Divertor Plasma Detachment:
Past and Future
Sergei Krasheninnikov
University of California at San Diego
23rd International Conference on Plasma Surface Interactions in Controlled Fusion Devices
June18-22, 2018, Princeton, NJ, USA

1

Introduction
n

n

n

n

n

Fusion reactions (e.g. D+T=He+n+17.6 MeV) require high energy(~ 10 keV) of colliding nuclei
Therefore, harvesting of fusion energy is only
possible in plasma
How we can confine such hot plasma to maintainHe-heated self-sustained fusion reaction?
We can either use magnetic field, causing
circulation of charged particle around magnetic
field lines (magnetic confinement), or
Do not confine hot plasma at all with a hope that
nuclei fuse before they fly out (inertial
confinement)2Introduction (cont.)nnn

To avoid hot plasma losses both along themagnetic field lines and due to drift effects
one could use:n either complex shape of “toroidal”
magnetic coils (stellarator, L. Spitzer)nor toroidal electric current should run
along simple donut-shaped plasma(tokamak, I. Tamm & A. Sakharov)As a result, magnetic field lines havehelical structure nestled on closed
magnetic flux surfaces
From the very beginning both Spitzer & Sakharov
were concerned about plasma-wall interactions but
their approaches to this issue were very different

3Dawn of research on plasma-wall interactionsnnn

We should remember that fusion grade plasma is very “fragile”!
Even tiny amount of highly radiative high-Z impurity (e.g. ~10-3 % of W) cancause radiative plasma cooling that exceeds fusion heating by alpha-particles
To avoid such catastrophic impurity effects Spitzer & Sakharov suggested:

hot
core

Spitzer: toroidal divertor

Sakharov: neutral cushion

4

Dawn (cont-ed)
n

n

n

n

Further studies have shown that both of thesesuggestions appear to be impractical!
However, GREAT IDEAS have a tendency to
the re-incarnations!
Today we are dealing with “poloidal divertors”
and “neutral cushions” near divertor targets

Today’s physics and synergistic effects ofpoloidal divertors and neutral cushions as wellas projection into the future are the main topics
of my review

5Poloidal divertor: first resultsnnn“First” data (1982) from ASDEX tokamak, after
installation of poloidal divertor, brought up two
unexpected results:
I.

Improved core plasma confinement H-mode,(Wagner, 1982), andII.

High recycling regime, (Keilhacker, 1982) of divertor
operationIII.

As the matter of fact, the first signatures of what wecall now “detached divertor” were also observed
around that time (Shimomura, 1983)

Since than (already for 36 years !) these topics
became the focal points of edge plasma physics infusion devices and this is why we are here today!
I will leave, largely, alone H-mode and concentrate
my talk on the physics of high recycling regime

SOL

6Poloidal divertor: high recycling regimennnWhat actually “high recycling regime” means?
Due to plasma neutralization on divertor targets
we have Γdiv=Γdiv

n

Γdiv

However, “ionization” trapping of neutrals indivertor region and “fast” recirculation
plasma ||B ->neutrals->plasma ||B, result in alarge plasma flux to the targets so that:Γdiv>>ΓSOLn

Neutral flux

(1)

This is what “high recycling regime” isEquation (1) looks simple, but it makes a huge
impact on both SOL and divertor physics!

ΓSOL

ΓSOL
7

Poloidal divertors: shapingnVery different divertor shapes were considered since thann

However, all of them should operate in “high recycling regime”!
8Main features of high recycling regime
For the case Γdiv>>ΓSOL, in a ball park, one can ignore ΓSOL,
neutral puffing and pumping and consider SOL and divertor
region as a “self-organized dissipative closed plasma box” which
I.

Contains fixed number of total amount of hydrogen andimpurity (ions+neutrals!) andII.

