Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 (2004)
6 (2004)139–143
000–000
J. Plasma Fusion

Large Plasma Pressure Perturbations andRadial Convective Transport in a Tokamak
KRASHENINNIKOV Sergei, RYUTOV Dmitri1 and YU Guanghui
University of California at San Diego, La Jolla, CA 92093, USA
1Lawrence Livermore National Laboratory, Livermore, CA 94551, USA(Received: 9 December 2003 / Accepted: 3 April 2004)

Abstract
Strongly localized plasma structures with large pressure inhomogeneities (such as plasma blobs in the scrape-offlayer (SOL)/shadow regions, pellet clouds, Edge Localized Modes (ELMs)) observed in the tokamaks, stellarators andlinear plasma devices. Experimental studies of these phenomena reveal striking similarities including more convective
rather than diffusive radial plasma transport. We suggest that rather simple models can describe many essentials ofblobs, ELMs, and pellet clouds dynamics. The main ingredient of these models is the effective plasma gravity caused
by magnetic curvature, centrifugal or friction forces effects. As a result, the equations governing plasma transport insuch localized structures appear to be rather similar to that used to describe nonlinear evolution of thermal convectionin the Boussinesq approximation (directly related to the Rayleigh-Taylor (RT) instability).
Keywords:
pressure perturbation, pellet cloud, ELM, blob, convective transport, Rayleigh-Taylor instability[16]. Therefore, it is not surprising that all these plasma
physics phenomena have many similar features.
The paper is organized as follows. In Sec. 2 we review
equations describing blob dynamics in the far SOL [6] and in
the shadows of linear devices [17], as well as discuss theresults of both analytic and numerical solution of these equations [6,18]. In Sec. 3 we derive equations describing blob
dynamics in the vicinity of the separatrix to account for theeffects of cross-field conductivity [19] caused by a strong
shear of the magnetic field near the X-point [20]. Based onthese equations we are also presenting the estimates of theblob radial velocity in the region close to separatrix. In Sec. 4
we discuss the equations, which can be used to model mainfeatures of the dynamics of blobs with large plasma beta andshow their similarity to studies of evolution of pellet clouds[9,10]. We also present some results of numerical modelingof these equations. In Sec. 5 we discuss the material presented in previous Sections in the contex of the studies of nonlinear evolution of RT instability and summarize our main conclusions.

1. Introduction
In many cases strongly localized, in the plane perpendicular to the magnetic filed lines, plasma pressure inhomogeneities emerge in tokamaks. The examples of such cases
are the plasma blobs in the SOL [1-8], pellet clouds [9,10],ELMs [11,12]. Even though at first glance they look very different, they all exhibit more convective rather than diffusive
radial transport of a plasma. Similar features of convective
crossfield plasma transport toward the wall were observed
also in stellarators [13] and linear plasma devices [14,15].
Rather detailed experimental study of these phenomena
shows striking similarities between them. For example, thetemporal profiles of ion saturation currents in plasma blobs,
measured by probes, in tokamak SOL [1] and in the shadowsof linear devices [15] look virtually the same. Another example is the similarities in probe measurements of plasma blobs
in both L- and H-modes and ELMs, although of relatively
small amplitudes which do not damage the probe, [11].
In this paper we suggest that rather simple two-dimensional (2D) models can describe many essentials of blobs,
small ELMs, and pellet clouds dynamics. The main ingredient of these models based on an ideal magnetohydrodynamics (MHD) is the effective plasma gravity caused by magnetic curvature, centrifugal or friction forces effects. As a result,the equations governing plasma transport in blobs, small
ELMs, and pellet clouds appear to be rather similar to thoseused to describe nonlinear evolution of the RT instability

