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.EURATOM 63305

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 x 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.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 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission

Towards a new image processing system at Wendelstein 7-X:From spatial calibration to characterization of thermal events

Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors.</p

Towards a new image processing system at Wendelstein 7-X: From spatial calibration to characterization of thermal events

Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors

Neutraalisuihkujen simuloiminen Wendelstein 7-X -stellaraattorissa

Stellarator fusion devices have seen a resurgence of interest in recent years, culminating in the building of the Wendelstein 7-X (W7-X) optimized stellarator in Germany. Compared to tokamaks, stellarators have the advantage of having no externally driven electric current. However, their non-axisymmetric nature makes it difficult to achieve closed particle orbits. The neutral beam injection (NBI) heating system will be a significant source of fast ions in W7-X, hence predictive modeling of the fast ion confinement is required for machine protection and scenario development. In this thesis, methods for numerical simulations of NBI ions in W7-X are established. Stellarator-specific magnetic field and radial electric field modules were implemented for ASCOT5, the latest development version of the Monte-Carlo orbit-following code ASCOT. Neutral beam injection in the W7-X high mirror ratio scenario was simulated with the BBNBI code, and subsequently the beam ion distribution and wall loads with ASCOT4. The results imply that the global confinement percentage of fast particles is higher than average, and the power loads low, in this scenario, although high local wall loads were also observed.Stellaraattori-tyyppiset fuusiolaitteet ovat viime vuosina tuottaneet yhä lupaavampia tuloksia, yhtenä tärkeimmistä Wendelstein 7-X stellaraattorin hyvin menestynyt ensimmäinen koekamppanja. Stellaraattorit ovat jatkuvatoimisia laitteita eivätkä vaadi ulkoisesti tuotettua plasmavirtaa, mutta niissä tarvitaan magneettikentän optimointia hiukkasten koossapidon varmistamiseksi. Erityisen vaarallisia ovat neutraalisuihkukuumennuksessa syntyvät nopeat ionit, joiden mallintaminen ennen kuumennuksen käyttöönottoa on tärkeää riskien kartoittamiseksi. Tässä työssä esittelemme menetelmiä neutraalisuihkujen mallinnukseen stellaraattoreissa. Stellaraattorien simuloimiseen vaadittavat magneetti- ja sähkökenttäominaisuudet lisättiin Monte Carlo-koodi ASCOT:n uusimpaan versioon. Neutraalisuihkujen toimintaa simuloitiin BBNBI-ohjelmalla, jonka jälkeen ionien ratoja seurattiin ASCOT4-ohjelmalla virtuaalisessa Wendelstein 7-X:ssä. Tulokset osoittivat, että nopeiden hiukkasten koossapito tutkituilla asetuksilla on kohtuullisen hyvä, mutta ionit saattavat paikallisesti aiheuttaa suuria seinäkuormia