Direct observation of ion acceleration from a beam-driven wave in a magnetic fusion experiment

Articles
https://doi.org/10.1038/s41567-018-0389-0
Direct observation of ion acceleration from a beam-driven wave in a magnetic fusion experiment
R. M. Magee *, A. Necas, R. Clary, S. Korepanov, S. Nicks, T. Roche, M. C. Thompson, M. W. Binderbauer and T. Tajima
Efficiently heating a magnetically confined plasma to thermonuclear temperatures remains a central issue in fusion energy research. One well-established technique is to inject beams of neutral particles into the plasma, a process known as neutral beam injection. In the classical picture, fast ions generated from neutral beam injection predominantly heat electrons as they are slowed by friction. This electron heat is then collisionally coupled to the plasma ions, which comprise the fusion fuel. Fast ions can also drive plasma waves, which divert energy from the fuel and can degrade confinement. Here we present new obser- vations from a field reversed configuration plasma in which a beam-driven wave in the open field line region couples directly to fuel ions, drawing a high-energy tail on subcollisional timescales that dramatically enhances the fusion rate. This mode there- fore allows the beam energy to bypass the electron channel and does so without having a deleterious effect on global plasma confinement. Our results demonstrate a means of directly and non-destructively coupling energy from fast ions to plasma ions, which may pave the way for improved neutral beam injection heating efficiency or the prevention of ash accumulation with alpha channelling.
The C-2U advanced, beam-driven field reversed configuration (FRC)1 experiment is composed of a high-density, prolate toroid with low internal magnetic field embedded in the lin-
ear open field line plasma of a magnetic mirror. Tangential neutral beam injection (NBI) 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 orbits (Fig. 1). These features of low field and tangential injection create a fast ion environment that is unique among magnetic fusion energy devices. In the toka- mak, for example, 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 fluctua- tions2, resulting in a fast ion lifetime near the classical limit3.
A further distinguishing feature of C-2U is the high NBI power density. The ratio of the injected beam power to plasma volume is roughly an order of magnitude larger than in the Joint European Torus, the most powerful tokamak in the world. High NBI power density coupled with good fast ion confinement results in the rapid accumulation of fast ions in the plasma. In fact, by about 1ms into the discharge (the time for a test ion to slow by 1/e), 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 plasma3,4, sustains the plasma against resistive decay5,6, and suppresses low-frequency turbulence7,8. Each of these has been described elsewhere. The focus of the present work is a remarkable, newly observed mode of ion energization. While beam-driven waves are common in magnetic fusion energy9, they usually degrade the confinement properties of the plasma10 or clamp the achievable fast ion density11. In our case, there is no observed confinement degradation or fast ion density limit with the appearance of the mode. Instead, we observe the creation of a large population of ‘tail’ ions: ions from the thermal populations that are accelerated to many times the thermal energy on subcollisional tim- escales. This is, to our knowledge, the first observation of plasma
TAE Technologies, Inc., Foothill Ranch, CA, USA. *e-mail: RMagee@TAE.com NATuRe PhySiCS | www.nature.com/naturephysics
ions gaining energy from beam-driven waves in a magnetic fusion device (although theoretical mechanisms have been proposed12).
Background
The Alfvén velocity is a fundamental plasma property, given by V = B∕ μ ρ where B is the magnetic field strength, μ is the per-
A00
meability of free space and ρ is the mass density of the plasma.
Beam particles injected into a plasma with velocity vb > VA can, like a speedboat through water, drive waves. The free energy source for the waves can be positive velocity space gradients (that is, ∂f/∂v > 0, where f is the ion velocity distribution function) or spatial gradients13. 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-amplitude waves can then resonate with plasma particles through one of a variety of mechanisms, and significant tail energization will occur. As we discuss below, these three conditions—(1) vb > VA, (2) ∂f/∂v > 0 and (3) vφ ≫ vT,i)—are precisely met in the open field line region of the C-2U plasma (yellow tubes in Fig. 1), and so it is there where we expect to find the most virulent beam-driven mode activity.
In this Article we present a collection of experimental obser- vations related to the newly discovered phenomenon, including magnetic field and density fluctuations, suprathermal neutron pro- duction and direct measurements of a high energy tail of plasma ions. An electromagnetic particle-in-cell (PIC) simulation in a sim- plified geometry is used to identify the mode, and connections are made to both the experimental observations and analytic theory. Finally, a dedicated experiment is conducted that illustrates how the mode can be controlled by manipulation of the free energy source, the neutral beam velocity distribution.
Observations
One product of the deuterium–deuterium fusion reaction com- prises 2.45MeV neutrons. The fusion rate of the plasma depends