guest ::
| Home > Publications database > Mitigation of disruptions in a Tokamak by means of large gas injection |
| Home > Publications database > Mitigation of disruptions in a Tokamak by means of large gas injection |
| Dissertation / PhD Thesis/Book | PreJuSER-37825 |
2004
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/250
Report No.: Juel-4122
Abstract: In a tokamak, the poloidal magnetic field provided by the toroidal plasma current forms an essential part of the magnetic field confining the plasma. However, instabilities of the magnetohydrodynamic equilibrium can lead to an uncontrolled sudden loss of the plasma current and energy, which is called a disruption. During disruptions the plasma energy is typically deposited on the vessel walls within 0.1 ms resulting in high heat loads and possible melting or evaporating of in-vessel components. The interaction of halo currents caused by displacements of the plasma column with the magnetic field results in j X B-forces which can lead to structural damages. The increased loop voltage can give rise to the appearance of multi-MeV electron beams, so-called runaway electrons, which cause local damage when hitting the vessel wall. In order to avoid these detrimental consequences, disruption mitigation is an essential part of tokamak research. In these thesis, mitigation of disruptions by a fast gas injection is investigated. A special gas valve has been developed by us with one of the fastest reaction times available (0.5 - 1 ms at p = 1 - 30 bar). In contrast to other valves, it contains no ferromagnetic materials and can be operated in the full magnetic field close to the plasma. If a sufficient amount of gas is injected into the tokamak discharge prior to an uncontrolled disruption, a substantial amount of the thermal plasma energy is radiated, resulting in a more uniform distribution of the power density over the vessel walls, minimizing possible excessive localized heat loads. The use of non-reactive gases for mitigation ensures their fast removal from the vessel after the termination of a tokamak discharge. A series of experiments on the tokamak ASDEX Upgrade with di®erent amounts and kinds of gases shows a reduction of the plasma current decay time and a suppression of halo currents. To study the basic physical processes of a disruption, a one-dimensional numerical model of particle and energy transport has been developed. Calculations for neon show that a fast penetration of the neutral gas can occur owing to the cooling of the plasma at the front of the neutral particle cloud. Assuming a large inward transport of the injected impurity of the order of 100 m$^{2}$/s, the radiated energy becomes equal to the thermal plasma energy prior to the disruption. After the thermal collapse, the plasma reaches an equilibrium temperature of several eV as a balance between ohmic heating, radiation and heat conduction losses.
|
The record appears in these collections: |
PDF
Fulltext by OpenAccess repository