power (0.7 MW ECRH power for 6 s) could be routinely established after proper wall conditioning. The fairly large set of diagnostic systems running in the end of the 10 weeks operation campaign provided first insights into expected and unexpected physics of optimized stellarators.

Investigation of pellet-triggered MHD events in ASDEX Upgrade and JET

To get a deeper insight into the MHD activity triggered by pellets we extended our previous analyses of standard type-I ELMs to pellets injected into discharge phases of the following types: Ohmic, L-mode, type-III ELMy H-mode, ELM-free, radiative edge scenarios with type-I ELMs, the quiescent H (QH)-mode regime. It turns out that pellet injection generally creates a strong local perturbation of the MHD equilibrium in the ablation region and even beyond. Regarding the triggering of ELMs, this initial perturbation can damp out, indicating that the plasma is stable in the corresponding regime even for finite-size perturbations. This behaviour is observed not only in Ohmic and L-mode phases but also in the QH-mode where the edge harmonic oscillations appear to keep the edge within or at the boundary of a stable regime. In case the plasma is prone to ELM growth, the large amplitude of the pellet perturbation can trigger the event even in situations where modes appear to be still linearly stable. The non-linear character of the ELM trigger process is also highlighted by the subsequent explosive growth of these events. For edge plasma conditions characterized by higher resistivity the growth time of spontaneously occurring ELMs increases when the plasma changes from type-I into type-III regime. Pellet-triggered ELMs, however, maintain the fast rise times otherwise typical for the hot edge type-I regime. In the discussion section we attempt to also relate these observations to core mode activity such as neoclassical tearing modes or snakes. Data were taken from ASDEX Upgrade and JET. Pellet triggering of mode activity can be shown to be a quite universal phenomenon, which, however, only for the case of ELMs can be unambiguously attributed to prompt direct excitation by the pellet.

Setup and initial results from the magnetic flux surface diagnostics at Wendelstein 7-X

Wendelstein 7-X is an optimized stellarator with superconducting magnetic field coils that just started plasma operation at the Max-Planck-Institut fur Plasmaphysik (IPP) Greifswald. Utilizing the electron beam technique the first vacuum flux surface measurements were performed during the commissioning of the magnet system. For the magnetic configurations investigated so far the existence of closed and nested flux surfaces has been validated. All features of the configuration designed for the initial plasma operation phase, including a predicted island chain, were confirmed. No evidence on significant magnetic field errors was found. Furthermore, the effect of the elastic deformation of the non-planar coils was confirmed by the measurements.

Cryogenic pellet launcher adapted for controlling of tokamak plasma edge instabilities

One of the main challenges posed recently on pellet launcher systems in fusion-oriented plasma physics is the control of the plasma edge region. Strong energy bursts ejected from the plasma due to edge localized modes (ELMs) can form a severe threat for in-vessel components but can be mitigated by sufficiently frequent triggering of the underlying instabilities using hydrogen isotope pellet injection. However, pellet injection systems developed mainly for the task of ELM control, keeping the unwanted pellet fueling minimized, are still missing. Here, we report on a novel system developed under the premise of its suitability for control and mitigation of plasma edge instabilities. The system is based on the blower gun principle and is capable of combining high repetition rates up to 143 Hz with low pellet velocities. Thus, the flexibility of the accessible injection geometry can be maximized and the pellet size kept low. As a result the new system allows for an enhancement in the tokamak operation as well as for more sophisticated experiments investigating the underlying physics of the plasma edge instabilities. This article reports on the design of the new system, its main operational characteristics as determined in extensive test bed runs, and also its first test at the tokamak experiment ASDEX Upgrade.

Spatio-temporal investigations on the triggering of pellet induced ELMs

Pellets injected into type-I ELMy H-mode discharges are known to trigger edge-localized modes (ELMs). In order to understand the underlying processes the triggering mechanism was investigated in this paper. The major questions of the investigations to be answered were: at which magnetic surface was the ELM initiated and what was the corresponding perturbation caused by the ablating pellet? During the investigations the natural ELM cycle was probed by injecting pellets from the high field side of the ASDEX Upgrade tokamak with significantly lower frequency than the natural ELM frequency. To determine the location of the seed perturbation of the ablating pellet triggering an ELM, the dynamics of the triggered ELMs was linked to the time history of the pellet position in the plasma. The ELM onset was determined by analysing magnetic pick-up coil signals and its delay relative to the time when the pellet crossed the separatrix was measured as a function of the pellet velocity. Supposing that to trigger an ELM a pellet has to reach a certain magnetic surface of the plasma independently of its mass and velocity, the most probable location of the seed perturbation was found to be at the middle of the pedestal—at the high plasma pressure gradient region. The onset of the MHD signature of the ELMs was detected about 50 µs after the pellet reached the seed position. According to our observations ELMs can be triggered either by the cooling of the pedestal region causing sudden increase of the pedestal plasma pressure gradient driving the plasma to the unstable region of the ballooning mode or by the strong MHD perturbation triggering an instability developing into an ELM.

