T. Stange

Plans for the first plasma operation of Wendelstein 7-X

Wendelstein 7-X (W7-X) is currently under commissioning in preparation for its initial plasma operation phase, operation phase 1.1 (OP1.1). This first phase serves primarily to provide an integral commissioning of all major systems needed for plasma operation, as well as systems, such as diagnostics, that need plasma operation to verify their foreseen functions. In OP1.1, W7-X will have a reduced set of in-vessel components. In particular, five graphite limiter stripes replace the later foreseen divertor. This paper describes the expected machine capabilities in OP1.1, as well as a selection of physics topics that can be addressed in OP1.1, despite the simplified configuration and the reduced machine capabilities. Physics topics include the verification and adjustment of the magnetic topology, the testing of the foreseen plasma start-up scenarios and the feed-forward control of plasma density and temperature evolution, as well as more advanced topics such as scrape-off layer (SOL) studies at short connection lengths and transport studies. Plasma operation in OP1.1 will primarily be performed in helium, with a hydrogen plasma phase at the end.

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.

Absolute calibration of sniffer probes on Wendelstein 7-X

Here we report the first measurements of the power levels of stray radiation in the vacuum vessel of Wendelstein 7-X using absolutely calibrated sniffer probes. The absolute calibration is achieved by using calibrated sources of stray radiation and the implicit measurement of the quality factor of the Wendelstein 7-X empty vacuum vessel. Normalized absolute calibration coefficients agree with the cross-calibration coefficients that are obtained by the direct measurements, indicating that the measured absolute calibration coefficients and stray radiation levels in the vessel are valid. Close to the launcher, the stray radiation in the empty vessel reaches power levels up to 340 kW/m(2) per MW injected beam power. Furthest away from the launcher, i.e., half a toroidal turn, still 90 kW/m(2) per MW injected beam power is measured.

Study of plasma start-up initiated by second harmonic electron cyclotron resonance heating on WEGA experiment

Although both 1st harmonic ordinary mode (O1) and 2nd harmonic extra-ordinary mode (X2) have been successfully used to initiate pre-ionization and breakdown in many devices, a complete theoretical model is still missing to explain the success of this method. Moreover, some experimental observations are not completely understood, such as what occurs during the delay time between the turn-on of ECRH power and first signals of density or light measurements. Since during this free period the ECRH power has to be absorbed by in-vessel components, it is of prime importance to know what governs this delay time. Recently, dedicated start-up experiments have been performed on WEGA, using a 28 GHz ECRH system in X2-mode. This machine has the interesting capability to be run also as a tokamak allowing comparative experiments between stellarator (ι/2π > 0) and tokamak (ι/2π = 0) configurations. Different scans in heating power, neutral gas pressure, and rotational transform (ι) show clearly that the start-up is a two...

Measurements of satellite modes in 140 GHz wendelstein 7-X gyrotrons: An approach to an electronic stability control

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.

Determination of electron thermal diffusivity at the WEGA stellarator

For determining the thermal diffusivity in WEGA the electron cyclotron resonance heating power from a 28?GHz gyrotron is square-wave modulated to provide a periodically varying energy source at the plasma centre causing a modulation mainly of the electron temperature Te. A fast Si-diode bolometer system, whose 16 channel view the entire plasma cross-section, is employed to monitor the heat propagation process. A fast Fourier transform analysis shows clear coherency of the line-integrated signals among different channels and monotonic radial increment of the phase delays of the fundamental components in the central channel signals, allowing the determination of the electron thermal diffusivity coefficient D. A purely diffusive heat transport is assumed and is simulated using a Monte Carlo method. The thermal diffusivity is determined by matching the simulated results to the measured ones. A typical value of D = 1.9?m2?s?1 is obtained. This value is then compared with the result based on a local power balance analysis. Within the error bars the local power balance calculation yields a similar diffusivity value. The limitations and conditions of using bolometric diagnostic for this purpose are discussed.

Stochastic acceleration of relativistic electrons and plasma heating and current drive with 2.45 GHz frequency at the WEGA stellarator

MeV electrons were produced with less than 6 kW of non-resonant 2.45 GHz microwave heating at magnetic fields of 0.3–0.5 T with no applied loop voltage at the WEGA stellarator. The maximum toroidal plasma current, carried by the electrons, was 1 kA. Broadband radiation, similar to synchrotron radiation, was measured in the 0.5–120 GHz range. X-ray radiation from the plasma and directed γ-ray emission from collisions of the electrons with nearby plasma-facing components was measured as well. The acceleration process was modeled by a tail formation due to stochastic interaction with the radio frequency field at the antenna mouth. The characteristic time for tail formation could be reproduced with power modulation experiments.

Electron Bernstein Wave Experiments at the WEGA Stellarator

Fundamental investigations on EBW excitation, propagation and absorption are presented. In particular the OXB mode conversion and EBW current drive with 2.45 GHz EBWs are investigated and compared with modeling. High density operation with 28 GHz EBWs could be achieved at 0.5 T (2nd harmonic). This regime is characterized by a additional strongly supra‐thermal electron distribution with energies >10 keV. These electrons were detected by measurement of the EBW emission and by X‐ray detection.