Wendelstein 7-X on the way to high-Performance plasmas
Institutskolloquium
- Datum: 18.12.2020
- Uhrzeit: 10:30 - 12:00
- Vortragender: Dr. Sergey Bozhenkov
- - born 25.06.1981, Oktyabrsky, Russia - Master of Applied Physics and Mathematics, 1998 - 2004 Moscow Institute of Physics and Technology - Phd student, 2004 - 2007 Forschungszentrum Juelich: disruption mitigation and runaway studies - Phd in Physics in 2007 from Bohum University - post-doc: 2008 - 2014, Forschungszentrum Juelich and IPP Greifwald - permanent stuff scientist 2014 - present, IPP Greifswald Worked on: disruption mitigation, runaway electrons, field line tracing for W7-X (web-service, error field correction, limiter design), Thomson scattering, high-performance pellet experiments, database analysis of plasma performance, prove of principle FILD measurements, coordination of the topical group on the fast-ion physics.
- Ort: Zoom Meeting Room 2
- Raum: Zoom
- Gastgeber: Dmitry Moseev
- Kontakt: dmitry.moseev@ipp.mpg.de
In this presentation an overview of most
prominent experimental results from the stellarator Wendelstein 7-X relevant to
the overall plasma performance is given and significance of these findings to
future progress is discussed. W7-X is the first large optimized stellarator
that was designed with reactor relevant requirements in mind. Its design criteria
include reduction of the neoclassical heat transport, minimization of the
bootstrap plasma current, improved fast-ion confinement and a suitable divertor
concept. The ultimate goal of the project is to validate the viability of this
design line as a fusion reactor that could allow a true steady state operation
without hazardous disruption events. W7-X came into operation in 2015 after a
lengthy construction phase and has now completed two experimental campaigns,
including a short limiter phase and a test divertor phase. This presentation
reviews the most prominent experimental results from the point of view of the
overall plasma performance.
Although the initial experimental time was largely devoted to technical but necessary aspects, such as commissioning of plasma diagnostics, error field correction, first wall conditioning and heating power upgrade; some prominent physics results that are likely to remain relevant in the future were already obtained. In particular, it is found that the plasma performance is well below pure neoclassical predictions in the vast majority of discharges. Typical energy confinement times are at the level at or below the empirical ISS04-scaling. The ion temperature in such discharges is limited below 1.6 keV, with profiles reminiscent of stiffness in tokamaks. Furthermore, almost no dependence on the efficiency of collisional coupling between ions and electrons and on magnetic configuration (effective helical ripple) is observed. But, a significant and serendipitous improvement of the plasma performance is observed after the injection of a series of frozen hydrogen pellets that leads to very peaked density profiles. Ion temperatures above 3 keV and energy confinement times above the ISS04-scaling, were achieved in post-pellet phases. This conditions correspond to the highest triple product realized in stellarators so far.
At
least two important conclusions can be drawn from the described observations.
Firstly, the record parameters obtained
with pellet injection strongly suggest that the W7-X transport optimization is
effective. Secondly, the findings in non-pellet cases imply that the plasma
performance in W7-X is otherwise dominated by anomalous losses. Even though a
detailed understanding of turbulent
losses still needs to be elaborated with the help of e.g. gyro-kinetic GENE
simulations, presently it appears that peaking of the density profile is
important for improving the plasma performance. Future experiments to validate
this conjecture will benefit from further improved experimental capabilities,
such as first wall cooling, active pumping, continuous pellet injection,
increased heating power. Apart from that, a truly steady state plasma operation
will soon become possible after installation of water cooled targets.