12/13/2023

Today Sebastian Thiede defended his Master's thesis on the topic 'Improvements of heat flux calculations for water-cooled plasma-facing components in W7-X. In his Master's thesis, Sebastian Thiede developed a code (layer THEODOR) for calculating the heat flow within the wall. The code can calculate components composed of different materials, such as the new divertor of Wendelstein 7-X. This now allows to calculate the heat flux from the plasma to the divertor to be determined from thermographic data. The code was validated using an analytical model, in comparison with ANSYS and with comparative measurements using caliometry. It was a great job. Congratulations. Special thanks at this point to Yu Gao, who supervised this work. That was great support.   

12/12/2023

Limit-cycle oscillations (LCOs) occur during the transition from low to high confinement. These occur in the low kHz range. Ondrej Grover and Peter Manz suggested that the frequency depends on the size of the tokamak and developed a model for this. In a recently published study, this was demonstrated using a database with data from the largest tokamak (JET) to ASDEX Upgrade, COMPASS and the smallest tokamak (Globus-M). 

12/08/2023

For future tokamak-based fusion reactors, only ELM-free confinement regimes are actually possible. Two of these - the EDA H-mode and the QCE regime - are characterised by a quasi-coherent mode (QCM) in the turbulence. This was systematically characterised in the work of Joey Kalis in ASDEX Upgrade. The QCM is a ballooning mode. The work was recently published in Nuclear Fusion. 

12/1/2023

Linear particle accelerators accelerate particles by gradients in the electric field. Plasma wakefield accelerators provide gradients several orders of magnitude higher than conventional linear particle accelerators significantly reducing the required size, thereby promising affordable and compact accelerators for various applications. The high electric fields require high plasma density. Helicon discharges are known to provide the highest densities in linear geometry. The prototype experiment PROMETHEUS-A developed in Greifswald with strong contributions by Dr. Fahrenkamp has demonstrated the required high density. The physics of helicon discharges at this particular high density is different from those in conventional helicon discharges. Based on code developments in Stuttgart, we will develop a code to understand the wave propagation and dissipation, the interplay with the gradients and the impact of the plasma boundaries. First, the code will be developed with the help of comparisons with experiments. We were able to win Oliver Lass for this project. It is Oliver Lass' job to carry out such experiments. As VINETA is not yet operational, he will carry out the first experiments on MAP in Madison and then later on VINETA and PROMETHEUS-A at CERN. The overall aim is to use the code to optimize helicon discharges towards particle-driven wakefield density requirements.

09/22/2023

The edge of fusion plasmas in characterized by steep gradients. In the presence of steep gradients, the applicability of local transport theory can break down. In general, when the scales of the mean field become comparable to the scales of the perturbations, the mean field no longer varies slowly in space. Hence, the higher spatial derivatives need to be retained. In this case, one can either use a series expansion or replace the expressions by convolutions of the mean field with some integral kernels. We investigated the turbulent transport in the presence of steep gradients by means of integral kernels in the stellerator TJ-K at the univeristy of Stuttgart.  This work, based on a Bachelor thesis by Nils Müller from the University of Hamburg, has been recently published in Phys. of Plasmas.  

we are looking for a PhD student in plasma physics 

The research group deals with dynamics in magnetized plasmas. High plasma densities are required for plasma wakefield accelerators, which can be achieved by means of helicon discharges. Helicon waves are electromagnetic waves in the radio frequency range that propagate in a magnetized plasma. The physics of helicon discharges at this particularly high density differs from conventional helicon discharges. The physical basis of helicon discharges with respect to the requirements of particle-driven accelerators will be investigated experimentally. First year work will be done at the University of Wisconsin-Madison. Finally, measurements at CERN are planned.

High willingness to travel and very good English skills are required. 

https://www.uni-greifswald.de/universitaet/information/stellenausschreibungen/wissenschaftliches-personal/stellenausschreibung/n/wissenschaftlicher-mitarbeiterin-institut-fuer-physik-23-b24-112669/

Your applications with the usual documents should preferably be sent by e-mail (one pdf file), quoting job vacancy number 23/B24, by 29.08.2023 to peter.manz@uni-greifswald.de

06-28-2023

The Charkiw Lectures are a series of lectures introducing the stellarator concept for magnetic confinement fusion. The virtual lectures cover the current state of research with emphasis on the latest developments in the field of helical confinement and the prospects for the reactor. Prof. Manz will open today with an introduction to the "Essentials of magnetic confinement fusion".

As part of the project subject 'Physics at the University' of the Alexander-von-Humboldt-Gymnasium in Greifswald, students were guests and learned about fusion research. After a preparation, they performed experiments at the tokamak GOLEM in Prague remote on 15.06.2023 in a competition to see who can achieve the highest energy confinement time. In one and a half hours, 14 discharges were performed with 33 attempts. We thank Vojtěch Svoboda for performing the experiments. 

2023/06/11

In future fusion reactors disruptions must be avoided at all costs. Disruptions due to the density limit (DL) are typically described by the power-independent Greenwald scaling. Recently, a power dependence of the disruptive DL was predicted by several authors (Zanca et al 2019 Nucl. Fusion59 126011; Giacomin et al 2022 Phys. Rev. Lett.128 185003; Singh and Diamond 2022 Plasma Phys. Control. Fusion64 084004; Stroth et al 2022 Nucl. Fusion62 076008; Brown and Goldston 2021 Nucl. Mater. Energy27 101002). It is investigated whether this increases the operational range of the tokamak. Increasing the heating power in the L-mode can induce an L-H transition, and therefore a power-dependent DL and the L-H transition cannot be considered independently. The different models are tested on a data base for separatrix parameters at the separatrix of ASDEX Upgrade and compared with the concept (SepOS) presented in Eich and Manz (2021 Nucl. Fusion61 086017). The disruptive separatrix density scales with the power n~P^0.38+/-0.08 in good agreement to all models. Also the back transition from high to low (H-L) confinement shows an approximately Greenwald scaling with an additional power dependence n~P^0.4 according to the SepOS concept. For future devices operating at much higher heating power such a power scaling may allow operation at much higher separatrix densities than are common in H-mode operation. Preconditions to extrapolation for future devices are discussed.