Powered by the heat flux QSOL coming from the core (recall
that hydrogen “ionization cost” Eion ~30 eV!)
Pigarov, 2017

B
QSOL
plasmarecycling
regionfirst walln

mid planeΓdiv = Γdiv
divertor target

9Main features of high recycling regime (con-ed)

mid plane

n

n

n

n

There is large plasma flux to divertor targets, causing strong
energy drain
Upstream plasma has virtually no particle source and is sustained
only due to plasma recycling in divertor region
For this case the only mechanism of heat transport fromupstream to divertor is the heat conduction parallel to magnetic
field linesq|| = −κ̂||∂T7/2 / ∂ℓ → Tup ∝ (q||L)2/7 → Tup >> Td
From a 2-point model, accounting for momentum balance along
the magnetic field lines, Bohm/Chodura condition for plasma flow
on divertor target, U|| ~ Td / M and energy balance in recycling
region we find the following scaling (Lackner, 1984)
3
2
Td ∝ n −2
,
n
∝
n
,
but
j
∝
n
up
d
up
d
up
But something “went wrong” in experiments!

n up

QSOL
plasmarecycling
regionn d Γdiv

Tup

B

q||

first walln

= Γdiv Td
10divertor target

LWhat “went wrong” in experiments: divertor plasma detaches!
n

n

n

n

n

n

2

Plasma flux to divertor targets, jd ∝ n up , instead of continuously
going up with increasing upstream plasma density, saturates andthen start to decrease (so-called “roll-over”!)
The roll-over in inner divertor starts at lower plasma density thenin outer one!
Why there is a roll-over ofplasma flux to divertor targets?
What is the key physics ofdetachment?
Two different views:
I.

Ion-neutral “friction”

II.

Plasma recombination &impurity radiation loss

(Loarte, 1998)(Shimomura, 1983)

Which one is correct?
11

Divertor plasma detachment and ion-neutral frictionnn

The very first explanation of detachment phenomena was based on ion-neutral
friction (Rebut, 1992; Stangeby, 1993). The basic idea is simple:
At small temperatures charge-exchange collisions dominatenTherefore, from plasma-neutral
momentum balance we find the following
expression for plasma fluxj|| = −nn

∂P / ∂ℓ
MK cx N

Which suggests that: a) plasma flux decreases with increasing
neutral density N, and b) plasma pressure falls toward thetarget, as, indeed, is observed in experiments!
In addition, experimental data from the PDX linear device also
demonstrated strong reduction of plasma flux to the target withincreasing neutral gas density (Hsu, 1982)
12

Detachment and ion-neutral friction (con-ed)

n

n

n
n

n

n

However, the results from linear divertor simulators, should betranslated to tokamak environment cautiously for the following reason
B
In linear divertor simulators plasma
is produced by source often having
separate chamber and puffing
QSOL
system (e.g. see PDX arrangement)
Only small part of total ionization
plasmasource, Γ ion , contributes to plasma
recycling
flux to the target, Γ div
regionIn tokamak the situation is very different!
Here entire ionization source contributes
Γ ion = Γ div + Γ recto plasma flux to the targets and plasma
particle balance has a very simple form
Therefore, reduction of Γ div during detachment could be either due toΓdiv = Γdiv
reduction of ionization source Γ ion or plasma recombination or both!!!
13
Neither of these effects was considered in ion-neutral friction model of detachment!!!

first walln

mid planeDetachment: SOL plasma energy and particle balances
mid planenConsider plasma energy balance accounting for impurity radiation loss, Qimp
hydrogen “ionization cost”, energy going to the target with plasma flux, andenergy loss carried to the wall by neutral QCX
Recalling also plasma particle balance, we have

QSOL = Qimp + Eion Γ ion + QCX + γTd Γ div

Γ ion = Γ div + Γ rec
n

n

n

n

For small Td we find(Krasheninnikov, 1996):

Γ div = (QSOL − Qimp ) / Eion − Γ rec

B

QSOL

first walln

impurity
radiation
regionIn other words, at small plasma Td plasma flux to the target is governedby power into SOL, impurity radiation loss and plasma recombination!!!
What is the role of ion-neutral collisions? They are helping to: a) maintainplasma momentum balance, and b) cool plasma to very low, ~1 eV,
Γdiv =
temperature where inelastic processes become ineffective!
However, ion-neutral collisions alone unable to reduce plasma flux to the target!!!

