Analysis of disruptions and their mitigation using ultra-fast observation systems
Analysis of disruptions and their mitigation using ultra-fast observation systems
A tokamak disruption represents a potential threat to the integrity and availability of fusion reactors and experiments to come. Disruption is a complex process in which the energy stored in the discharge is lost in two steps. First the thermal energy is released on a submillisecond time scale. Then...
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Personal Name(s): | Bozhenkov, Sergey (Corresponding author) |
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Contributing Institute: |
Plasmaphysik; IEF-4 |
Imprint: |
Jülich
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
2009
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Physical Description: |
133 p. |
Dissertation Note: |
Zugl.: Diss., Univ., Bochum, 2007 |
Document Type: |
Report |
Series Title: |
Berichte des Forschungszentrums Jülich
4288 |
Link: |
OpenAccess |
Publikationsportal JuSER |
A tokamak disruption represents a potential threat to the integrity and availability of fusion reactors and experiments to come. Disruption is a complex process in which the energy stored in the discharge is lost in two steps. First the thermal energy is released on a submillisecond time scale. Then the magnetic energy associated with the plasma current is dissipated in a resistive way. Fast release of the thermal energy during the first stage can lead to a significant erosion of the wall components - up to 100 μm per event. In elongated plasmas loss of the vertical control is typical in a disruption and leads to excitation of halo currents. Since these currents are closed in the wall, the vessel is subjected to strong $\vec{j}$ x $\vec{B}$ forces. High induced fields during the current decay can accelerate a beam of runaway electrons with energies of tens of MeV. A local deposition of such beam can lead to melting of the wall. To prevent machine damage it was proposed to soften the disruption consequences by a fast injection of impurities. Such plasma quenching by massive gas injection is the topic of this thesis. Massive gas injections are performed with the aid of a fast valve activated by eddy currents. The absence of any ferromagnetic materials in the construction makes the valve suitable for the magnetic fusion environment. The valve was developed several years ago at the Forschungzentrum Jülich, however its main characteristics remained poorly known. For this reason the first part of the thesis is devoted to study of the valve characteristics. The study is based on direct observation of the piston motion by means of a fast framing camera. The piston stroke and the injection duration are shown to strongly depend on the operational pressure and the used gas. The same is true for the valve throughput. The knowledge of the injection duration is also used to deduce the pressure decay rates and the gas outflow rates. The dependence of gas outflow rate on the piston stroke shows that the outflow rates can be increased by a factor of 4 by modifying the diameter of output orifice by a factor of 2. The modified valve is currently in operation at TEXTOR. Disruption mitigation experiments with the fast valve were conducted at TEXTOR. The superior ultra-fast framing camera system that is available at TEXTOR allows detailed and systematic studies of the impurities dynamics. Together with complementary diagnostics (ECE, Thomson scattering, soft X-ray camera) these data reveal that the plasma quenching is gradual only at the beginning of the gas puff. As soon as the edge safety factor becomes equal to 2 the plasma is destabilized and the core of the discharge collapses. This phenomenology is confirmed in a wide range of gas pressures 1.5 − 20 bar and for different used gases: D$_{2}$, He, Ar, 5% Ar + 95% D$_{2}$, 10% Ar + 90% D$_{2}$, 20% Ar + 80% D$_{2}$. The thermal quench of the induced disruption, i.e. cooling of the core plasma, lasts about 0.5 ms for argon and mixtures of argon with deuterium and more than 1 ms for helium. The inward mixing of impurities during this stage accelerates the following current decay, which is known to be preferable for the reduction of the electromagnetic stresses due to halo currents. The current decay in argon experiments is almost 2 times faster than in helium experiments. In the case of low pressure (p$_{w}$ < 10 bar) argon injections the generation of runaway electrons carrying up to 25% of the initial plasma current is registered. Increase of the amount of injected argon suppresses runaway electrons. Since runaway electrons are potentially the most dangerous consequence of a disruption in ITER, the generation of runaway electrons in TEXTOR experiments is analyzed in the framework of a 0D model. The model consists of equations for the plasma current, the currents induced in the vessel, the generation of runaway electrons and the evolution of plasma thermal energy in coronal equilibrium. The main free parameters of the model are the density of deuterium and density of injected atoms. These parameters are chosen in such a way as to provide the best matching between the modeled current evolution and that measured experimentally. The analysis shows that the runaway electrons arise because of the incomplete inward mixing of atoms. The mixing efficiency is estimated to be 3% for pure argon injections, 15% for injections of argon mixtures and 40% for helium. |