Inertial confinement fusion | EPFL Graph Search

Inertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with fuel. The targets are small pellets, typically containing deuterium (2H) and tritium (3H). Energy is deposited in the target's outer layer, which explodes outward. This produces a reaction force in the form of shock waves that travel through the target. The waves compress and heat it. Sufficiently powerful shock waves generate fusion. ICF is one of two major branches of fusion energy research; the other is magnetic confinement fusion. When first proposed in the early 1970s, ICF appeared to be a practical approach to power production and the field flourished. Experiments demonstrated that the efficiency of these devices was much lower than expected. Throughout the 1980s and '90s, experiments were conducted in order to understand the interaction of high-intensity laser light and plasma. These led to the design of much larger machines that

Theoretical study of electronic transport in TCV ELMy H-mode

Gaël Induni

High-confinement mode (H-mode) is a promising reference scenario for ITER. But we are still facing major issues because of instabilities. They expel periodically some of the energy, which can damage the device. These instabilities are called the edge localized modes (ELM) and are not yet fully theoretically understood. The present work is a study on the profiles evolution in between ELMs and on the ELM effects. This may help to have a better understanding of the conditions before the ELM. We use the simulations as theoretical tool. For the purpose of the simulations, we build an H-mode χ e profile according to a standard L-mode one that we truncate at the edge to create a transport barrier. This gives a good agreement with the experimental data. Several scaling laws were successfully used. The first one is the energy confinement time scaling which was used for the thermal diffusivity to scale the temperature profile. A scaling between the core and pedestal energies was found recently. It was used to compute the pedestal χ e to scale the temperature pedestal, which was successful. Finally, we used a scaling for transport barriers which links the density gradient length to that of the temperature to compute the density in the pedestal. It was already found to be good in TCV electron internal transport barriers and in ASDEX Upgrade H-mode pedestals. Looking at the MHD stability parameters, it was found that for our reference case, ELMs are not likely to be triggered by the time evolution of the pressure gradient and the current density profiles in our model, as these are only varying significantly during the first millisecond after the crash, and are almost constant during the long remaining time until the next crash. Studying different cases, we investigate the behavior of the plasma when replacing the edge heating by central one to observe the influence of the heating profile, but no significant difference was found, neither in the MHD stability parameters. Further we change the particle diffusion coefficient to compare the dynamic behavior of the density. Slowing down the density dynamic behavior also slows down the pressure one, this can be seen on the MHD stability parameters. We also vary the ELM period to compare to the change due to the variation of the particle diffusivity. It was found that there may be a sort of relation between the particle diffusivity and the ELM period at least for the density, since both cases change the density recovery time with respect to the ELM period. A last case considered is doubling the radial ELM interaction range. This is done in order to observe the difference to the reference simulation that takes the density top of pedestal as ELM range, and to compare the spatial range influenced by the MHD activity and the one by the transport improvement. It was found that the MHD stability parameters in the pedestal exhibit a different behavior with the pressure gradient starting to increase very fast.

2011

Advanced RF heating schemes in preparation for ICRF heating and fastion experiments in the W7-X stellarator

Jonathan Graves, Mike Machielsen, Hamish William Patten

Fast ion confinement is crucial for the demonstration of the stellarator approach towards fu- sion energy. To study confinement of fast ions with today’s stellarators advanced RF heating schemes can be used to generate fast ions. Secondly, advanced RF heating schemes are de- veloped to improve ion heating performance. Advanced ICRH schemes (e.g. the 3-ion species scheme [1]) have the advantage of improved polarization, so the wave energy is nearly com- pletely carried by the left hand polarized wave at resonance, thereby providing good power transfer to the ions. However, modelling of minority and 3-ion species heating in W7-X has shown that the fast ion tails are limited, and core heating is modest [3]. Additionally, the highly anisotropic distributions generated by the minority and 3-ion species schemes are not well con- fined. Furthermore, the high density plasma of W7-X creates difficulty heating thermal particles because of the high collisionality. An advantage of the so called RF-NBI synergetic scheme is that NBI born ions are weakly collisional at birth. Secondly, compared to other ICRH schemes the RF-NBI scheme heats more in the parallel direction, generating less trapped ions. Thirdly, a more isotropic velocity distribution is also desirable for fast ion studies since fusion born alphas are isotropic as well [2, 3]. The code SCENIC [4] is used to model ICRH scenarios in 3D. The code is comprised of a magnetic equilibrium code, a full wave code and a Fokker-Planck code. It is able to determine wave propagation and absorption in hot plasma with full FLR effects. Lastly, effects of finite orbit width effects and wave absorption on the distribution function are taken into account. This contribution will explore RF-NBI and other advanced heating schemes in W7-X relevant plasma scenarios prior to ICRF heating and fast ion experiments. [1] Ye. O. Kazakov, D. Van Eester, R. Dumont and J. Ongena, Nucl. Fusion 55, 3 (2015) [2] H. Patten et al., in preparation of Phys. Rev. Lett (2019) [3] H. Patten, "Development and optimisation of advanced auxiliary ion heating schemes for 3D fusion plasma devices", EPFL, (2018) [4] M. Jucker, "Self-consistent ICRH distribution functions and equilibria in magnetically confined plasmas", EPFL, (2010)

European Physical Society2019