2. Blob dynamics in far SOL of tokamak andshadow of linear device
In order to explain experimental results [21] of fast radial transport of plasma through the SOL of main chamber asimple 2D model was suggested in [6]. The main idea of this©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author’s e-mail: skrash@mae.ucsd.edu
1139Krasheninnikov S. et al., Large Plasma Pressure Perturbations and Radial Convective Transport in a Tokamakmodel can be described as follows. Time to time, due tosome nonlinear processes, the plasma blobs (filaments,
extended along the magnetic field lines and seen on visual
diagnostics [3] as a blob) are peeled off from the bulk plasma
and move radially through the SOL toward the wall with highspeed due to ∇B plasma polarization and corresponding E ×
B drift. The plasma density in the blob is much higher thatambient plasma density in far SOL and comparable to that of
the bulk plasma in the vicinity of the saparatrix. Therefore,
even though blobs are peeled off the bulk not often, theircontributions to plasma energy and particle transport in far
SOL can be dominant. In linear devices the role of ∇B in
plasma polarization can be played by centrifugal force orneutral wind effects [17]. We notice that this physical pictureof dominant role of blobs in plasma transport in tokamak far
SOL and the shadows of linear devises was later supported
by experimental observations in both tokamaks and linear
devices.
To describe blob dynamics in tokamak far SOL we use
quasi-slab approximation of the outer side of the torus, with x
and y coordinates being radial and poloidal directions andstraight magnetic field lines intersecting material surfaces situated at distance Lc (with the subscript “c” standing for “connection”) from each other (see Fig. 1). We assume cold ionsand fixed electron temperature (Te) which is uniform andconstant in time. Then in electrostatic approximation fromelectron and ion momentum balance equations and ∇·j = 0,
where j is the electric current, we find⎧ cnTe d∇⊥ φ ⎧ 2 cTe ∂ n∇⊥ . ⎨= ∇ || j || ,⎨+⎩ Ω i B dt ⎩ R B ∂ yAssuming low plasma volumetric resistivity and constant plasma density along the magnetic field lines we integrate the equation (1) along parallel coordinate and fine

j||(1) − j||(2)⎧ cnTe d∇⊥ φ ⎧ 2 cTe ∂ n∇⊥ . ⎨
=
, (2)
⎨+
Lc
⎩ Ω i B dt ⎩ R B ∂ ywhere j||(1) and j||(2) are the currents to the walls⎛ M ⎞ 1/2⎧⎧
j||(wall) = swenCs⎨1 − ⎜⎟ exp(φw − φ )⎨ ,
⎝
⎠
2π m
⎩
⎩

(3)

M and m are the ion and electron masses respectively,
Cs = (Te /M)1/2, sw = ±1 depending on the orientation of thewall with respect to the coordinate frame, φw is the potentialof the wall (here we assume φw = const.). For relatively small
fluctuating part of electrostatic potential, |φ| < 1, from (3) wehave j||(wall) = σwenCsφ. Substituting this expression in (2)we find⎧ d∇ φ ⎧ 2ρ C ∂ n 2Csρ2s ∇⊥ . ⎨ n ⊥ ⎨ + s s
=
nφ ,
R ∂y
Lc
⎩ dt ⎩(4)

where ρs = Cs /Ωi. Equation (4) with the continuity equation∂n
2Cs+ uE × B .∇n = −
n,
∂t
Lc

(5)

govern blob dynamics in far SOL region (the term in righthand side of eq. (5) describes plasma leakage to material surface along the magnetic field lines).
In the absence of ambient plasma the eqs. (4) and (5)allow the solution in the form of traveling wave (blob) [6],(1)⎧ ⎛ y ⎞ 2 2Cst ⎧n(x,y,t) = nb(x − Vbt)exp ⎨ − ⎜ ⎟ −⎨,Lc ⎩⎩ ⎝ δy ⎠where R is the tokamak major radius, n is the plasma density,
B ∝ 1/R is the toroidal magnetic field strength, Ωi is the ioncyclotron frequency, c is the light speed, φ = eϕ/Te, e is theelementary charge, ϕ is the electrostatic potential, d(…)/dt =∂(…)/∂t + uE × B·∇(…), uE × B = c(B × ∇ϕ)/B2, j|| is the parallel
current.

