Effect Of Fast Ions Research Articles

Achieving a long-lived high-beta plasma state by energetic beam injection.

Developing a stable plasma state with high-beta (ratio of plasma to magnetic pressures) is of critical importance for an economic magnetic fusion reactor. At the forefront of this endeavour is the field-reversed configuration. Here we demonstrate the kinetic stabilizing effect of fast ions on a disruptive magneto-hydrodynamic instability, known as a tilt mode, which poses a central obstacle to further field-reversed configuration development, by energetic beam injection. This technique, combined with the synergistic effect of active plasma boundary control, enables a fully stable ultra-high-beta (approaching 100%) plasma with a long lifetime.

Mechanical properties of carbon steel depending on the rate of the dose build-up of nitrogen and argon ions

The effect of pulsed irradiation with argons and nitrogen ions on the mechanical properties, mor� phology, and structure of the surface layers of carbon steel St3 (0.2% C, 0.4% Mn, 0.15% Si, and Fe for bal� ance) has been investigated depending on the rate of dose buildup at an average ion current density of 10, 20, and 40 μA/cm 2 . It has been established that the fatigue life and microhardness of surface layers increase in the entire studied range of dose buildup rates. This seems to be due to the hardening of the surface layers, which resulted from the generation of radiation defects and the irradiationdynamic effect of fast ions. The sample irradiated by argon ions at the lowest of the selected dose buildup rates j av = 10 μA/cm 2 withstands the largest number of cycles to failure.

Effect of re-entering fast ions on NBI heating power in high-beta plasmas of the Large Helical Device

We calculate the heating power of the neutral beam injection (NBI) in a 〈β〉 = 4.8% high-beta discharge achieved in the Large Helical Device (LHD). We investigate the difference of the heating efficiency and the heating power profile with and without the re-entering fast ion effects. When the re-entering fast ion effects are taken into account, the heating efficiency of the co-injection of the NBI (co-NBI case) is improved and it is about 1.8 times larger than that without the re-entering effects. In contrast, the heating efficiency with the re-entering effects in the counter-injection of the NBI (ctr-NBI case) scarcely differs from that without the re-entering ones. We also study the re-entering fast ion effects on the transport properties in the LHD high-beta discharges. It is found that the tendency of the thermal conductivities on the beta value is not so much sensitive with and without the re-entering effects. In addition, we investigate the difference in the re-entering fast ion effects caused by the field strength and the magnetic configuration. In the co-NBI case, the heating efficiency with the re-entering effect was improved with a decrease in the field strength. In contrast, in the ctr-NBI case, it barely differs by changing the field strength.

Fast-ion effects during test blanket module simulation experiments in DIII-D

Fast beam-ion losses were studied in DIII-D in the presence of a scaled mock-up of two test blanket modules (TBM) for ITER. Heating of the protective tiles on the front of the TBM surface was found when neutral beams were injected and the TBM fields were engaged. The fast-ion core confinement was not significantly affected. Different orbit-following codes predict the formation of a hot spot on the TBM surface arising from beam ions deposited near the edge of the plasma. The codes are in good agreement with each other on the total power deposited at the hot spot, predicting an increase in power with decreasing separation between the plasma edge and the TBM surface. A thermal analysis of the heat flow through the tiles shows that the simulated power can account for the measured tile temperature rise. The thermal analysis, however, is very sensitive to the details of the localization of the hot spot, which is predicted to be different among the various codes.

Advances in validating gyrokinetic turbulence models against L- and H-mode plasmas

