H. Niemann

Efficient injection of an intense positron beam into a dipole magnetic field

We have demonstrated efficient injection and trapping of a cold positron beam in a dipole magnetic field configuration. The intense 5 eV positron beam was provided by the NEutron induced POsitron source MUniCh facility at the Heinz Maier-Leibnitz Zentrum, and transported into the confinement region of the dipole field trap generated by a supported, permanent magnet with 0.6 T strength at the pole faces. We achieved transport into the region of field lines that do not intersect the outer wall using the drift of the positron beam between a pair of tailored plates that created the electric field. We present evidence that up to 38% of the beam particles are able to reach the intended confinement region and make at least a 180° rotation around the magnet where they annihilate on an insertable target. When the target is removed and the plate voltages are switched off, confinement of a small population persists for on the order of 1 ms. These results lend optimism to our larger aims to apply a magnetic dipole field configuration for trapping of both positrons and electrons in order to test predictions of the unique properties of a pair plasma.

Synthetic plasma edge diagnostics for EMC3-EIRENE, highlighted for Wendelstein 7-X

Interpretation of spectroscopic measurements in the edge region of high-temperature plasmas can be a challenge since line of sight integration effects make direct interpretation in terms of quantitative, local emission strengths often impossible. The EMC3-EIRENE code-a 3D fluid edge plasma and kinetic neutral gas transport code-is a suitable tool for full 3D reconstruction of such signals. A versatile synthetic diagnostic module has been developed recently which allows the realistic 3D setup of various plasma edge diagnostics to be captured. We highlight these capabilities with two examples for Wendelstein 7-X (W7-X): a visible camera for the analysis of recycling, and a coherent-imaging system for velocity measurements.

A divertor scraper observation system for the Wendelstein 7-X stellarator

Two graphite divertor elements called scrapers have been installed on the Wendelstein 7-X stellarator in the throat of the magnetic island divertor. To diagnose one, we have designed, built, calibrated, and installed a new infrared/visible imaging endoscope system to enable detailed observations of the plasma interactions and heat loads at one of the scrapers and the nearby divertor surfaces. The new system uses a shuttered pinhole-protected pair of 90° off-axis 228 mm focal length aluminum parabolic mirrors, and two flat turning metal mirrors, to send light to a sapphire vacuum window 1.6 meters away, beyond which we have co-located telephoto lens-based infrared and visible cameras. The back-to-back off-axis parabolas serve to cancel out most aberrations, enabling the use of off-the-shelf commercial optics outside of the vessel. For the infrared, we use a 3-5 μm 1-megapixel FLIR SC8303HD camera and for the visible, a 5-megapixel CMOS PCO 5.5 edge camera. A short 1-m quartz pickoff fiber is used to send 200-1100 nm light to a compact spectrometer, also located in the same iron shield box as the cameras. The camera field of view covers the 700 mm length of the scraper, and includes locations monitored by thermocouples and Langmuir probes embedded in some of the scraper tiles. Predicted and actual optical test performances of the overall system are compared.

Infrared imaging systems for wall protection in the W7-X stellarator (invited)

Wendelstein 7-X aims at quasi-steady state operation with up to 10 MW of heating power for 30 min. Power exhaust will be handled predominantly via 10 actively water cooled CFC (carbon-fiber-reinforced carbon) based divertor units designed to withstand power loads of 10 MW/m2 locally in steady state. If local loads exceed this value, a risk of local delamination of the CFC and failure of entire divertor modules arises. Infrared endoscopes to monitor all main plasma facing components are being prepared, and near real time software tools are under development to identify areas of excessive temperature rise, to distinguish them from non-critical events, and to trigger alarms. Tests with different cameras were made in the recent campaign. Long pulse operation enforces additional diagnostic design constraints: for example, the optics need to be thermally decoupled from the endoscope housing. In the upcoming experimental campaign, a graphite scraper element, in front of the island divertor throat, will be tested as a possible means to protect the divertor pumping gap edges during the transient discharge evolution.

Density related operational limit in the limiter phase of Wendelstein 7-X

The stellarator Wendelstein 7-X (W7-X) has been designed to show that optimized stellarators can achieve and sustain fusion relevant plasma conditions. One of the main optimization criteria was the reduction of the neoclassical transport, which is considered as one of the most critical issues of the stellarator concept. While the demonstration of the neoclassical optimization will definitely be one of the most important aspects of W7-X, several additional topics are equally important due to the boundary condition that fusion relevant conditions should be demonstrated and sustained. Hence, low neoclassical transport needs to be achieved in a scenario which is compatible with divertor operation, high densities, and an acceptable impurity concentration without further accumulation during stable operation. So far, these issues have mostly not been tackled experimentally in the first experimental campaign. W7-X had its first plasma in December 2015 and the first operational campaign (OP1.1) was mainly intended for testing and commissioning purposes. In OP1.1, W7-X was operated in a limiter configuration without divertor and at low densities. Nevertheless, valuable insights could be gained which help to prepare and understand the first experiments with a test divertor planned to start in the second half of 2017. As will be shown in this contribution, one of the interesting observations in OP1.1 was the presence of an operational limit, where above a critical density the power balance seems to be dominated by radiative losses. Such a behavior is well-known from other stellarator experiments and while it can be expected that this critical density is particularly low for OP1.1 (high impurity concentration connected to the limiter operation and conservative wall conditioning), this observation also shows that impurity related radiation losses will be an important issue to keep track of as the density is progressively increased in the next experimental campaigns of W7-X.