Investigation of pellet-driven magnetic perturbations in different tokamak scenarios

Magnetic perturbations directly driven by pellets were studied in three different plasma scenarios in the ASDEX Upgrade tokamak to gain a deeper insight into the triggering process of type-I ELMs. In the type-I ELMy H-mode, promptly after the ELM, a mode with toroidal mode number n = ?6 (the negative sign denoting the ion drift direction) was detected in the 100?150?kHz frequency range, for both spontaneous and triggered ELMs. For triggered ELMs with pellets ablating longer than the ELM crash, this mode was observed for a longer time?therefore this could be identified as the pellet-driven perturbation. However, pellets promptly trigger ELMs after entering the plasma, and the large-amplitude ELM footprint masks the pellet-driven perturbation at the instance of the trigger event, i.e. the pellet-driven mode can only be studied after the ELM in a type-I ELMy H-mode. In L-mode plasmas the pellet was observed to drive broadband Alfv?n waves, detected in the 80?300?kHz frequency range with a toroidal mode number of n = ?6, similar to the mode after type-I ELMs, confirming that the mode seen in the H-mode after ELMs is indeed the pellet-driven perturbation. The magnitude of the pellet-driven perturbation was observed to increase monotonically with pellet penetration, and showed an exponential decay after pellet burn-out. Similarities and differences are discussed for the type-III ELMy H-mode scenario, which resulted in the finding that the pellet only drives and/or triggers modes which can be naturally present in the target plasma. Concerning type-I ELM triggering, the pellet-driven magnetic perturbation is unlikely to be the trigger for ELMs, since the structure of the pellet-driven modes is completely different from that of the observed pre-ELM modes (coherent modes with toroidal mode number n = 3 and 4, similar to Washboard modes) or type-I ELMs themselves (also n = 3 and 4).

Volume measurement of cryogenic deuterium pellets by Bayesian analysis of single shadowgraphy images.

In situ commissioning of the Blower-gun injector for launching cryogenic deuterium pellets at ASDEX Upgrade tokamak was performed. This injector is designed for high repetitive launch of small pellets for edge localised modes pacing experiments. During the investigation the final injection geometry was simulated with pellets passing to the torus through a 5.5 m long guiding tube. For investigation of pellet quality at launch and after tube passage laser flash camera shadowgraphy diagnostic units before and after the tube were installed. As indicator of pellet quality we adopted the pellet mass represented by the volume of the main remaining pellet fragment. Since only two-dimensional (2D) shadow images were obtained, a reconstruction of the full three-dimensional pellet body had to be performed. For this the image was first converted into a 1-bit version prescribing an exact 2D contour. From this contour the expected value of the volume was calculated by Bayesian analysis taking into account the likely cylindrical shape of the pellet. Under appropriate injection conditions sound pellets with more than half of their nominal mass are detected after acceleration; the passage causes in average an additional loss of about 40% to the launched mass. Analyzing pellets arriving at tube exit allowed for deriving the injectors optimized operational conditions. For these more than 90% of the pellets were arriving with sound quality when operating in the frequency range 5-50 Hz.

Pellet cloud characterisation, scaling and estimation of the material- and temperature distribution inside the cloud

Using spatially calibrated images of fast visible cameras, a database was established containing pellet cloud images and the related pellet- and plasma parameters. Using this database, two scalings were derived for the cloud size along the magnetic field lines as a function of pellet speed and ablation rate (first case) and pellet speed, pellet volume, plasma temperature and plasma density (second case). Using the images—based on the number of radiation maxima—the four main cloud shapes were also categorized. The isotope effect (the effect of hydrogen pellets in hydrogen or helium plasma) was also investigated with particular attention devoted to the cloud characteristics. Finally, a synthetic diagnostic—which simulates the measurement system and produces a synthetic pellet cloud image based on the output of the pellet cloud simulation—was developed to reveal the underlying density- and temperature distributions of the observed pellet cloud images. Using this synthetic diagnostic, one of the main identified cloud shapes was reconstructed. Our goal is to derive a scaling law for the toroidal extension of the pellet cloud at different pellet- and plasma conditions, to give a more reliable input for the pellet ELM triggering simulations and using these two results—a better understanding of the pellet-caused pressure perturbation.