Γdiv
14

Detachment: SOL plasma energy and particle balances
Experimental datanExperimental data seem to be, in a ball park, consistent with this physical picture:(Napiontek, 1997)(Terry, 1998)(Lipschultz, 1999)(Fujimoto, 2009)15

Detachment: SOL plasma energy and particle balances
radiation trappingnnn

At high plasma/neutral densities hydrogen radiation lines (mainly Lyman lines) becometrapped (Krasheninnikov, 1986), which, indeed, was observed in experiments (e.g. Terry,
1998)
It affects both plasma particle and energy balances by altering hydrogen atomic rate
constants (e.g. ionization and recombination) and “ionization cost”
In addition, it introduces
non-local effects,(Kukushkin)associated with radiationtransport, which
complicate
Ø spectroscopic plasma
diagnostics, andØ numerical edge
plasma transport
simulations
16(Terry, 1998)

Detachment: SOL plasma energy and particle balances
Numerical simulationnn

The results of numerical simulations, in a ball park, also consistent with physical picture ofdetachment following from SOL plasma energy and particle balances
E.g. see data from DIII-D – like plasma simulations (Pshenov, 2017)17

n

To find the conditions for an onset of ultimate detachment we should
recall that in high recycling regime upstream plasma is supported byplasma recycling in divertor, which is powered by the heat flux

q recycl < qSOLnnHowever, it seems obvious that given q recycl can support upstream
plasma with pressure Pup which is below some critical value
impurity
As a result we find (Krasheninnikov, 1999)
radiation
! P
$
Pup
up &
#
<!
plasmaq recycl #" q recycl &%recycling
crit
1/2
! P
$
!
$
M
# up & ~ # H & ~ 20 N
#
&
#q
&
MW
" recycl %crit " I %mid plane

Pup
qSOL
first wall

Detachment onset

qrecyclΓdiv = Γdiv

2D simulations support this scaling (Krasheninnikov, 2016)18

Detachment onset (con-ed)nnAlthough such local scaling for detachment onset is useful for check up our theoreticalunderstanding, it is not very practical for comparison with experimental data
From what we found, is clear that detachment onset is characterized by almost complete
radiation of QSOL by both impurity Qimp and hydrogen QH=Eion Γion

QSOL=Qimp+QHnFollowing (Tokar, 2003) we could introduce an effective “ionization cost” for impurity, Eimp,
which accounts for impurity radiation loss (we notice that Eimp~few keV >> Eion~30 eV)
2D simulations(Pshenov, 2017)§ Assuming that both impurity and H fluxes are proportional to
impurity and H pressures we arrive to the scaling for
detachment onset similar to that deduced from AUG data
(Kallenbach, 2015)

PH + (Eimp / Eion ) M H / M imp Pimp ~~ PH + 25⋅ Pimp >! QSOL / C norm

19Intermediate conclusions: Detachment-101
I.

High recycling regime is an interesting self-organization of the SOL plasma powered by theheat flux coming from the coreII.

Ultimate divertor plasma detachment is the most dissipative version (The End!) of highrecycling regime

III. Reduction of plasma flux to the target could only be either due to increasing impurity
radiation loss or plasma recombination
IV. Ion-neutral friction, although important for both plasma momentum balance and divertor
plasma cooling to sub-eV temperature, cannot alter plasma flux to the target
V.

For a given magnetic flux tube an onset of detachment is determined by(

Pup / q recycl >! Pup / q recycl
VI.

