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SUMMARY:Impact of plasma flow velocity shear and neutrals on edge plasma i
nstabilities
DTSTART;VALUE=DATE-TIME:20210513T101000Z
DTEND;VALUE=DATE-TIME:20210513T103000Z
DTSTAMP;VALUE=DATE-TIME:20210417T022724Z
UID:indico-contribution-17483@conferences.iaea.org
DESCRIPTION:Speakers: Sergei Krasheninnikov (University California San Die
go)\nWhereas it is widely believed that velocity shear could suppress plas
ma instabilities and stimulate the transition from low (L-) to high (H-) c
onfinement modes\, the underlying physics of plasma instability suppressio
n is still not clear. Often it is assumed that the stabilization of plasma
instability characterized by the growth rate $\\gamma _{inst} $ occurs wh
en by the velocity shear $| V'_{0}|$ exceeds $\\gamma _{inst} $ (e.g. see
Refs. 1\, 2). One of the complications of the analysis of the velocity she
ar effect on plasma instabilities is the non-Hermitian nature of the diffe
rential equations describing an impact of velocity shear on plasma/fluid i
nstabilities [3]. However\, we find that the situation is more complex and
just effective Richardson number $Ri=(\\gamma _{inst} /| V'_{0} |)^{2} $
cannot describe overall impact of $|V'_{0} |$. Employing ``radial'' densit
y profile $n(x)=\\bar{n}+(\\delta n/2)tanh(x/w$) (where $w$ is the effecti
ve width of the density profile and $\\bar{n}\\gg\\delta n$ are some const
ants) and analyzing the localized modes\, we find [4] that for $\\kappa =|
k_{y} w|\\gg 1$ ($k_{y} $ is the ``poloidal'' wave number) the growth rat
es of both fluid Rayleigh-Taylor (RT) and plasma interchange (I) modes cou
ld be significantly reduced even for $Ri\\gg 1$ (see Fig. 1a).\n![The grow
th rate of the most unstable I mode. a) The curves of solid black\, dashed
blue\, dash-dot red are for $| V'_{0} |/\\bar{\\gamma }=0$\, $0.2$\, and
$0.4$. b) The growth rate and $|V'_{0} |/\\gamma _{inst} $ of the most uns
table RDW mode. The solid\, doted\, and dashed are for $k_{y} \\rho _{s} =
0.5$\, $0.3$\, and $1$.][1] \n\nOn the contrary\, the resistive drift wave
s (RDW) are not stabilized even for $Ri\\ll 1$ (see Fig. 1b\, where $\\hat
{\\omega }_{*} =k_{y} \\rho _{s} C_{s} /L_{n} $\, $\\nu _{\\parallel} /\\h
at{\\omega }_{*} =50$\, $\\nu _{\\parallel} =k_{z}^{2} T_{e} /m\\nu _{ei}
$ is the effective parallel electron diffusion frequency\, and $w/\\rho _{
s} =30$). However\, the localized RDW modes cease to exist at $| V'_{0} |
>| V'_{0} |_{loc} \\approx 0.66(1+k_{y}^{2} \\rho _{s}^{2} )^{-1} (\\rho _
{s} /w)C_{s} /L_{n} $. In addition\, we find that\, whereas the eddies of
both RT and I modes in the presence of $| V'_{0} |$ become tilted into y-d
irection\, Fig. 2a\, those of the RDW become just shifted into radial dire
ction\, Fig. 2b\, The results of numerical analysis of non-modal solutions
of the RDW for $| V'_{0} |>|V'_{0} |_{loc} $ will be presented.\n![Eddies
of a) the I mode and b) the RDW mode w/o (left) and with (right) velocity
shear.][2] \nUnlike the effect of velocity shear\, the results of the stu
dies of an impact of neutrals on edge plasma instabilities and turbulence
are somewhat controversial. Whereas some experiments show no effect of neu
trals on edge plasma turbulence\, others demonstrate an importance of neut
rals for L- to H-mode transition (e.g. see Refs. 5\, 6). Similarly\, where
as some simulations show that neutrals result in increasing edge plasma tu
rbulence\, some others claim opposite effect (e.g. see Refs. 7\, 8). One o
f the complexities of the incorporation of neutral effects into plasma ins
tabilities\, turbulence\, and transport is the wide range of neutral-plasm
a interaction regimes (from kinetic to fluid) defined by the ratio of the
wave length (frequency) of different plasma modes to neutral-ion collision
mean free path (neutral-ion collision frequency).\nWe report here the res
ults of a careful analysis of the effect of neutrals (ranging from kinetic
to fluid transport regimes) on interchange\, RDW\, and grad($T_{e}$ ) ins
tabilities [9] and find that in practice neutral make a very minor impact
on these instabilities\, although in dense divertor plasma an impact of ne
utrals on plasma stability could be important (see Ref. 10 and the refernc
es therein). However\, we find that neutrals can significantly alter the g
eneration of zonal flow by plasma turbulence (e.g. by DW turbulence [11])
and by that modify edge plasma turbulence and transport.\n\n[**1**] W. Hor
ton\, ``Turbulent transport in magnetized plasmas'' (World Scientific\, Se
cond Edition\, 2018)\; [**2**] J. Kinsey\, R. Waltz\, and J. Candy\, Phys.
Plasmas **12** (2005) 062302\; [3] L. N. Trefethen\, et al.\, Science **1
2** (1993) 578\; [4] Y. Zhang\, S. I. Krasheninnikov\, and A. I. Smolyakov
\, Phys. Plasmas\, **27** (2020) 020701\; [5] M. A. Pedrosa\, et al.\, Phy
s. Plasmas **2** (1995) 2618\; [6] D. J. Battaglia\, et al.\, Nucl. Fusion
**53** (2013) 113032\; [7] D. P. Stotler\, et al.\, Nucl. Fusion **57** (
2017) 086028\; [8] N. Bisai\, R. Jha\, and P. K. Kaw et al.\, Phys. Plasma
s **22** (2015) 022517\; [9] Y. Zhang\, S. I. Krasheninnikov\, submitted t
o Phys. Plasmas\, 2020\; [10] A. Odblom\, et al.\, Phys. Plasmas **6** (19
99) 3239\; [11] A. I. Smolyakov\, et al.\, Phys. Rev. Lett. **84** (2001)
491.\n\n\n [1]: https://live.staticflickr.com/65535/49533754862_a0a28d288
1_m.jpg\n [2]: https://live.staticflickr.com/65535/49533034738_1cc9376fd2
_m.jpg\n\nhttps://conferences.iaea.org/event/214/contributions/17483/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17483/
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