Plugging of multi-mirror machines by a traveling rotating magnetic field

Abstract

Axial plugging is a critical challenge for fusion in open-ended magnetic confinement systems. Multi-mirror systems, consisting of a series of axially aligned magnetic mirrors, aim to enhance axial confinement by increasing the effective diffusion coefficient; however, additional plugging is required to meet the Lawson criterion. In [T. Miller et al., Phys. Plasmas 30, 072510 (2023)], it was found that applying a traveling and rotating electric field in multi-mirror machines can significantly suppress axial loss due to a selectivity effect induced by the Doppler shift of the ion cyclotron resonance. However, this method is energetically expensive and vulnerable to plasma screening effects. Here, we propose using a traveling, rotating magnetic field that can achieve comparable plugging effectiveness while offering better penetration and lower energy costs. Two limiting scenarios, with and without an induced electric field, were considered. The confinement enhancement is calculated using a semi-kinetic rate equation model, in which the flux coefficients are determined from single-particle simulations. While both scenarios exhibit significant confinement enhancement, the scenario without an induced electric field is much more energetically efficient, as it relies on phase-space mixing rather than on energy deposition in the escaping particles. The decoupling of confinement from plasma collisionality enables fusion conditions in the central cell while allowing affordable and efficient confinement enhancement in the multi-mirror sections.

Figures (11)

• Figure 1: An illustration of one section of an MM system (top) and the amplitude of the axial magnetic field (Eq. (\ref{['eq: Bz mirror']})) of two MM cells with $R_m=3$ (bottom). The illustration shows the right half of the MM system, where the left half is assumed to lie to the left of the vertical dashed line.
• Figure 2: Population conversion $\Delta \bar{N}_{ij}$ plots for tritium in TRMF for different values of $k,\omega$. Colors indicate different transitions between the three populations (see legend). In each subplot, the horizontal axis represents time over the interval $[0,\, \tau_{\mathrm{th}}]$, while the vertical axis shows the population–conversion metric over the range $[0,\, 1]$.
• Figure 3: Population conversion $\Delta \bar{N}_{ij}$ plots for tritium in TRMF--noE for different values of $k,\omega$. Colors indicate different transitions between the three populations (see legend). In each subplot, the horizontal axis represents time over the interval $[0,\, \tau_{\mathrm{th}}]$, while the vertical axis shows the population–conversion metric over the range $[0,\, 1]$.
• Figure 4: Population conversion $\Delta \bar{N}_{ij}$ plots for tritium in TREF for different values of $k,\omega$. Colors indicate different transitions between the three populations (see legend). In each subplot, the horizontal axis represents time over the interval $[0,\, \tau_{\mathrm{th}}]$, while the vertical axis shows the population–conversion metric over the range $[0,\, 1]$.
• Figure 5: Smoothed and dimensionless RF rates, $\bar{N}_{ij}$, as a function of $k,\omega$ for tritium in TRMF (with induced electric field). The overlaid dashed black lines indicate the theoretical resonance condition for right- and left-going particles.