)crit

It seems that for practical purposes, an onset of detachment could be determined fromPH + (Eimp / Eion ) M H / M imp Pimp ~ PH + 25⋅ Pimp >! QSOL / C norm

20

Detachment and SOL plasma pressure/density limitnCondition for an onset of ultimate detachment for fixedqrecycl results in upper limit of upstream plasma pressure(

Pup / q recycl <! Pup / q recycl
n

n

n
n

(Borrass, 1997))crit

Taking into account of weak dependence of upstream
plasma temperature on the heat flux, Tup ∝ (q||L)2/7 , thelimitation on plasma pressure is effectively causing thelimitation on upstream plasma density
This is so-called “edge plasma density limit” (e.g.
Borrass, 1997; Krasheninnikov 1999)
It exists only with plasma recombination turned on!
Unfortunately, it is still not clear either “edge” density limit
is related or not to “density limit” or H-mode “density limit”(Pshenov, 2017)21

In- Out- detachment asymmetrynnn

Due to fast equilibration of plasma pressure along themagnetic field lines we have
in
out
Pup
= Pup
However, due to ballooning nature of anomalous transport,heat flux into outer divertor is larger than that into inner one
As a result, an onset of detachment starts first in inner divertor

Pup / q recycl
n
n

(
in

~ Pup / q recycl

)crit

QSOL
in
out
Pup
= Pup

Simultaneously, it “freezes” upstream plasma pressure
As a result, outer divertor can only be detached by in- outre-distribution of the heat flux22In- Out- detachment asymmetry (con-ed)
nAn in- out- redistribution of the heat flux could
be related toØ Neutral flux from inner to outer divertor
Ø ExB drifts (in particular) through private
flux region (e.g. Du, 2017)
w/o ExB

DIII-D like

ITER

with ExB(Pshenov, 2017)(Pitts, 2009)23

In- Out- detachment asymmetry (con-ed)
nAn impact of drifts in the simulations of AUG and JET (Aho-Mantila, 2017, 2012, etc.)

AUG

forward B

reversed B 24

In- Out- detachment asymmetry (con-ed)
nAn impact of drifts in DIII-D divertors (Rognlien, 2017)

forward B

reversed B

25

In- Out- detachment asymmetry (con-ed)nAsymmetric detachment of inner and outer divertors in AUG
causes strong fluctuation (f~5 kHz) of radiation losses near
X-point (Potzel, 2014; Manz, 2017)nSuch fluctuations disappear when outer divertor detachesnCurrently there are two plausible explanations:Ø Ionization-recombination instability of detachment front(Manz, 2017; Krasheninnikov, 1999)
Ø Current convective instability (Krasheninnikov, 2016)

Potzel, 201426

In- Out- detachment asymmetry (con-ed)nnnCurrent convective instability (CCI) in cold inner divertor could be driven by the current
between “hot” outer and “cold” inner divertors (Harbour, 1988; Steabler, 1989)
Due to low temperature in inner divertor
CCI is not stabilized by parallel electron
heat conduction
Nonlinear numerical simulations givereasonable agreement withexperimental data (Stepanenko, 2017)eU ≡ U e ~ 4Td(out) ~ 30 eV >> Tcold ~ 3 eV

27

Bifurcation-like transition to detachmentnnBifurcations were observed in DIII-D
divertor (McLean, 2015)n

Drifts might be responsible for such bifurcationtransition to detachment (Rognlien, 2017)

However, increasing cross-field plasma transport withdecreasing divertor plasma temperature(observed inexperiments, Sun, 2015), also can cause bifurcation-like
transition to detachment (Krasheninnikov, 2017)28

Detachment and baffling of neutralsnnExperimental data show pronounced effect of neutral baffling on detachment (e.g. see
Lipschultz, 2007)
Whereas at tight neutral baffling plasma flux is limitedby qrecycl, neutrals bypassing impurity radiation region
have ionization energy reservoir q|| >> qrecycl

impurity
radiationq||verticle
plate

slot
geometry

flat
plate

bypassingneutrals

qrecyclplasmarecyclingtarget(Pshenov, 2017)29

Detachment and impuritiesnnn

It is widely accepted that impurity radiation vitally importantfor power dissipation and divertor detachment
For reactor relevant regimes
Ø impurity radiation should not reduce core plasma
confinement
Ø to avoid core plasma dilution/poisoning with impurity it ispreferably to confine impurity in divertor region, andØ to avoid local overheating of the first wall (e.g. inreactors), impurity radiation should be spread over entire
divertor legs (e.g. in new advanced long-leg divertors)
However,
Ø So far experimental data from both AUG and JET
suggest that impurity radiation is usually clamped near
X-pint and outer divertor target (Reimold, 2015, Bernert,
2017)(e.g. Reimold, 2015)30

