Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

TLDR: The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s, and many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH.

Abstract: Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

Figures

FIG. 11. Cooling of the plasma edge detected with Langmuir probes at different positions in the plasma vessel as reaction on N2 injection into the edge plasma.

FIG. 10. Geometry of the helium beam diagnostic at the upper plasma edge. Adjustment of the 8 los was supported by the shown EMCR-EIRENE simulation of HeI emission at 706 nm.27 The LCFS is indicated by the dashed red line.

FIG. 9. HeII emission detected by the filterscope viewing the limiter in module 3. τp*He is derived from the indicated time decay of the emission after a He puff at ∼200 ms.

FIG. 5. Neutral gas pressure in four machine modules in a discharge terminated by strong edge radiation.

FIG. 8. CIII emission at limiter in module 5 (double-stripe structure at top left) detected with a visible camera and an interference filter at 467±5 nm. The bright spot at bottom right is due to NeI emission from neon injected by the helium beam injection system for edge cooling experiments.

FIG. 6. (a) Infrared image of limiter in module 5 shows heterogeneous temperature distribution due to 3D structure of the scrape-off layer; (b) heat flux density profiles calculated at three tiles of limiter in module 3.

Frequently asked questions

Q1. What is the effect of the electromagnetic forces on the magnetic field?

As modelled by finite element analysis the electromagnetic forces are causing an elastic deformation of the non-planar field coils depending on the field strength, thus influencing the rotational transform and the radial position of that island chain as well. 

Q2. What are the contributions in "Overview of diagnostic performance and results for the first operation phase in wendelstein 7-x (invited)" ?

Wendelstein 7-X ( invited ) M. Krychowiak, A. Kaczmarczyk, T. Kocsis, P. Kremeyer, I. Książek, M. Dzikowicka, F. Jakubowski, J. 

Q3. What was the use of the infrared camera?

The infrared camera was used to derive space and time resolved power fluxes, as well as in a calorimeter mode to infer total energy deposited on the limiter. 

Q4. What is the main purpose of the detectors?

The aperture plates and detector holders are water cooled as protection from thermal overheating as well as to minimize thermal drifts of the detector offset. 

Q5. What is the intense resonance line of a HEXOS?

The most intense resonance lines of medium ionization stages of expected intrinsic impurities as well as injected tracer impurities (e.g., Ar, Ne, N) emit light at wavelengths covered by HEXOS. 

Q6. What is the main purpose of the OP1.1 system?

Two endoscopes with horizontal and vertical bolometer cameras based on metal-resistive detectors have been operated during OP1.1.42,43 

Q7. What is the filtered visible camera used to measure?

The filtered visible camera provided absolutely calibrated measurements of carbon and hydrogen light, and therefore inputs to particle balance calculations. 

Q8. What is the phase shift of the 5.3 m signals?

After frequency doubling the residual 10.6 µm beam is frequency doubled in a second AgGaSe2 crystal, the phase shift of the 5.3 µm signals being detected. 

Q9. How many diagnostic systems have been used at W7-X?

About 20 diagnostic systems have been modeled and used at JET, MAST, TJ-II, TCV, KSTAR, ASDEX Upgrade and W7-X, making Minerva the default diagnostic modeling and analyzing system at W7-X. 

Q10. What is the concept of Bayesian graphical models?

It uses the concept of Bayesian graphical models45 to fully describe forward models, statistical and systematic uncertainties, regularizing priors and their dependencies on each other. 

Q11. What is the definition of a dispersion interferometer?

Dispersion interferometers are intrinsically insensitive to vibrations along the optical beam path, only vibrations perpendicular to that path increase theReuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. 

Q12. How many diagnostic systems have been commissioned?

The first experimental campaign with five uncooled graphite inboard limiters defining the LCFS has been conducted between December 2015 and March 2016 allowing the integral commissioning of all machine components and installed diagnostics. 

Q13. How wide is the viewing angle of the limiter?

The viewing angle of 86◦–136◦ allows the monitoring of a wide range of the plasma vessel interior with a spatial resolution of 4 mm (at the optical axis) up to 10 mm (at the far view) being sufficient for the monitoring purposes. 

Q14. How many diagnostic systems have been installed during OP1.1?

More than 20 diagnostic systems have been installed,2 commissioned and almost in all cases successfully operated during OP1.1, allowing many physics investigations. 

Q15. How much heat flux is generated by the limiter?

For a typical discharge of the first campaign with 3 MW of injected power, there is about 1.5 MW/m2 heat flux near the watershed (see Fig. 6(b)). 

Q16. What is the way to verify the existence of magnetic flux surfaces in a vacuum case?

In stellarator like magnetic configurations the verification of the existence and quality of magnetic flux surfaces is possible for the vacuum case, i.e., without theexistence of a plasma. 

Q17. What is the spectral distribution of the HEXOS?

Download to IP: 130.56.97.58 On: Mon, 28 Nov2016 03:50:27wavelength region between 2.5 and 160 nm, distributed over four spectral channels, with high spectral and temporal (up to 1 kHz).