Analysis of the neutral fluxes in the divertor region of Wendelstein 7-X under attached and detached conditions using EMC3-EIRENE

This paper analyzes the neutral fluxes in the divertor region of the W7-X standard configuration for different input powers, both under attached and detached conditions. The performed analysis is conducted through EMC3-EIRENE simulations. They show the importance of the horizontal divertor to generate neutrals, and resolve the neutral plugging in the divertor region. Simulations of detached cases show a decrease in the number of generated neutrals compared to the attached simulations, in addition to a higher fraction of the ion flux arriving on the baffles during detachment. As the ionization takes place further inside the plasma during detachment, a larger percentage of the generated neutral particles leave the divertor as neutrals. The leakage in the poloidal and toroidal direction increases, just as the fraction of collected particles at the pumping gap. The fraction of pumped particles increases with a factor two, but stays below one percent. This demonstrates that detachment with the current target geometry, although it improves the power exhaust, is not yet leading to an increased particle exhaust

Heat and particle exhaust in high-performance plasmas in Wendelstein 7-X

The paper reports for the first time the heat and particle exhaust at the plasma boundary through various edge diagnostics for the high-performance plasma obtained after pellet injection on Wendelstein 7-X. The plasma density at the edge is found to be reduced by a factor of 2 in the high-performance phase, supporting the previously reported density peaking at the plasma centre. The plasma beta effect on the magnetic topology is reflected by the appearance of the second strike line, which is well understood with simulation. However, during the rapid decay phase of the enhanced confinement, a transient localized heat flow of up to 16 MW m-2 is observed at the leading edge of a poorly cooled divertor component, which has not been understood but raises concerns about machine safety

Heat and particle exhaust in high-performance plasmas in Wendelstein 7-X

The paper reports for the first time the heat and particle exhaust at the plasma boundary through various edge diagnostics for the high-performance plasma obtained after pellet injection on Wendelstein 7-X. The plasma density at the edge is found to be reduced by a factor of 2 in the high-performance phase, supporting the previously reported density peaking at the plasma centre. The plasma beta effect on the magnetic topology is reflected by the appearance of the second strike line, which is well understood with simulation. However, during the rapid decay phase of the enhanced confinement, a transient localized heat flow of up to 16 MW m−2 is observed at the leading edge of a poorly cooled divertor component, which has not been understood but raises concerns about machine safety

Analysis of the gas balance for Wendelstein 7-X

This work presents the first experimental investigation of the gas balance on the optimized modular stellarator Wendelstein 7-X (W7-X). A balance of all injected and removed particles and a measurement of internal particle reservoirs allows inference of the bound particle reservoir in the wall, which is of interest due to its effects on plasma density control and fuel retention. Different scenarios of the gas balance are presented with data from the operation campaign 1.2 with an inertially cooled graphite divertor. Both net outgassing and net retention scenarios are presented and W7-X is found to operate stable in a wide range of scenarios with varying wall conditions. Since fusion experiments are conducted in ultra-high vacuum, suitable gauges are required for total and partial pressure measurement. The challenges and opportunities of the operation of pressure gauges in the steady magnetic field extending beyond plasma pulses are discussed. The performance of newly improved neutral pressure gauges, based on crystal cathode emitters is quantified. These provide improved operational robustness since they can be operated for long periods of time in strong magnetic fields. A crystal cathode setup and and its operation performance is presented along with a fast calibration scheme. Partial pressure measurements provide additional important information complementing the total neutral pressure measurements, and allowing additional physics insights. As part of this thesis work, a new diagnostic of this kind was implemented on W7-X, the so-called diagnostic residual gas analyzer (DRGA). It provides a wealth of information on various neutral gas species, with a relatively high time resolution - of order a few seconds. The diagnostic setup and its first results are presented in this thesis.Diese Arbeit stellt die erste experimentelle Untersuchung der Gasbilanz für den optimierten modularen Stellarator Wendelstein 7-X (W7X) vor. Eine Bilanz aller injizierten und entfernten Partikel und eine Messung der internen Partikelreservoirs erlaubt die Rückschlüsse auf das in der Wand gebundene Partikelreservoir, was wegen seiner Auswirkungen auf die Plasmadichtekontrolle und die Brennstoffrückhaltung von Interesse ist. Verschiedene Szenarien der Gasbilanz werden mit Daten aus der Betriebskampagne 1.2 mit einem inertialgekühlten Graphitdivertor vorgestellt. Es werden sowohl Netto-Ausgasungs- als auch Netto-Rückhalteszenarien vorgestellt, und es wird festgestellt, dass W7-X in einem breiten Spektrum von Szenarien mit unterschiedlichen Wandbedingungen stabil arbeitet. Da Fusionsexperimente im Ultrahochvakuum durchgeführt werden, sind geeignete Messgeräte zur Messung des Gesamt- und Partialdrucks erforderlich. Die Herausforderungen und Möglichkeiten des Betriebs von Druckmessgeräten im stationären Magnetfeld über die Plasmapulse hinaus werden diskutiert. Die Leistung neu verbesserter Neutraldruckmessgeräte, die auf Kristallkathoden-Emittern basieren, wird quantifiziert. Diese bieten einen robusteren Betrieb, da sie über lange Zeiträume in starken Magnetfeldern betrieben werden können. Ein Kristallkathodenaufbau und seine Betriebsleistung wird zusammen mit einem schnellen Kalibrierungsschema vorgestellt. Partialdruckmessungen liefern zusätzliche wichtige Informationen, die die Messungen des Totaldrucks ergänzen und zusätzliche physikalische Erkenntnisse ermöglichen. Im Rahmen dieser Doktorarbeit wurde eine neue Diagnostik dieser Art auf W7-X implementiert, der sogenannte diagnostischen Restgasanalysator (DRGA). Sie liefert eine Fülle von Informationen über verschiedene neutrale Gasspezies mit einer relativ hohen Zeitauflösung - in der Größenordnung von wenigen Sekunden. Der Diagnostikaufbau und seine ersten Ergebnisse werden in dieser Arbeit vorgestellt

Tuning of the rotational transform in Wendelstein 7-X

The control of rotational transform in Wendelstein 7-X (W7-X) is key to both the island divertor operation and safety of plasma facing components. The island divertor concept in W7-X relies on an edge flux surface with rotational transform of ι - = 1 resonating with an intrinsic n/m = 5/5 resonance to form a five lobed island chain. This island chain intersects with divertor plates to give rise to the island divertor. Changes in the relative position of the rational surface and the divertor plates can result in changes in divertor performance, thus the control of the rotational transform is essential to operation of the W7-X device. During the first divertor campaign electromagnetic loads resulted in elastic deformations of the shaped modular stellarator coils. Such deformations made these coils more planar, reducing the vacuum rotational transform, subsequently shifting the ι - = 1 resonance outward. Unintended plasma wall interactions provided the first clear evidence of this effect during plasma operation. Flux surface measurements were utilized to estimate the correct level of current in the planar coils for correction of ι -, and found to be around ∼−100 A. Scans of International the planar coil current for iota correction were performed during plasma operation. These measurements suggest planar coil currents between −250 and ∼0A would place the strike lines at the designed distance from the pumping gaps. Divertor Langmuir and upstream probe measurements confirm these estimates along with measurements of divertor neutral gas pressure.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019-2020 under grant agreement No. 633053

Major results from the first plasma campaign of the Wendelstein 7-X stellarator

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 1019 m-3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.Peer reviewe