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Enhanced fusion reactivity in the beam-driven field reversed configuration
R. M. Magee,1, a) A. Necas,1 R. Clary,1 M. C. Thompson,1 T. Roche,1 S. Korepanov,1 S. Nicks,1 and T. Tajima1
Tri Alpha Energy, Inc., Rancho Santa Margarita, CA, 92688, USA
(Dated: 22 March 2017)
A dramatic enhancement of the deuterium-deuterium fusion reaction rate - 100x beyond thermonuclear - is observed from a field reversed configuration plasma during high power hydrogen beam injection. This fusion rate enhancement is the result of a high energy tail drawn from the bulk ion energy distribution by the interaction of the beam with the plasma on sub-collisional timescales. The appearance of magnetic field and density fluctuations near the ion cyclotron frequency and the timescale of interaction suggest that an energetic particle mode is the mediator of the beam-plasma energy transfer. Remarkably, this mode is observed to not have any deleterious effect on global plasma confinement. This is likely due to the high phase velocity of the wave relative to the thermal velocity of the plasma in the interaction region. Additionally, the large orbit, beam-injected particles themselves are observed to be beneficial to both the lifetime and confinement properties of the plasma. The experimental observations of enhanced fusion, a high energy tail, and fluctuations in the range of the ion cyclotron frequency are reproduced with a particle-in-cell code in which the mode is identified as a finite beta ion Bernstein mode.
The C-2U advanced, accelerator-driven field reversed con- figuration (FRC)1 experiment is composed of a high density, prolate toroid with low internal magnetic field embedded in the linear open field line plasma of a magnetic mirror. Tan- gential neutral beam injection creates a population of fast ions whose orbits encircle the toroid, dipping in and out of the closed flux surface region as they execute betatron-like or- bits (see Figure 1). These features of low field and tangential injection create a fast ion environment unique among mag- netic fusion energy (MFE) devices. In the tokamak, for exam- ple, the ratio of the fast ion orbit radius to the plasma radius is typically a few percent. Here, it is ∼ 1. This large fast ion orbit size allows the particles to average over small scale fluctuations,2 resulting in a fast ion lifetime near the classical limit.3
A further distinguishing feature of C-2U is the high neutral beam injection (NBI) power density. At 10 MW/m3, the ratio of the injected beam power to plasma volume is roughly 30
FIG. 1. An illustration of the FRC (solid body with cut-away) with closed field ine surfaces (red shells) embedded in the mirror plasma (yellow tubes). A sample 7 keV fast ion trajectory is shown in blue.
a)Electronic mail: rmagee@trialphaenergy.com
times larger than in the Joint European Torus, the most pow- erful tokamak in the world. High NBI power density coupled with good fast ion confinement results in the rapid accumula- tion of fast ions in the plasma. In fact, by about 1 ms into the discharge, the fast ion pressure becomes approximately equal to the plasma thermal pressure.
This dominant fast ion population affects the global plasma in several ways. It stabilizes the plasma,3,4 sustains the plasma against resistive decay,5 and suppresses low frequency turbulence.6 Each of these has been described elsewhere. The focus of the present work is on the remarkable, newly- observed fusion reactivity enhancement, some two orders of magnitude over the thermonuclear yield, postulated to be the result of ion energization from a beam-driven mode. Beam- driven waves are common in MFE, and are usually detrimen- tal to confinement and stability.7 Here, despite such a large reservoir of free energy, detriment is not observed. Further- more, although theoretical mechanisms have been proposed8, plasma ions gaining energy from beam-driven waves has not, to our knowledge, been observed.
The Alfve´n velocity is a fundamental plasma property, given by VA = B/√μ0ρ where B is the magnetic field strength, μ0 is the permeability of free space, and ρ is the mass density of the plasma. Beam particles in a plasma mov- ing faster than this velocity can, like a speedboat through wa- ter, drive waves. The free energy source for the waves can be positive velocity space gradients (i.e., ∂f/∂v > 0, where f is the ion velocity distribution function) or spatial gradients.9 If the phase velocity of these waves, vφ, is much larger than the plasma ion thermal velocity, vT,i (and much smaller than the electron thermal velocity), the wave is not immediately damped and can grow to large amplitude. These large ampli- tude waves can then resonate with plasma particles through one of a variety of mechanisms, and significant tail energiza- tion will occur. As we discuss below, these three conditions (i. vb > VA, ii. ∂f/∂v > 0, and iii. vφ >> vT,i) are precisely met in the open field line region, or scape-off layer (SOL), of the C-2U plasma (yellow tubes in Figure 1), so, it is there where we expect to find the most virulent beam-driven mode