(6)

where n b (x) is an arbitrary function, δ y is the effective
poloidal width of the blob, and⎛ ρ ⎞2 L
Vb = 2Cs ⎜ s ⎟ c⎝ δy ⎠ R(7)

is the velocity of the blob. For Te ~ 30eV, B ~ 2T, δy ~ 1cm,
Lc ~ qR, where q is the safety factor (~3), from eq. (7) wefind Vb ~ 105 cm/s, which is close to the experimental observations [1-5].
In the case of linear devices, where there are no magnetic field curvature effects, plasma polarization can be due tocentrifugal force or neutral wind. The later one is related toasymmetry in neutrals distribution function [17]. The neutrals
coming to the wall from plasma side experienced some plasma-neutral interactions. Therefore, they are hotter than theneutrals coming from the wall. As a result of this asymmetry,in the shadow regions of linear devices there is neutral-ion
friction force (neutral wind) even though neutral particle flux
is negligibly small. In a sense, neutral wind is an extension ofthermal force to the semi-collisional situation.
Taking neutral wind effects into account, the analog ofeq. (4) can be written as follows
Fig. 1 Schematic view of the SOL region.

2
140

Krasheninnikov S. et al., Large Plasma Pressure Perturbations and Radial Convective Transport in a Tokamak

the divertor leg. Due to a very strong squeezing of the tube in
the vicinity of X-point, its minimal width for standard tokamak conditions decreases from ~1 cm around the mid-plane
to less than the ion gyro-radius close to X-point [20]. As aresult, the effects of finite cross-field resistivity, otherwise
small, are strongly magnified [19,22] and play an importantrole.
In order to incorporate these effects into our simplified
physical picture of blob dynamics we will use a heuristic
model developed in [19]. The essence of this model is thesubstitution of exact solution of penetration of the electrostatic potential into X-point region by an effective boundary condition relating parallel current and potential at the entrance to
the X-point region. To derive this relation, we take into
account squeezing of the flux tube we introduce: a) squeezing function S(r) ≈ exp(−r/LX), where r is the length along
the magnetic field line and LX is the effective squeezing
length (usually in current large tokamaks LX ~ 103 cm), andb) the effective wave number of the potential perturbation
K(r) = k/S(r), where k is the wave number at the entrance
into the X-point region. Then, we balance perpendicular, j⊥ =−iσ⊥K(r)ϕ, and parallel, j|| = −σ||(∂ϕ/∂r), currents via the ∇·j2= 0 equation, ∂ 2ϕ /r2 = −( σ ⊥/ σ ||)K 2(r) ϕ , where σ ⊥ = ω pe22νei/(4πΩe ) and σ⊥ = ω pe /(4πνei) (notation is standard). As aresult, we find a relation between parallel current and electrostatic potential at the entrance to the X-point region [19]⎧ d∇ φ ⎧∂ n 2Csρ2s ∇⊥ . ⎨ n ⊥ ⎨ + Csηpolar
=
nφ ,
(8)
dt
Lc
∂y
⎩
⎩
where ηpolar ~ νiN/Ωi is the plasma polarization factor and νiNis the ion-neutral collision frequency. Notice that withreplacement ηpolar ~ νiN/Ωi on ηpolar = 2ρs /R the eqs. (4) and(8) are virtually the same. It explains the similarities of blobs
in tokamak SOL and shadows in linear devices seen in experiments.
Study of blob dynamics governed by eqs. (8) and (5)shows [18] that the blobs with crossfield scale⎛ L ⎞ 2/5
δb ~ δ ∗ = 2 ρs ⎜ e⎟ (ηpolar)1/5⎝ ρs ⎠(9)(we assume here that δb ≡ δy ~ δy) are very stable structurally
and propagate radially on large distance keeping its shapeintact. Blobs bigger than δ* are the subjects of the RT instability, which splits them in a few smaller ones. Blobs smaller
than δ* are quickly transformed into mushroom shape withthin front like structures [8] and their further evolution is sensitive even to weak plasma diffusion.

