Divertor plasma detachment : a fresh look at a familiar problem…

1. Divertor plasma detachment : a fresh look at a familiar problem… W. Fundamenski (UKAEA, EFDA-JET) with contributions from M.Wischmeier (IPP), A.Kukushkin (ITER), S.Krashenninikov (UCSD), B.Lipschultz (MIT), S.Wiesen (FZJ)

2. Scope of the present session Growing consensus in the DSOL ITPA community, that • It is the right time to revisit the issue of divertor plasma detachment, which has gradually fallen off the radar, since its glory days of 1990’s • Aside from basic physics curiosity, the motivation for this re-examination comes from the need to make quantitative predictions of power and particle fluxes to divertor tiles in ITER • Since these predictions are ultimately made with 2-D multi-fluid plasma / MC neutral codes, such as SOLPS (B2/EIRENE), validation and verification of these codes (ideally against a multi-machine database) is highly desirable, as is the resolution of any discrepancies between code and experiment • One cannot do justice in a single DSOL ITPA session to the full complexity of this problem, especially all its ramifications for ITER. • It is thus preferable to use this session to re-open (re-postulate) the problem, perhaps reviewing new experimental data and/or progress made in code development in recent years, and thus provide a basis for a follow-up session(s) at future meetings. In light of the above, the present session will focus on the basic physics understanding of divertor detachment, rather than its implications for ITER

3. Basic mechanisms understood… • Detachment is defined variously as • the loss of SOL plasma pressure along B from upstream to target, p_u / p_t, • the reduction of the plasma flux to the target wrt the two-point model (high-recycling regime) scaling, Gamma_t ~ n_u^2 / (1-f_rad). Expressed as DOD. • the movement of radiation, ionisation (and recombination) fronts away from the target • Requires removal of both plasma momentum and energy • Momentum removed by CX and elastic collisions with neutrals in the divertor volume • Hence, divertor closure facilitates detachment, e.g. vertical vs horizontal targets • Energy removed by above, plus hydrogenic and impurity line radiation • Hence, impurity seeding facilitates detachment • Particles additionally removed by volumetric recombination for very high densities • In-out asymmetry in lines with that of power into the divertor volume • For normal field direction (grad B down), more power to outer target • For comparable length of inner and outer target legs, the inner target becomes colder & denser, and hence detaches at lower upstream density • This is not true when the inner leg is much shorter than the outer leg, e.g. TCV • For reversed field direction (grad B up), comparable power to both targets • Hence, roughly symmetric target conditions and detachment behaviour • Asymmetry consistent with GC drifts and diamagnetic flows

4. Upstream profiles broaden with density

5. Basic mechanisms understood… • Upstream collisionality is the governing parameter • For relatively open divertors, detachment occurs for n* ~ 50-100 • Divertor neutral pressure increases during the onset of detachment • Later saturates and/or decreases as the ionisation front moves away from target • Detachment of the outer target concurrent with formation of an X-point MARFE • One of the mechanisms proposed for the density limit (K.Borrass) • Preceded by a Type I-III back transition when in H-mode (G. McCracken) • Under Type-I H-mode conditions, detachment only during the inter-ELM phase • Type-I ELMs ‘burn through’ the neutral gas buffer and transiently reattach the plasma • Limited amount of ‘energy buffering’ possible on the time scale of the ELM • Steady detachment possible only for smallest, typically Type-III, ELMs

6. Pressure profiles in L & H modes

7. …but details are proving elusive • Most of the above tendencies can be reproduced by 2D multi-fluid/MC codes • However, the codes are not able to reproduce all the measurements simultaneously, even under relatively simple Ohmic and L-mode conditions • Possible reasons for discrepancies, i.e. the physics missing from these codes • SOL plasma turbulence involves strong thermodynamic perturbations, e.g. dn/n ~ 1 • Hence, the mean field formulation on which the 2D codes are based may be inaccurate, e.g. <nv> = <n><v> + <dn dv>, with the second terms no longer small • Similarly, the turbulent closure schemes which are used, typically the Prandl mixing length scheme with D_perp ~ const, may also generate large errors • Finally, the problem becomes 3D rather than 2D, as axisymmetry broken by turbulent plasma filaments • Kinetic effects (non-local heat transport) become important in detachment fronts • Spitzer-Harm (Braginskii) closure for heat flux and viscosity become invalid • Heat flux limit (flux limiting factor) corrections are likewise inadequate, since heat flux can actually exceed the S-H value when hot electrons stream into a cold region • Neutral effects (n-n collisions, neutral flows and turbulence) • Under dense divertor conditions, neutrals become fluid-like and most likely turbulent • Photon effects (e.g. L_alpha opacity) and radiation transport • Under dense divertor conditions, the plasma/neutrals becomes opaque in certain lines • Impurity effects (erosion, transport, i-Z interaction) involve many approximations

8. Detachment issues for ITER In ITER, detached divertor operation must be compatible with a sufficiently strong edge transport barrier to achieve Q=10. This raises the key question, what is the impact of divertor detachment on the H-mode pedestal & ELMs ?! and a series of related issues: • The role of extrinsic impurity seeding in facilitating plasma detachment, and the contribution of seeded impurity radiation required in ITER with W divertor tiles,i.e. in the absence of Carbon as the main radiating species? • The impact of seeded impurities on W erosion, i.e. the trade-offs between the beneficial effect of reducing Te_div and harmful effect of providing higher Z ions? • The impact of detachment on X-point radiation and hence on the plasma pedestal temperature, and ELM characteristics (including I-III), in present tokamaks & ITER • The transient effect of ELMs on the detached divertor plasma, specifically the impurity transport processes in the post-ELM phase • Finally, the effect of the so-called “active ELM mitigation” techniques, such as pellet injection and magnetic field perturbations (RMP, EFCC, TF Ripple) on plasma detachment during the inter-ELM phase