Detachment

Summary

The plasma is said to be detached from the divertor target plate when the primary plasma-neutral interaction takes place away from the plate because a region of high neutral density buffers the plate from the plasma. Detachment happens when physical processes (e.g. radiation, charge exchange, recombination, ...) in the Scrape-Off Layer (SOL) dissipate enough energy and momentum upstream of the divertor target plate. These processes can be activated at high density or with radiating impurities. Intrinsic impurities can be sputtered from plasma facing components by the plasma fuel ions, and extrinsic impurities can be added to the plasma by gas puffing.

Motivation for detaching

Detachment is associated with reduced heat flux reaching the divertor target plate, which helps avoid melting and thermal stress, and reduced electron temperature $T_e$ at the divertor target, which reduces sputtering. This is important for prolonging the lifetime of plasma facing components in the divertor. The heat that would've reached the divertor is distributed across a wider wall area by radiation, instead of being concentrated in a small area, such as a narrow annulus near the magnetic strike point in the case of a tokamak.

Quantifying detachment

A common way of measuring detachment is the Degree of Detachment (DOD) metric, which is the ratio of expected particle flux for an attached plasma to measured particle flux. Particle flux reaching the divertor target ${\mathrm{\Gamma }}_t$ is proportional to the ion saturation current $I_{sat}$ measured by Langmuir probes, which is a straightforward measurement from a ubiquitous diagnostic. So, DOD can be recast as the ratio of expected $I_{sat}$ to measured $I_{sat}$. Attached $I_{sat}$ vs. density can be fit with a simple empirical function, such as $I_{sat}=C{n}_e^2$, where $C$ is the fitting coefficient. Then expected attached $I_{sat}$ can be calculated later, when the plasma is detached, by just evaluating $C{n}_e^2$. Average density such as from an interferometer is often used in the scaling, although the two-point model indicates that density at the separatrix $n_{e,sep}$ would be better. However, average density is much easier to obtain. Then DOD is just

$DOD=\frac{C{n}_e^2}{I_{sat}}$

Detachment can be described as complete or total when the particle flux becomes negligible. A plasma can be described as simply "detached" or "deeply detached" if it is detached enough, where enough is subjective but could mean $DOD\ge 10$. Partial detachment refers to a state with $DOD$ greater than 1 but less than what would be considered deep detachment; this could mean $2\le DOD\le 10$, but there is no universal standard. Partial detachment can also refer to part of the divertor plate being at a high DOD with another part of the late still at low DOD. Detachment tends to begin near the magnetic strike point and spread outward into the farther SOL as detachment deepens.

Detachment can also be assessed via electron temperature $T_e$ at or near the divertor target plate, as measured by Langmuir probes or divertor Thomson scattering. The physical processes associated with deep detachment occur at low $T_e$, and many of them are strong functions of $T_e$, making $T_e$ a useful metric. Sputtering of tungsten by deuterium ions drops sharply at $\sim 8$ eV, so $T_e\le 8$ eV could be a useful threshold. However, 5 eV is more widely used as a threshold for "pretty detached". Since language such as partially or deeply detached is not strictly defined, it may be advantageous to instead quote $DOD$ and $T_e$ at the strike point or the location of a peak in heat flux, particle flux, or $T_e$ in order to define how detached a plasma is. $DOD$ (actually attachment fraction $A_{frac}=1/DOD$) and $T_e$ have been used as control variables in detachment control systems.

Side effects / disadvantages

Detachment is achieved via high density and/or high impurity content. High density can lead to disruption because of the density limit, or be suboptimal for fusion performance. Impurities, whether intrinsic (sputtered from the walls) or extrinsic (added intentionally, such as by gas puffing), dilute the fuel and reduce fusion gain. This dilution also increases effective charge state $Z_{eff}$, which affects many physical processes: resistivity is increased, making it harder to maintain adequate current drive and pulse length, and some instabilities may be excited. At higher impurity radiation levels, heat can be removed from the core plasma, reducing energy confinement time and thus fusion gain. High impurity content also brings the risk of radiation condensation, where a decrease in $T_e$ causes increased radiating efficiency, which cools the plasma and reduces $T_e$ even further in a feedback loop. The radiation condensation effect can be localized to a relatively small volume at the edge of the plasma; one manifestation of this is a MARFE (Multi-faceted Asymmetric Radiation From the Edge). At the extreme, radiation condensation can lead to a radiative collapse, a type of disruption where the entire plasma is cooled and stops carrying current.

Control

To balance the need to mitigate heat loads on the divertor with the disadvantages of extreme density and impurity content, detachment control systems are developed.