3. Blob dynamics in the vicinity of separatrix
In previous section we analyzed blob dynamics in tokamak far SOL where magnetic field lines have a very simple
geometry. However, blobs are peeled off from bulk plasma inthe vicinity of the separatrix (in diverted tokamaks). To
describe blob dynamics there we need to account for geometrical effects of a strong shear of the magnetic field near the
X-point. In [20] it was shown that magnetic shear in thevicinity of X-point results in dramatic squeezing of magnetic
flux tubes. Figure 2 shows this schematically. The shadowed
regions represent the same flux tube when it passes near the
X-point, from the position 1 in the main SOL, to position 3 in

j|||entr ≈ σ eff |k|ϕ | entr ,(10)2
where σeff = Gω pe/(4πΩe), and G is order unity phenomenological coefficient. Notice that the squeezing of the magnetic
flux tube occurs near the separatrix in both open and closed
flux surfaces and, therefore, the expression (10) can beapplied at both sides from the separatrix.
We can use the relation (10) to close eq. (1) after the
integration along the magnetic field like as we did in Sec. 2,
where magnetic field lines were going through the wall and
we used relation (3) to close eq. (2). For simplicity, we consider only a symmetric double-null divertor, so that eq. (10)should be applied at both ends of the flux tube (with the obvious change of the sign). Then, approximating wave numberof the blob at the entrance into the X-point region as k ~ 1/δb
we find⎧ d∇ φ ⎧ 2ρ C ∂ n 2Cs ρsρ2s ∇⊥ . ⎨ n ⊥ ⎨ + s s=
Gnφ , (11)R ∂yLb δ b⎩ dt ⎩
where Lb is the parallel length of the blob. From eq. (11) we
estimate radial velocity of the blobVb ~2Cs ρs Lb.
G δb R(12)

Thus we find that strong squeezing of magnetic flux tube in
the vicinity of X-point do not prohibit the blob radial motion.
By taking into account effective X-point resistivity [19] we
describe blob motion in the vicinity of the separatrix in bothclosed and open magnetic flux surfaces. Moreover, compar-

Fig. 2 Evolution of the cross-section of magnetic flux tube in
the vicinity of separatrix

3141Krasheninnikov S. et al., Large Plasma Pressure Perturbations and Radial Convective Transport in a Tokamak

ing the expressions (7) and (12) we see that close to the separatrix Vb ∝ ρs /δb << 1, while in the far SOL Vb ∝ (ρs /δb)2.
Therefore, it can explain experimental observations [5] ofhigher blob velocity in the region close to the separatrix thanin far SOL.

4. Dynamics of blobs with large beta
In previous sections we neglected an impact of blob on
the structure of the magnetic field. However, in case of large
beta of blob plasma, βb, such impact can be very important.
In order to address this issue we estimate the perturbation of
the magnetic field due to the blob motion in a tokamak. In
this case, the plasma polarization current is balanced by parallel current dipole

j|| ~ enCs

ρs Lb
,
δb R

Fig. 3 Strong bending of magnetic field line causes touch offirst wall without intersection of limiters.

(13)

which gives the following magnitude of radial perturbation of
the magnetic field strength, Br,

Br 4π
L
~
j||δ b ~ βb b .
B
cR

Substituting (18) in (2) we have2V⎧ d∇ φ ⎧ 2C ∂ nρs∇⊥ ⋅ ⎨n ⊥ ⎨ + s= − A namb ρs∇⊥2 φ , (19)Lb
⎩ dt ⎩ R ∂ y(14)where the Alfven velocity is calculated with the density ofambient plasma namb = const. The eq. (19) is just the same as
the equation describing the evolution of pellet clouds in [9,10]. From eq. (19) it is possible to show that for large blobs,
where inertial term in (19) is small, the velocity of the blob
described by eq. (19) does not depend of its spatial scale andcan be estimated as follows

As a result, the magnetic field line bends in radial direction.
Since bending of magnetic field like propagates along B with
the Alfven velocity VA = B/(4πnM)1/2 such quasi-steady state
approximation of the magnetic field line structure can beconsidered if