Detachment and impurities (con-ed)nAlthough, numerical simulations of new MAST divertor performance demonstratedesirable impurity radiation spread in long-leg divertors (Havlíčková, 2015)
demonstrate

31

Detachment and impurities (con-ed)nnnHowever, the physics behind such N radiation spread is not clear yet
Taking into account that neutral radiation in these simulations is also spread along
divertor leg (Havlíčková, 2015)

It is plausible that such neutral and N ion radiation spread is due to strong plasma
recycling on the side walls
32

Detachment and impurities (con-ed)nnSimilar impurity radiation spread along divertor leg was reported in (Umansky, 2016)
However, it is known that so-called “fixed
impurity fraction”, used in these simulations,has a tendency to over-estimate radiation
loss at high temperatures
%
(1/2
T
−2
Q rad (T) = Pel '' 2ξimp ∫ L rad (T)κ|| (T)T dT **&)0(Post, 1995)

33

Detachment and impurities (con-ed)nIn addition to “fast” (~5 kHz) divertor plasma oscillation discussed before,
relatively “slow” (~10-100 Hz) oscillations were observed in both experiments
on JET and ITER simulationsn
Although the physics of JET oscillations is notvery clear, oscillations found in ITER simulations
JET
were driven by the interplay of impurity transportand radiation loss

(Loarte, 1999)(Kukushkin, 2015)34

Detachment and anomalous transportnAlready long ago experimental data demonstrated strong increase of anomalous
cross-field transport when plasma density approaches density limitn

It results in the formation of plasma density shoulder in far
SOL and very large plasma flux on main chamber wall inboth L- and H- modes(McCormick, 1992)(Umansky, 1998)

35

Detachment and anomalous transport (con-ed)nnMoreover, from experimental data it was clear that far SOL
plasma transport is not consistent with diffusive process(Umansky, 1998), but could be related to advection ofisolated plasma filaments (“blobs”) on the outer midplane(Krasheninnikov, 1998), which, actually, were observed inexperiments long time ago (Goodall, 1982; Zweben, 1985;
Endler, 1999) and also were noticed in 2D interchange
driven edge plasma turbulence simulations (Benkadda,
1994; Sarazin, 1998)
Thus, in edge plasma anomalous transport we should be dealing with an impact ofdivertor detachment on H-modeØ near SOL transport, determining a footprint of power flux to the target (and, therefore,
altering detachment!), andØ far SOL transport, determining plasma interactions with the main chamber wall
Ø although, we should keep in mind that such separation of anomalous plasma
36transport on near- and far- SOL is somewhat artificial

Detachment and near SOL anomalous transport
n

n

n

n
n

With increasing edge plasma pressure the SOL“width”, ΔSOL , might be controlled by ideal ballooning
2
RβSOL / ΔSOL , which could strongly
instability, α = q cyl
increase SOL plasma transport (M. Peng, 1994)
Experimental data show that complete detachment causes
significant increase of near SOL anomalous transport. Is it
indeed controlled by ideal ballooning effects?

(Sun, 2015)

It seems that 3D edge plasma turbulence simulations suggest that at small diamagnetic
parameter, αd ∝ T / n , edge plasma transport, mainly governed by resistive ballooning
modes at outer side of the torus, strongly increases with increasing α (Rogers, 1992;
Pogutse, 1994; Hu, 2003; Rici, 2012; B. Chen, 2017)
Experimental data look consistent with these conclusions (LaBombard, 2005; Gunn, 2007)
Simpler 2D turbulence simulations implying the interchange drive also show (at least
qualitative) agreement with experimental data (Garcia, 2007; Naulin, 2007; Myra, 2011)37

Detachment and near SOL anomalous transport (con-ed)nnnBased on experimental fit for the SOL “width” ΔSOL (Eich, 2011) and “heuristic” SOL widthmodel (Goldston, 2012) it was suggested that H-mode density limit is also governed byballooning modes (Eich, 2018)
According to this model, after αsep reaches critical value corresponding to the destabilization of ballooning modes, confinement degrades, which sets the H-mode density
limit
However,Ø is the SOL turbulence generated in SOL?
Ø or it is comes from inside of the separatrix?

