# Intrinsic suppression of turbulence in linear plasma devices

## TL;DR

This paper investigates how the balance between parallel and perpendicular plasma lifetimes suppresses turbulence in linear devices, revealing a threshold where turbulence is intrinsically suppressed due to this balance.

## Contribution

The study identifies a specific lifetime ratio threshold (~11) that correlates with turbulence suppression, providing new insights into plasma stability in linear devices.

## Key findings

- Turbulence suppression occurs when $ au_{ ext{parallel}} ext{ is about 11 times } au_{ ext{perp}}$.
- Instability must persist for roughly 100 growth times to reach non-linear saturation.
- A correlation exists between the saturated density perturbation level and the ratio of lifetimes.

## Abstract

Plasma turbulence is the dominant transport mechanism for heat and particles in magnetized plasmas in linear devices and tokamaks, so the study of turbulence is important in limiting and controlling this transport. Linear devices provide an axial magnetic field that serves to confine a plasma in cylindrical geometry as it travels along the magnetic field from the source to the strike point. Due to perpendicular transport, the plasma density and temperature have a roughly Gaussian radial profile with gradients that drive instabilities, such as resistive drift-waves and Kelvin-Helmholtz. If unstable, these instabilities cause perturbations to grow resulting in saturated turbulence, increasing the cross-field transport of heat and particles. When the plasma emerges from the source, there is a time, $\tau_{\parallel}$, that describes the lifetime of the plasma based on parallel velocity and length of the device. As the plasma moves down the device, it also moves azimuthally according to $E\times B$ and diamagnetic velocities. There is a balance point in these parallel and perpendicular times that sets the stabilisation threshold. We simulate plasmas with a variety of parallel lengths and magnetic fields to vary the parallel and perpendicular lifetimes, respectively, and find that there is a clear correlation between the saturated RMS density perturbation level and the balance between these lifetimes. The threshold of marginal stability is seen to exist where $\tau_{\parallel}\approx11\tau_{\perp}$. This is also associated with the product $\tau_{\parallel}\gamma_*$, where $\gamma_*$ is the drift-wave linear growth rate, indicating that the instability must exist for roughly 100 times the growth time for the instability to enter the non-linear growth phase. We explore the root of this correlation and the implications for linear device design.