Vb ~ B r
L<~ βb b .
VA B
R

(15)
Vb ∼ Cs

Then, taking into account the expressions (7) and (12) forblob velocity, we find from inequality (15):

Vb < Cs β 1/2
b

Lb
.
R

R∆w
,
L2c

(16)

5. Discussions
Thus we find that the blob dynamics in edge plasmas
can be described with rather similar eqs. (8), (11) and (19),depending on plasma conditions and geometry. In somesense these equations are similar to the Boussinesq approximation of 2D thermal convection (related to the RayleighTaylor instability of a stratified medium)(17)

where we assume Lb ≈ Lc, ∆w is the distance from last closed
flux surface to the first wall. In order to describe the evolution of blobs with βb > βcrit within the framework of simple
2D model, we use the approach adopted in the studies of the
dynamics of pellet clouds [9,10]. Introducing the vector
potential A|| and taking into account that the bending of magnetic field propagates along the field line with Alfvén speed,from relation E|| = −∇||ϕ − c-1∂A||/∂t = 0 we find ϕ = (VA/c)A||and, correspondingly,

j|| = −c2
∇ 2⊥ϕ .
4πVA

(20)

The results of numerical solution of eq. (19) support estimate(20). In Fig. 4 we show how seeded blob, described by eq.
(19), coherently moves in the radial direction (R = 175 cm, Lb= 4000 cm, ρs = 0.06 cm, VA = 4 × 109 cm/s, Cs = 3 × 106cm/s).

However, at relatively large βb bending of the magnetic
field lines becomes so strong that the magnetic field line
would “touch” first wall without even going through thematerial surfaces of divertor targets or limiters (see Fig. 3).
Taking into account expression (14) from Fig. 3 one sees thatsuch situation occurs for

β b > β crit ~Cs Lb
L= Cs β b .
VA RR

dt ∇ ⊥2ψ + g∂T= µ∇ ⊥4ψ,∂y

dt T = 0 ,(21)where T is the temperature, g is the effective gravity acceleration, u = ez × ∇ψ is the velocity, µ is the effective viscosity.
However, “dissipative” terms in right hand side of the blob
equations are different than in (21). In case where dissipation
is not important we see that mushroom shape of originallycircular blob develops (see for example [8,18]) similar to thetypical mushrooms shapes in the dynamics of the RT instability [16]. But, “dissipative” terms can bring structural stability of blobs when they propagate on large distance as a

(18)

4
142

Krasheninnikov S. et al., Large Plasma Pressure Perturbations and Radial Convective Transport in a Tokamak

Fig. 4 Density contours of blob.

coherent structure [6,8,18].
We also find that that dynamics of blobs with high beta
can be described by the same equations as the evolution ofpellet clouds [9,10]. As a matter of fact, this is not surprisingsince in both cases we consider the evolution of plasma structures with the pressure, which significantly exceeds the pressure of a surrounding tokamak plasma. Therefore, this may
be a reasonable model for the studies of nonlinear dynamics
of large pressure perturbations of relatively small spatial
scale when perturbations can be treated like isolated filaments.
One of the examples of such process can be ELM ofsmall amplitude, when tokamak edge plasma is not perturbed
too much, and isolated features of ELM structure are clearlyseen in the experiment [5,11]. According to our findings
described above, small isolated ELM propagates in convective manner through the edge and SOL plasmas somewhat
similar to what was observed in experiment [5,11]. This isdifferent from the results of [23] where early nonlinear stageof ballooning instabilities was considered and explosive typeof behavior was predicted. It is plausible that early nonlinear
evolution goes with some acceleration and then, in a deeply
nonlinear stage, evolution converges into nonlinear advectionof pressure perturbation which is characterized by more orless constant speed. Clearly additional studies are needed toclean this issue up.
The work was performed for the U.S. DoE under contract W-7405-Eng-48 at the LLNL and Grant No. DE-FG0300ER54568 at the UCSD.

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