Beam-beam Reactions Research Articles

TCV Tokamak Neutron Shielding Upgrade for Dual NBI Operation

The Tokamak à Configuration variable (TCV) is equipped with two neutral beam injection (NBI) systems delivering up to 1.2 MW each for pulse durations of up to 2 s. The first system (NBI1), designed for an injection energy in the range of 25 to 30 keV has been operational since 2016. The existing concrete neutron shielding of the experimental hall proved insufficient for fully protecting human accessible areas, limiting the number of daily plasma pulses using NBI1. The recently commissioned second system (NBI2) is designed for injection synergies in the range 50 to 60 keV. Both systems are tangentially oriented in opposite directions in order to permit experiments with low or no net torque. Calculations with the TRANSP and ORBIS heating codes show that neutron rates from deuterium-deuterium fusion reactions may be as high as 1014 n/s, up to 10 times higher than with the lower energy beam only. This is due both to the ~five times larger beam-plasma neutron rates from the higher energy beam and to an exceptionally high contribution from beam-beam reactions between the opposing beams. The radiation protection policy at the Swiss Plasma Center is that all staff members be considered as members of the general public, limiting the daily personal dose to 4 µSv. This is also the maximum admissible daily dose in any publicly accessible zone, whether occupied or not. Currently, with only the lower energy beam, this limit can be attained in the control room adjacent to the device hall after only five NBI pulses out of a possible 30 daily pulses. To allow for exploitation of the two beams at full specifications, the source side of the existing barite concrete walls of the 15 × 20 × 8 m large TCV hall will be covered with 20-cm-thick polythene (PE) cladding and a ceiling made of 35-cm-thick PE will be added. The total mass of PE will be 200 tons. The usage of PE at this scale for neutron shielding is unprecedented at any fusion research facility.

Study of inertial electrostatic confinement fusion using a finite-volume scheme for the one-dimensional Vlasov equation.

While the majority of fusion energy research is focused on magnetic confinement, there have been several alternative confinement methods aimed at the development of smaller and less expensive reactors. A number of these alternative reactors are based on a spherically convergent beam of recirculating ions and include designs such as inertial electrostatic confinement (IEC), multigrid IEC, and the periodically oscillating plasma sphere concept. Here, a fully time-dependent GPU-based Vlasov solver was developed in order to study these spherically convergent devices. This code solves the Vlasov equation for a spherically symmetric system using a finite-volume method with a modified flux to account for electrode transparency. The solver accounts for secondary electron emission, interactions between the charged particles, and collisional effects such as ionization and charge exchange. This code was used to investigate a system similar to the ion-injected device described by Hirsch (see [R. L. Hirsch, J. Appl. Phys. 38, 4522 (1967)10.1063/1.1709162]), who had reported a neutron production rate for deuterium-deuterium reactions in the range of 10^{6}to10^{7} neutrons per second, which was attributed to the formation of a virtual electrode structure near the center of the chamber. Attempts to reproduce this experiment [B. J. Egle, Ph.D. thesis, 2010] yielded similar fusion rates, though the majority of the reactions were found not to occur near the center of the chamber. The results of this Vlasov solver, considering only beam-beam and beam-background fusion reactions, show that beam-background reactions would be dominant in such an ion-injected device. This result is consistent with work by Baxter and Stuart, who proposed a simplified steady-state Boltzmann model. However, the result of both models are inconsistent with the experimental results, which indicate a higher neutron production rate, and an inverse pressure scaling trend. It is shown that the higher experimental rates may be explained by beam-target fusion between the ion beam and deuterium embedded on the inner surface of the cathode.

Analytical investigation of neutron production rate in spherical IECF devices

The radial profile of the neutron production rate in spherical inertial electrostatic confinement fusion devices (IECF) is investigated. The electrostatic potential is determined by solving the Poisson equation, and by using the potential; the fuel ion energy distribution function is determined at each radial point. From the energy distribution function, the fusion reaction rate is evaluated. Dependence of beam-beam and beam-background reaction rate on some important parameters as the spreads in the normalized angular ion energies, working pressure, and grid transparency are investigated.

Fusion Reaction Rate Coefficient for Different Beam and Target Scenarios

Fusion power output is proportional not only to the fuel particle number densities participating in reaction but also to the fusion reaction rate coefficient (or reactivity), which is dependent on reactant velocity distribution functions. They are usually assumed to be dual Maxwellian distribution functions with the same temperature for thermal nuclear fusion circumstances. However, if high power neutral beam injection and minority ion species ICRF plasma heating, or multi-pinched plasma beam head-on collision, in a converging region are required and investigated in future large scale fusion reactors, then the fractions of the injected energetic fast ion tail resulting from ionization or charge exchange will be large enough and their contribution to the non-Maxwellian distribution functions is not negligible, hence to the fusion reaction rate coefficient or calculation of fusion power. In such cases, beam-target, and beam-beam reaction enhancement effect contributions should play very important roles. In this paper, several useful formulae to calculate the fusion reaction rate coefficient for different beam and target combination scenarios are derived in detail.

