1,147 research outputs found
Particle Orbits in a Force-Balanced, Wave-Driven, Rotating Torus
The wave-driven rotating torus (WDRT) is a recently proposed fusion concept
where the rotational transform is provided by the E x B drift resulting from a
minor radial electric field. This field can be produced, for instance, by the
RF-wave-mediated extraction of fusion-born alpha particles. In this paper, we
discuss how macroscopic force balance, i.e. balance of the thermal hoop force,
can be achieved in such a device. We show that this requires the inclusion of a
small plasma current and vertical magnetic field, and identify the desirable
reactor regime through free energy considerations. We then analyze particle
orbits in this desirable regime, identifying velocity-space anisotropies in
trapped (banana) orbits, resulting from the cancellation of rotational
transforms due to the radial electric and poloidal magnetic fields. The
potential neoclassical effects of these orbits on the perpendicular
conductivity, current drive, and transport are discussed.Comment: 13 pages, 7 figure
Wave-Driven Torques to Drive Current and Rotation
In the classic Landau damping initial value problem, where a planar
electrostatic wave transfers energy and momentum to resonant electrons, a
recoil reaction occurs in the nonresonant particles to ensure momentum
conservation. To explain how net current can be driven in spite of this
conservation, the literature often appeals to mechanisms that transfer this
nonresonant recoil momentum to ions, which carry negligible current. However,
this explanation does not allow the transport of net charge across magnetic
field lines, precluding ExB rotation drive. Here, we show that in steady state,
this picture of current drive is incomplete. Using a simple Fresnel model of
the plasma, we show that for lower hybrid waves, the electromagnetic energy
flux (Poynting vector) and momentum flux (Maxwell stress tensor) associated
with the evanescent vacuum wave, become the Minkowski energy flux and momentum
flux in the plasma, and are ultimately transferred to resonant particles. Thus,
the torque delivered to the resonant particles is ultimately supplied by the
electromagnetic torque from the antenna, allowing the nonresonant recoil
response to vanish and rotation to be driven. We present a warm fluid model
that explains how this momentum conservation works out locally, via a Reynolds
stress that does not appear in the 1D initial value problem. This model is the
simplest that can capture both the nonresonant recoil reaction in the
initial-value problem, and the absence of a nonresonant recoil in the
steady-state boundary value problem, thus forbidding rotation drive in the
former while allowing it in the latter.Comment: 17 pages, 2 figure
Confinement Time and Ambipolar Potential in a Relativistic Mirror-Confined Plasma
Advanced aneutronic fusion fuels such as proton-Boron tend to require
much higher temperatures than conventional fuels like deuterium-tritium. For
electrons, the bulk plasma temperature can approach a substantial fraction of
the rest mass. In a mirror confinement system, where the electrons are confined
by an ambipolar potential of at least five electron temperatures, the tail
electrons which can escape the potential are fully relativistic, which must be
taken into account in calculating their confinement. In this paper, simple
estimates are employed to extend the scaling of the confinement time into the
relativistic regime. By asymptotically matching this scaling to known solutions
in the non-relativistic limit, accurate forms for the confinement time (and
thus the the ambipolar potential) are obtained. These forms are verified using
finite-element-based Fokker-Planck simulations over a wide range of parameters.
Comparing relativistic and nonrelativistic mirror-confined plasmas with the
same ratio of confining potential to electron temperature and
the same mirror ratio , the net result is a decrease in the confinement time
due to relativistic effects by a factor of .Comment: 9 pages, 7 figure
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