Robust validation of predictive turbulent transport models requires quantitative comparisons to experimental measurements at multiple levels, over a range of physically relevant conditions. Toward this end, a series of carefully designed validation experiments has been performed on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] to obtain comprehensive multifield, multipoint, multiwavenumber fluctuation measurements and their scalings with key dimensionless parameters. The results of two representative validation studies are presented: an elongation scaling study performed in beam heated L-mode discharges and an electron heating power scan performed in quiescent H-mode (QH-mode) discharges. A 50% increase in the elongation κ is observed to lead to a ∼50% increase in energy confinement time τe and accompanying decrease in fluctuation levels, qualitatively consistent with a priori theoretical predictions and nonlinear GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] simulations. However, these simulations exhibit clear quantitative differences from experiment in the predicted magnitudes and trends with radius of turbulent fluxes and fluctuation levels which cannot be fully accounted for by uncertainties due to transport stiffness. In the QH-mode study, local nonlinear GYRO simulations that neglect fast ion effects show a similar proportional response to the applied electron cyclotron heating as the experiment, but overpredict the magnitudes of transport and fluctuation levels by a factor of 10 or more. Possible sources of this overprediction, namely nonlocal effects and self-consistent fast beam ions, are identified and discussed.

Rotation driven by fast ions in tokamaks

Collective fast ion effects on flows in tokamaks are investigated analytically and numerically. A general analysis of noncollisional electrodynamic momentum transfer from fast ions to bulk plasma is presented, with polarization effects and dissipation in the bulk plasma taken into account. The analysis is illustrated using idealized simulations of fast ion orbits and radial electric fields in the Mega-Ampère Spherical Tokamak (MAST) [A. Sykes, R. J. Akers, L. C. Appel et al., Nucl. Fusion 41, 1423 (2001)], the Joint European Torus (JET) [P. H. Rebut et al., Nucl. Fusion 25, 1011 (1985)], and ITER [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)]. In the MAST simulation, prompt losses of beam ions injected counter to the plasma current drive up a radial electric field that saturates at a level such that beam ions subsequently injected are confined electrostatically. Although the actual radial electric fields in counterinjected MAST discharges are lower than this, the scenario explored in the simulation would be approached in MAST plasmas with sufficiently low collisionality. The JET simulation, although unrealistic, shows that a similar process could be driven by losses of fusion α-particles from a burning plasma. Test-particle simulations of α-particles in ITER suggest that performance-limiting instabilities such as neoclassical tearing modes and resistive wall modes could be affected significantly by flows associated with radial fast particle currents.

Laser-Produced Plasma Light Source Development for Extreme Ultraviolet Lithography

We present recent results of our laser-produced plasma light source development for next-generation lithography. The plasma target of the extreme ultraviolet (EUV) source system is a liquid xenon jet and the driver laser is a 600 W Nd:YAG laser operating at a repetition rate of 10 kHz. A EUV output power of 2.2 W at 13.5 nm (2% bandwidth, 2π sr) having a stability of 0.72% (1σ, 50-pulse moving average) has been achieved. Related to future collector mirror lifetime considerations, fast ions from the laser-produced plasma have been characterized by time-of-flight (TOF) measurements. Using a low repetition rate 8-ns, 100-mJ Nd:YAG laser Xe+ to Xe6+ ions were observed with Xe2+ being the main charge state. In addition, the effects of fast ions on Mo/Si multilayer mirrors have been studied using a Xe ion gun. Ion sputtering of the multilayer structure is the main damage mechanism but layer boundary mixing and surface roughness increase are also observed.

Impact of pellet injection on extension of the operational region in LHD

Pellet injection has been used as a primary fuelling scheme in the Large HelicalDevice. With pellet injection, the operational region of NBI plasmas has been extended to higher densities while maintaining a favourable dependence of energy confinement on density, andseveral important values, such as plasma stored energy of 0.88 MJ, energy confinementtime of 0.3 s, β of 2.4% at 1.3 T and density of 1.1 × 1020 m -3, have beenachieved. These parameters cannot be attained by gas puffing.Ablation and the subsequent behaviour of the plasma have been investigated. The measured pelletpenetration depth estimated on the basis of the duration of the Hα emission is shallower than the depth predicted from the simple neutral gas shielding (NGS) model. It can be explained by the NGS model with inclusion of the effect of fast ions on the ablation. Just after ablation,the redistribution of the ablated pellet mass was observed on a short timescale (∼400 ms). The redistribution causes shallow deposition and low fuelling efficiency.

