search

ukaea / PROCESS

PROCESS is a systems code at UKAEA that calculates in a self-consistent manner the parameters of a fusion power plant with a specified performance, ensuring that its operating limits are not violated, and with the option to optimise to a given function of these parameters.
https://ukaea.github.io/PROCESS/
MIT License
30 stars 11 forks source link

Physics-based density limit #970

Closed jonmaddock closed 1 week ago

jonmaddock commented 4 years ago

In GitLab by @mkovari on Dec 4, 2019, 10:20

Origin of Tokamak Density Limit Scalings
D. A. Gates and L. Delgado-Aparicio, Phys. Rev. Lett. 108, 165004 – April 2012

They argue that the density limit is due to radiative effects.

For future tokamaks, which may operate at higher temperature and density, bremsstrahlung radiation may become dominant giving a lower density limit.

This suggests to me that we should be using a physics-based density limit, rather than the empirical Greenwald limit.

Any thoughts - @rkemp @msiccini @skahn ?

PS Michael Fitzgerald's comment - "If someone had figured out the physical origin of the density limit we would have heard about it."

jonmaddock commented 2 years ago

In GitLab by @mkovari on May 6, 2022, 16:23

See also Phys. Rev. Lett. 128, 185003 (2022) - First-Principles Density Limit Scaling in Tokamaks Based on Edge Turbulent Transport and Implications for ITER

A quick reading of this paper seems to show that it is thoroughly dishonest. Although they say their proposed formula applies to H-mode and L-mode, this isn't true. Their actual definition of the density limit of discharges with an H-mode phase is as follows.

the plasma undergoes first an L-H transition, then as the density is increased, plasma confinement degrades until an H-L transition occurs and, once in L-mode, a density limit is attained.

In other words, this is actually an L-mode density limit!

@stuartmuldrew @ajpearcey

jonmaddock commented 2 years ago

In GitLab by @mkovari on May 17, 2022, 10:40

In principle, their formula could be used for L-mode reactors, and it would allow higher densities for suitable values of the parameters. However, the issue of confinement degradation with increasing edge density remains.

stuartmuldrew commented 1 week ago

Comment from Francis Casson:

The Greenwald limit is now understood to be a L-mode or separatrix density limit:

  • Most present machines limited by fuelling from SOL

  • Reactor devices require core fuelling (pellets)

H-mode operation above Greenwald limit has been demonstrated in devices including DIIID and AUG

stuartmuldrew commented 1 week ago

Given there is no well understood alternative, and the Greenwald fraction is still used as the main density reference, I propose we just close this issue with no change. It can be moved to a discussion if needed.