38

Detachment and near SOL anomalous transport (con-ed)nn

It is known that even in H-mode (in between ELMs) ~50% of anomalous plasma transport
is associated with blobs (Boedo, 2001)
With increasing plasma density an impact of “blobby” transport increases (Bernert, 2015;
Carralero, 2017), which results in the “erosion” of the pedestal(Bernert, 2015)39

Detachment and near SOL anomalous transport (con-ed)
nHowever, it seems that blobs are formed inside of or at the separatrix (e.g. Terry, 2003;
Maqueda, 2011; Chang, 2017)!
Blob in C-Mod
originatedinside the LCFS
and propagating into
outboard SOL(Terry, 2003)(Maqueda, 2011)40

Detachment and near SOL anomalous transport (con-ed)nAlso, statistical properties of blobin near and far SOL are virtually
identical (Garcia, 2018)(Terry, 2018)41

Detachment and near SOL anomalous transport (con-ed)

nWhere SOL turbulence “starts”?

n

Do we have clear evidence supporting “local” SOL turbulence?

n

Or SOL turbulence comes from the “core”?

nWhy “heuristic” model for the SOL width, which assumes “noturbulence”, fits experimental data?

42

Detachment and far SOL anomalous transportnAlready early analysis of experimental data on high density
edge plasma transport (Umansky, 1998) has shown a strong
plasma flux into far SOL, which can only be consistent withplasma radial advection due to blobs

SOL plasma density shoulder(LaBombard, 2005)(Carralero, 2017)43

Detachment and far SOL anomalous transport (con-ed)
n

n

n

n

It is often assumed (Manz, 2013; Carralero, 2014, 2017; ) that at high plasma density the formation of
the SOL plasma density shoulder could be associated with:
Ø effective “insulation” of plasma blob from divertor target at small divertor temperatures (Myra,2006), which could boosts up blob’s speed
Ø or neutral ionization in far SOL (D’Ippolito, 2002)
However, actual dynamics of blobs is much more complex than simple analytic models often used in
the literature. For example, “sheath connected” and “disconnected” models do not account for very
high effective X-point resistivity related to magnetic shear (Farina, 1993; Ryutov, 2004), which resultsin different scaling for blob’s speed (Krasheninnikov, 2004)
In addition, two important issues are usually disregarded:Ø variation of blob generation rate with increasingplasma density, andØ energy balance related to neutral “ionization cost”
It seems that very recent studies of shoulder formation(Wynn, 2018) show that more work should be done!

σ|| / σ sh < 1→ λ C / L|| < m / M ≈ 1/ 60

σ|| / σ X < 1→ λ C / L|| < k ⊥ρe ≈ 10−344Intermediate conclusions: Detachment-201
I.

By adding impurity radiation and drifts, details of detachment physics become much more
complex:
Ø In- out- asymmetryØ Could impurity radiation be spread over divertor leg?
Ø Bifurcations
Ø Detachment oscillations

II.

Anomalous edge/SOL plasma transport adds additional complexity:
Ø Increase of near SOL width at high plasma density is due to what?
Ø How it affects divertor detachment? Any synergistic effects?
Ø Formation of far SOL shoulder is due to blob’s transport (e.g. increase of generationrate) or neutral ionization? Any impact on detachment?
45

Detachment-301
nApproaches to divertor design in tokamak-reactors:
Ø Main trend is to use multi-X-point long leg divertors

X-point target divertor
LaBombard, APS, 2013
Snow-flake divertor
Although, no monsters weresuggested yet!