Effects of Fast Ions and an External Inductive Electric Field on the Neoclassical Parallel Flow, Current and Rotation in General Toroidal Systems.

Effects of external momentum sources, i.e., fast ions produced by the neutral beam injection and an external inductive electric field, on the neoclassical ion parallel flow, current, and rotation are analytically investigated for a simple plasma in general toroidal systems. It is shown that the contribution of the external sources to the ion parallel flow becomes large as the collision frequency of thermal ions increases because of the momentum conservation of Coulomb collisions and sharply decreasing viscosity coefficients with collision frequency. As a result, the beam-driven parallel flow of thermal ions becomes comparable to that of electrons in the Pfirsh-Schluter collisionality regime, whereas in the 1/ν or banana regime it is smaller than that of electrons by the order of √me/mi (me and mi are electron and ion masses). This beam-driven ion parallel flow can not produce a large beam-driven current because of the cancellation with electron parallel flow, but produces a large toroidal rotation of ions.As both electron and ions approach the Pfirsh-Schluter collisionality regime the contribution of thermodynamical forces becomes negligibly small and the large toroidal rotation of ions is predominated by the beam-driven component in the non-axisymmetric configuration with large helical ripples.

Approximate expression for beam driven current in tokamak plasmas

An approximate expression for the beam driven current in which the trapping effects of fast ions and electrons are included is derived by means of a variational principle. The beam driven current due to arbitrary fast ions can be calculated easily from this expression in an axisymmetric magnetic field with finite aspect ratio. It is shown that in a tokamak with circular cross-section this expression gives a good approximation to the results obtained by numerically solving the Fokker-Planck equation. The beam driven current due to energy diffusion, which is neglected in the approximate expression, is also estimated

Characteristics of fusion reaction rate and neutron yield in deuterium plasma

Characteristics of the fusion reaction rate and the fusion neutron yield have been investigated taking into account the effect of fast ions. The dependences of the fusion reactivity (〈σν〉B) and normalized fusion reaction rate (QB) on the fast ion energy and target plasma temperature are clarified. The following reactions are considered: ( 1 ) D 0 → D + ( D ( d , n ) 3 He ) , ( 2 ) D 0 → D + ( D ( d , p ) T ) , ( 3 ) D 0 → T + ( T ( d , n ) 4 He ) , ( 4 ) T f → D + ( T ( d , n ) 4 He ) , ( 5 ) D 0 → H 3 e + ( H 3 e ( d , p ) 4 He ) , ( 6 ) H 3 e f → D + ( H 3 e ( d , p ) 4 He ) . The mutual relation of 〈σν〉B and QB for each reaction is also clarified. Furthermore, the fusion neutron yield (Fn) has been evaluated using a simple steady-state plasma model in which the radial profiles of the plasma temperature and density or the fast-ion velocity distribution are the same as those observed in recent experiments. The results of this model agree with those of the other calculational model. We have studied the following dependence for JT-60 plasma size: F n vs. n ¯ e , F n vs. τ E , F n vs. n ¯ e ( using ⁡ Kaye − Goldston or INTOR scaling ) , F n vs. n D / ( n D + n H ) , F n vs. E B , and F n vs. T i / T e .

An analysis of the ablation rate for solid pellets injected into neutral beam heated toroidal plasmas

A new model for calculating the ablation rate of a solid pellet injected into a neutral-beam-heated toroidal plasma is discussed. It is an extension of the neutral cloud shielding model introduced by Parks and Turnbull. Fluid equations describing a transonic stationary flow of neutral particles ablated from the pellet surface in the presence of both electron heat flux and of flux due to fast ions are derived and solved as an eigenvalue problem. – The effect of fast ions on the ablation rate is significant in low electron temperature and low density plasmas such as edge plasmas when the fast ion density is rather high. When the ablation due to fast ions is dominant and electron heat flux is negligible, the ablation rate is given by a scaling relation similar to that obtained in the case of electron heat flux only. – An example of an ablation rate profile is shown for a PDX tokamak-like plasma in a self-limiting approximation for electron temperature and density. The results are compared to those of the Milora model which includes fast ions.