Modeling the time variation of beam–grid fusion reaction rates in an Inertial Electrostatic Confinement device driven by a ring-shaped magnetron ion source

We use the two-dimensional analysis code KUAD2 to simulate trajectories in an Inertial Electrostatic Confinement (IEC) device driven by a ring-shaped magnetron ion source (RS-MIS). This aims to maximize the path length λCX for ion-gas charge exchange by operating at just units of mPa D2 gas pressures; however, under these conditions simulations reveal a surprisingly small path length for ion loss to the (Mo) cathode grid λgrid ∼ 30 cm ≪ λCX. By developing an ad hoc model relating the time variation of cathode temperature and absorbed D2 surface density, we use the simulated flux Γgrid of ions striking the cathode grid to obtain the neutron production rate (NPR) arising from ‘beam-grid’ reactions. Results indicate that at units of mPa pressures, beam-grid NPR may dominate over the ‘beam-gas’ and ‘beam-beam reactions,’ providing a qualitative explanation for earlier experimental observations of time-varying NPR-dependence on cathode grid current and gas pressure.

Improved Confinement in Inertial Electrostatic Confinement for Fusion Space Power Reactors

Multiple improvements to the inertial-electrostatic-confinement (IEC) fusion concept are presented. Prior efforts have consisted of considerable theoretical effort and numerous ground experiments resulting in the fusion of D-D, D-T, and D- 3 He at rates on the order of 10 1 0 reactions per second. Theory developed in the paper shows that prior experiments, which observed a linear relationship between reaction rate and background pressure, were limited. With improved ion confinement, the reaction rate and the pressure should decouple at high pressure where the beam-beam reactions are not yet dominant. Highly efficient beam-beam reaction rates are found to dominate the total reaction rate as ion lifetime is increased. The ion lifetime is limited mainly by collisions with background particles and by defocusing. The naturally low background pressure available in the space environment effectively eliminates background pressure as a constraint on ion lifetime, leaving defocusing as the main ion lifetime limiter. To improve ion confinement, multiple grids are introduced to produce focusing channels for ions. The other main loss mechanism is electrons streaming from the core region to the anode. An additional grid is placed within the cathode, providing a central trap for core electrons. Theory predicts that improvements in confinement should be verifiable using existing relatively high-pressure ground systems and the proposed experiment design is presented. The buildup of space charge in the focusing lenses is used to estimate improvements in confinement from < 10 passes to the order of 1000 passes, boosting the performance of IEC reactors by two orders of magnitude, yielding in the near term, a fast neutron source suitable for medical, security, research, and industrial applications. Further increases of confinement will enable net power production in reactors ideally suited for spacecraft power production as a result of low system mass in comparison to magnetic confinement fusion.

Observation of CH A 2 Delta -->X 2 Pi r and 2 Sigma --->X 2 Pi r emissions in gas-phase collisions of fast O(3P) atoms with acetylene.

Optical emissions in single-collision, beam-beam reactions of fast (3--22-eV translational energy) O${(}^{3}$P) atoms with ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$ have been measured in the wavelength range 300--850 nm. Two features were observed, one with a peak wavelength at 431 nm, corresponding to the CH A $^{2}\mathrm{X}$ $^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{r}}$ transition, and a second weaker emission in the range 380--400 nm corresponding to the B $^{2}\mathrm{\ensuremath{\Sigma}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}X $^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{r}}$ transition. Both the A\ensuremath{\rightarrow}X and B\ensuremath{\rightarrow}X emissions were fit to a synthetic spectrum of CH(A) at a vibrational temperature ${\mathit{T}}_{\mathit{v}}$ of 10 000 K (0.86 eV) and a rotational temperature ${\mathit{T}}_{\mathit{r}}$ of approximately 5000 K (0.43 eV); and CH(B) to ${\mathit{T}}_{\mathit{v}}$=2500 K (0.22 eV) and ${\mathit{T}}_{\mathit{r}}$=1000 K (0.09 eV). The energy threshold for the A\ensuremath{\rightarrow}X emission was measured to be 7.3\ifmmode\pm\else\textpm\fi{}0.4 eV (lab) or 4.5\ifmmode\pm\else\textpm\fi{}0.2 eV (c.m.). This agrees with the energy threshold of 7.36 eV (lab) for the reaction O${(}^{3}$P)+${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$\ensuremath{\rightarrow}CH(A)+HCO.

Fusion-neutron production in the TFTR with Deuterium neutral-beam injection

We report measurements of the fusion reaction rate in the Tokamak Fusion Test Reactor (TFTR) covering a wide range of plasma conditions and injected neutral-beam powers up to 6.3 MW. The fusion neutron production rate in beam-injected plasmas decreases slightly with increasing plasma density ne, even though the energy confinement parameter neτE generally increases with density. The measurements indicate and Fokker-Planck simulations show that with increasing density the source of fusion neutrons evolves from mainly beam-beam and beam-target reactions at very low ne to a combination of beam-target and thermonuclear reactions at high ne. At a given plasma current, the reduction in neutron source strength at higher ne is due to both a decrease in electron temperature and in beam-beam reaction rate. The Fokker-Planck simulations also show that at low ne, plasma rotation can appreciably reduce the beam-target reaction rate for experiments with coinjection only. The variation of neutron source strength with plasma and beam parameters is as expected for beam-dominated regimes. However, the Fokker-Planck simulations systematically overestimate the measured source strength by a factor of 2–3; the source of this discrepancy has not yet been identified.

Neutron emission profiles in the beam-heated Princeton large tokamak

Neutron emission profiles in neutral-beam-heated deuterium plasmas in the PLT are strongly peaked on the magnetic axis, as expected for thermonuclear reactions during hydrogen beam injection, and for beam-target and beam-beam reactions during deuterium beam injection.