First ECRH plasmas in the Wendelstein 7-X Stellarator
Event details
| Date | 20.06.2016 |
| Hour | 14:00 › 15:00 |
| Speaker | Dr. Torsten Stange - Max-Planck-Inst. für Plasmaphysik, Greifswald, D |
| Location |
PPB 019
|
| Category | Conferences - Seminars |
On 10th December 2015, after 15 years of construction, the first plasma was created in Wendlstein 7-X (W7-X). It is the world's largest optimized stellarator, with 3-D shaped superconducting modular coils, and aims to achieve reactor relevant plasma parameters in steady state operation. The exclusive heating method in the first operational phase of W7-X (OP1.1) is electron cyclotron resonance heating (ECRH) at 140 GHz in X2 mode. In combination with the steady-state confining magnetic field at 2.5 T, the ECRH system allowed for very fast progress in OP1.1, with keV plasmas generated by 4 MW of input power already after 3 days of operation, even though the vacuum vessel was not conditioned by glow discharge cleaning in advance resulting in an early radiation collapse of the plasma. However, OP1.1 was a campaign primarily dedicated to the integral commissioning of the heating system, the diagnostics and the critical components of the device itself, such as the vacuum vessel and coil system. The discharge time was limited by the heat capacity of the uncooled graphite limiters installed on the high-field side (HFS) in each of the five W7-X modules, whereas the gyrotrons are capable of steady-state operation. Up to six gyrotrons were used in OP1.1 at power levels usually between 2 and 4 MW. This talk will focus on physical investigations based on the flexible use of the ECRH system, e.g. shaping of electron temperature and density profile by change of on- and off-axis power deposition, current drive scenarios for compensation of the bootstrap current and advanced heating scenarios like O2-heating. Furthermore, ECRH power modulation was used for first transport studies by use of the ECE diagnostic. The ECE radiometer of W7-X is, unlike many other ECE diagnostics, absolutely calibrated and consists of 32 channels with additional 16 flexible channels with higher radial resolution. The resultant confinement times can be compared with an analytical model for the power balance. The non-absorbed power is determined by stray-radiation probes mounted in each of the five modules, as well as an electron cyclotron absorption (ECA) diagnostic, consisting of 128 mono-mode waveguide antennas incorporated in the HFS graphite tiles opposite to the ECRH launchers. The excellent reliability of the stray radiation monitors enables the use as a plasma operation interlock. Comparing the stray radiation level with the electron cyclotron absorption measurements opposite to the ECRH launchers allows distinguishing poor absorption due to a decaying plasma from poor absorption because the cut-off density has been reached. The operation of W7-X was started with helium as discharge gas, followed by hydrogen, allowing a direct comparison of start-up and plasma performance for both gas species.
Practical information
- Informed public
- Free
Organizer
- Prof. P. Ricci
Contact
- Prof. P. Ricci