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

Abstract

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.

Scientific and technical challenges on the road towards fusion electricity

“…The first campaign of Wendelstein 7-X was very successful and has already provided information on the key physics issues for stellarators, such as plasma confinement [36]. The coming campaigns will look to more fully address the key physics issues and bring the concept towards maturity.…”

Nonlinear fatigue crack propagation in a baffle module of Wendelstein 7‐X under cyclic bending loads

“…Currently, Wendelstein 7‐X (W7‐X) is the world's largest modular stellarator to conduct nuclear fusion experiments at the Max Plank Institute (Greifswald, Germany). A hydrogen plasma is generated inside a plasma vessel (PV), and extremely high heat fluxes are radiated on the first wall of the plasma facing components . The associated baffles and heat shields are both made of graphite tiles that are bolted with low prestress onto a heat sink of CuCrZr .…”

“…A hydrogen plasma is generated inside a plasma vessel (PV), and extremely high heat fluxes are radiated on the first wall of the plasma facing components. [7][8][9]32 The associated baffles and heat shields are both made of graphite tiles that are bolted with low prestress onto a heat sink of CuCrZr. [7][8][9]32 The heat sink was brazed onto a water-cooled steel pipe of 12-mm outer diameter and 1-mm thickness.…”

“…[7][8][9]32 The associated baffles and heat shields are both made of graphite tiles that are bolted with low prestress onto a heat sink of CuCrZr. [7][8][9]32 The heat sink was brazed onto a water-cooled steel pipe of 12-mm outer diameter and 1-mm thickness. The baffles are needed to cool the PV under operative conditions, and they were supported rigidly on a steel structure while the heat shields were connected flexibly with pins onto the PV wall.…”

The new W7-X logbook – A software for effective experiment documentation and collaborative research at Wendelstein 7-X

“…The stellarator experiment Wendelstein 7-X (W7-X) [1] completed the first part of its second operation phase (OP 2.1a) in December 2017. Even in these early phases, nearly 40 diagnostic systems were already operated by an international research team [2]. In the first operational phases -with discharges limited to 100 s -up to 50 experiment programs (XP) can be conducted per day.…”