Ryutov, PoP, 2007

46

Detachment-301 (con-ed)nnn

It is anticipated that the fusion power generated in future reactors will besignificantly (~5 times?) higher then that in ITER
In case it’d be possible to prevent high-Z impurity accumulation in core plasma,an ideal solution of heat removal would be almost complete re-radiation of thepower from the core with high-Z (to reduce fuel dilution) impurity
If it appears not possible, we will have the following issues with heat removalfrom SOL/divertors and plasma facing materials in magnetic fusion:Ø Protection of divertor targets
Ø Protection of main chamber first and long-leg divertor side walls
Ø Prevention of localized overheating of divertor side-walls

47

Detachment-301 (con-ed)nI believe that in the absence of violent transient
effects:
Ø Protection of divertor targets
Ø Possible
Ø Protection of main chamber first and long-leg
divertor side walls
Ø Virtually impossible due to large span ofplasma parameters along divertor legs (could
sacrificial protection layer help? Stangeby)
Ø Prevention of localized overheating of divertor
side-walls
Ø Difficult to assess with present knowledge of
impurity transport involving both classical andanomalous transport

Soukhanovskii, 201548

Detachment-301 (con-ed)nRecently suggested Li vapor box divertor looks
interesting alternative to “conventional”
magnetic divertors
Ø High Li atom polarization constant should
provide large stopping power of Li vapor forincoming hydrogen plasmaØ Potentially, Li could provide sacrificial protection
layer for entire first wall
Ø However, Li ionization potential is very low,which could result in predominant build up of
Li plasma, and its ejection from the box
toward the coren

But much more should be done in thisdirection, before any quantitative predictions
could be madeGoldston, 2017

Kulcinski, 1974

49

Final conclusions
n

n

n

n

n

n

The detached divertor regime is a natural continuation of the high recycling conditions to higher density
and higher impurity radiation loss
Both theoretical analyses and experimental data show clearly that mutually complementary effects of
the increase of the impurity radiation loss and volumetric plasma recombination cause the rollover ofthe plasma flux to the target, which is the manifestation of ultimate detachment
Plasma-neutral friction (neutral viscosity effects), although important for sustainment of the highdensity, high pressure plasma upstream, is not directly involved in the reduction of the plasma flux tothe targets
In- Out- detachment asymmetry and drifts paly important role in detachment phenomena and could
cause detachment “oscillations” and bifurcations
Transition to detachment results in increasing intermittent SOL plasma transport which increases near
SOL width, facilitates plasma interactions with main chamber wall, and, finally, is responsible fortermination of H-mode. However, the cause of increased intermittent SOL plasma transport is not clear
yet (although there are some plausible explanations of this effect)
New new divertor concepts are under assessment. However, at this moment it is difficult to predict
their futures as possible divertor solutions in future reactors

50

Final conclusion (personal opinion)
I believe that today the main “showstopper” is our poor understanding of anomalous
plasma transport in detached regimes!
In particular:
I. Is the SOL plasma turbulence in current divertors is “locally” driven turbulence? or
II. It is a “fossil” turbulence driven by some instabilities inside the separatrix? orIII. The truth is somewhere at the separatrix?
IV. Turbulent plasma transport within long divertor legs in “advanced” divertors?

51

Final conclusion (personal opinion, cont-ed)
However, obviously there are other gaps in our understanding of detachments
physics and imperfections in the codes we use. Such as:
I. How important edge plasma kinetic effects in both perp and parallel plasmatransport? Can they cause any “big” effects?
II. More quantitative assessments of an impact of radiation trapping on detachment
II. Bifurcations in edge/SOL plasmas: drifts vs anomalous transport
III. Impact of dust on detached plasma, what is the limit for disruption?
IV. Edge plasma coupling to dynamic wall response: synergistic effects?
V. Is any way (or no way!) to “marry” edge/SOL turbulence and transport codes?
VI. What is a role of electron emission from the target in divertor detachment?
VII. ???

52

Acknowledgementsnn

I acknowledge the discussions of different aspects of edge plasma physics with many mycolleagues all around the world
Work is performed under auspices of USDOE, grants DE-FG02-04ER54852, DESC0018302, DE-SC0016548, and DE-FG02-04ER54739 at UCSD

53