Ideal Magnetohydrodynamic Simulations of Low Beta Compact Toroid Injection into a Hot Strongly Magnetized Plasma

📝 Abstract

We present results from three-dimensional ideal magnetohydrodynamic simulations of low $\beta$ compact toroid (CT) injection into a hot strongly magnetized plasma, with the aim of providing insight into CT fueling of a tokamak with parameters relevant for ITER (International Thermonuclear Experimental Reactor). A regime is identified in terms of CT injection speed and CT-to-background magnetic field ratio that appears promising for precise core fueling. Shock-dominated regimes, which are probably unfavorable for tokamak fueling, are also identified. The CT penetration depth is proportional to the CT injection speed and density. The entire CT evolution can be divided into three stages: (1) initial penetration, (2) compression in the direction of propagation, and reconnection with the background magnetic field, and (3) coming to rest and spreading in the direction perpendicular to injection. Tilting of the CT is not observed due to the fast transit time of the CT across the background plasma.

💡 Analysis

We present results from three-dimensional ideal magnetohydrodynamic simulations of low $\beta$ compact toroid (CT) injection into a hot strongly magnetized plasma, with the aim of providing insight into CT fueling of a tokamak with parameters relevant for ITER (International Thermonuclear Experimental Reactor). A regime is identified in terms of CT injection speed and CT-to-background magnetic field ratio that appears promising for precise core fueling. Shock-dominated regimes, which are probably unfavorable for tokamak fueling, are also identified. The CT penetration depth is proportional to the CT injection speed and density. The entire CT evolution can be divided into three stages: (1) initial penetration, (2) compression in the direction of propagation, and reconnection with the background magnetic field, and (3) coming to rest and spreading in the direction perpendicular to injection. Tilting of the CT is not observed due to the fast transit time of the CT across the background plasma.

📄 Content

arXiv:0902.2485v2 [physics.plasm-ph] 25 Jun 2009 Ideal Magnetohydrodynamic Simulations of Low Beta Compact Toroid Injection into a Hot Strongly Magnetized Plasma Wei Liu1, Scott C. Hsu2, Hui Li1 1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM, USA, 87545 2 Physics Division, Los Alamos National Laboratory, Los Alamos, NM, USA, 87545 E-mail: wliu@lanl.gov Abstract. We present results from three-dimensional ideal magnetohydrodynamic simulations of low β compact toroid (CT) injection into a hot strongly magnetized plasma, with the aim of providing insight into CT fueling of a tokamak with parameters relevant for ITER (International Thermonuclear Experimental Reactor). A regime is identified in terms of CT injection speed and CT-to-background magnetic field ratio that appears promising for precise core fueling. Shock-dominated regimes, which are probably unfavorable for tokamak fueling, are also identified. The CT penetration depth is proportional to the CT injection speed and density. The entire CT evolution can be divided into three stages: (1) initial penetration, (2) compression in the direction of propagation, and reconnection with the background magnetic field, and (3) coming to rest and spreading in the direction perpendicular to injection. Tilting of the CT is not observed due to the fast transit time of the CT across the background plasma. PACS numbers: 25.60.Pj, 28.52.Cx, 52.30.Cv,52.55.Fa,52.65.Kj Submitted to: Nuclear Fusion Compact Toroid Fueling 2

1. Introduction It is important to deliver fuel into the core of a tokamak fusion plasma to maintain steady-state operation, achieve more efficient utilization of deuterium-tritium fuel, and optimize the energy confinement time [1]. Several fueling schemes have been proposed, such as edge gas puffing, pellet injection [2], and compact toroid (CT) injection [3]. Among them, CT fueling is considered to be the most promising method for core fueling because the injection speed via this method is far higher than those of the other methods. Although extensive worldwide efforts have been devoted to study CT fueling theoretically [1, 3, 4], numerically [5, 6, 7, 8], and experimentally [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27], the dynamics of core CT fueling of large devices like ITER (International Thermonuclear Experimental Reactor) [28] is not well understood. CT injection has the potential to deposit fuel in a controlled manner at any point in the machine, from the edge to the core. Tangential (toroidal) injection can impart momentum for improving plasma β and stability [29]. In a burning plasma device with only radio-frequency (rf) for auxiliary current drive, a CT injection system may be the only internal profile control tool for optimizing bootstrap current and maintaining optimized fusion burn conditions. CT fueling also provides a good chance to study core transport in present machines, helium ash removal, and Edge Localized Mode (ELM) [30] control. In this work, we employ a simple idealized model of a low β CT propagating into a uniform slab plasma with a uniform magnetic field perpendicular to the CT injection direction, mimicking CT fueling into a tokamak with infinite aspect ratio. This model helps us identify different regimes of operation in terms of CT injection speed, density, and magnetic field strength, as well as understand the essential physics occurring during CT injection. More realistic scenarios, including the use of realistic tokamak profiles and geometry in the background plasma, as well as high β CT’s and dense plasma jets, are planned for follow-on research. Compared to past work on CT injection simulations, we have investigated new regimes especially in terms of higher injection velocity and a more ITER-relevant ratio (at least for low β CT’s such as spheromaks) of CT-to- background magnetic field (∼0.1). Simulations with higher injection velocity were made possible by the shock-handling capability of our three-dimensional (3D) ideal magnetohydrodynamic (MHD) code [31]. The lower CT-to-background magnetic field ratios, compared to past work, was enabled by the higher resolution of our code which allowed the boundary layer between the CT and background plasma to be properly resolved. The paper is organized as follows. In Sec. 2, we describe the the problem setup including initialization of the CT and background slab plasma and the numerical model. We present the simulation results in Sec. 3, and our conclusions and implications for future CT fueling experiments are given in Sec. 4. Compact Toroid Fueling 3
2. Problem setup and numerical model A low β CT with spherical radius rb = 1, centered initially at xb = 0, yb = 0 and zb = zb,0 = −12, is injected along the z axis into a lower density background plasma with injection velocity vinj (see Figure 1). The basic model assumptions and numerical treatments are briefly summarized here; they are essentially the same as those in Li et al.[32] where mor

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