Losses of neutral beam (NB) injected fast ions from the confinement region of a field-reversed configuration (FRC) with a strong magnetic mirror are numerically analyzed for parameters relevant to NB injection experiments on the FIX (FRC injection experiment) device [T. Asai et al., Phys. Plasmas 7, 2294 (2000)]. Ionization processes of beam particles are calculated by the Monte Carlo method. The confinement of beam ions is discussed with the concept of accessible regions that restrict the ion excursion and are determined from two constants of motion, the kinetic energy and canonical angular momentum, in the case of an axisymmetric and a steady state FRC without an electrostatic field. From the calculation of the accessible regions, it is found that all the fast ions suffer from the orbit loss on the wall surface and/or the end loss. Single particle orbits are also calculated to find a difference of confinement properties from the results by employing the accessible regions. The magnetic moment is observed to show nonadiabatic motions of the beam ions, which cause a gradual orbit loss on the wall even in a case that a strong magnetic mirror is applied. The results show that the correlation of the magnetic moment disappears as the fast ions experience the density gradient around the separatrix surface and the field-null points.

A discrepancy persists between field-reversed configuration experiments, which are generally stable, and theoretical predictions of instability. The common consensus has been that the stability is the result of finite Larmor radius (FLR) effects. An FLR analysis is presented that finds the self-consistent displacement functions and complex frequency. This is done using the linear gyroviscous model, a fluid-based representation of FLR that allows a wide range of equilibria and modes to be examined with modest computations. The conclusion is that FLR in static FRC fails to explain the observed stability. The cause of stability must lie elsewhere.

The effects of Coulomb collisions on neutral beam (NB) injected fast ions into field-reversed configuration (FRC) plasmas are investigated by calculating the single particle orbits, where the ions are subject to the slowing-down and pitch-angle collisions. The Monte Carlo method is used for the pitch-angle scattering, and the friction term is added to the equation of motion to show the effects of the slowing-down collision, such as the deposited power profile. The calculation parameters used are relevant to the NB injection on the FRC injection experiment device [T. Asai, Y. Suzuki, T. Yoneda, F. Kodera, M. Okubo, and S. Goto, Phys. Plasmas 7, 2294 (2000)]. It is found that the dominant local power deposition occurs in the open field region between the X point and the mirror point because of a concentration of fast ions and a longer duration travel at the mirror reflection point. In the present calculation, the maximum deposited power to the FRC plasma is about 10% of the injected power. Although the pitch-angle scattering by Coulomb collision destroys the mirror confinement of NB injected fast ions, this effect is found to be negligible. The loss mechanism due to nonadiabatic fast ion motion, which is intrinsic in nonuniform FRC plasmas, has a much greater effect than the pitch-angle scattering by Coulomb collision.

Experimental evidence for the global stabilization effect of neutral beam injection on field-reversed configuration (FRC) plasma has been found for the first time. The translation of FRC plasma, i.e., transferring the formed plasma into another vessel along a gradient of guiding magnetic field, is an important technique for both the current FRC experiments and a future fusion reactor to separate formation and confinement regions. However, the translation’s dynamic process causes global wobble movement of the plasma column. The stabilization effect induced by obliquely injected neutral beams on this global movement has been observed by both interferometry and magnetic measurement. The behavior of injected beam ions has been calculated numerically and suggests that the injected beam ions confined under a strong mirror field could produce effective ring-current at both ends of the plasma column. The stabilization effect of this ring-current on the global movement has been discussed by using a rigid body model.

The marginal stability of a static plasma with finite-Larmor-radius (FLR) effects depends on a combination of the FLR effect and the ideal magnetohydrodynamic (MHD) potential energy. For the tilt mode in a field-reversed configuration (FRC) previous computations of these two factors led to a prediction of stability for S*⩽(3−5)E where S* is the macroscale parameter (separatrix radius/ion skin depth) and E is the elongation (separatrix half length/separatrix radius). This prediction explained the observed stability of most experiments. However, recent computations of actual MHD eigenfunctions indicate that the MHD growth rate has a much weaker scaling with elongation than previously believed. As a consequence, most of the long-lived, stable FRC experiments lie in the region predicted to be unstable. It appears then that the stability of FRC experiments is not explained by FLR effects in a static equilibrium.