J. Plasma Fusion Res. SERIES, Vol.2 (1999) 85-88

Experimental lnvestigations of Plasma Turbulence
Suppression in the DIII-D Tokamak
MCKEE George R., BURRELL Keirh H.r, DOYLE Edward J.:, GREENFIELD Charles M.r, FONCK Raymond J.,
JACKSON Gary L.t, MURAKAMI Masanori.3, RETTIG Curt L.2 and STAEBLER Gary M.r
University of Wisconsin-Madison, Madison, WI 53706, USA
lGeneral Atomics, San Diego, CA, 92186, USA
2Iln-iversity of Califurnia-Los Angeles, Los Angeles, CA, 90095 USA'Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA(Received: 8 December 1998 / Accepted:22 July 1999)

Abstract
Reducing turbulence has been shown to improve confinement under a variety of plasma operationalmodes. Turbulence measurements during the formation of internal transport barriers in plasmas withnegative central magnetic shear have shown that turbulence-driven fluctuations are dramatically reduced
inside the barrier while energy transport is also reduced. This is consistent with the E x .B shearing modelof turbulence suppression during formation of an internal transport barrier. A complementary path toturbulence suppression is suggested by recent experiments in which impurities are puffed into a quiescent

L-mode discharge. Turbulence is dramatically reduced in response to the impurity puff, and energy
confinement increases simultaneously. This behavior is consistent with gyrokinetic simulations whichindicate that dilution of the working ions by impurities can reduce growth rates of ion temperature
gradient driven turbulence.

Keywords:
turbulence, fluctuations, internal transport barrier, flow shear, suppression, impurity effects

1. lntroduction

transport and improve confinement.
The E x B shearing model [] successfully explains

Anomalous cross-field transport, observed in mostthe suppression of turbulence in a wide variety ofplasma modes. In this model, a radial electric field ismagnetically confined plasmas, reduces energy andparticle confinement from that which would be obtained

if transport were driven solely by neoclassicalformed through a core charge imbalance generated by a
combination of tangential and poloidal plasma rotation
as well as the plasma pressure gradient. Radial shear incollisional processes. Plasma turbulence is widelybelieved to be responsible for this anomalous transport.
Turbulence and the resulting electrostatic fluctuations

the radial electric field, dE"/dr, results in sheared

induce an anomalous flux, 7 = (fii,) that can be
described by a diffusive transport coefficient that is

poloidal flows,

E,X.B,"t-often orders of magnitude above the neoclassical values.
Recent advances in experimental methods andapplications of new techniques have demonstrated theability to suppress plasma turbulence and thereby reduceg''that alter turbulence through two mechanisms: turbulent
eddies are sheared apart by the poloidally varying flow
@1999 by The Japan Society of Plasma
Science and Nuclear Fusion Research85

Mckee G.R. et al., Expeimental Investigations of Plasma Turbulence Suppression in the DIII-D Tokamak

velocity, reducing their amplitude, and density andsuppress turbulence through decorrelation and eddy

potential fluctuations are decorrelated by a change in thephase angle between density and potential fluctuations[2]. This is formalized by writing the shearing rate,

shearing [1]. Measurements of fluctuations in suchplasmas are broadly consistent with the thesis thatturbulence is suppressed by E x B shear. Measurementswith far infrared scattering [10], which observes density
fluctuations over the central region of the plasma, showa sharp reduction in fluctuations as an internal transport(na,l' a / r. \@E,B=B|
RBeldvl\ -=-l[3]. It has been shown [4] that as the shearing ratebarrier developes, along with an increase in the average

becomes comparable to or greater than the growth rate

lab-frame frequency of the fluctuations, indicating anincrease in the core rotation velocity of the plasma [ 1].
Spatially localized measurements of densityof the largest unstable drift wave modes, the turbulenceis suppressed or eliminated and transport is reduced.

fluctuations obtained with BES, shown in Fig. I,

Turbulence manifests itself as fluctuations inplasma parameters such as the density, electrostatic
potential, temperature, and magnetic fields. The
development of advanced plasma diagnostics has

demonstrate that long-wavelength density fluctuations
are significantly reduced deeper in the core of theplasma, while outside of a given radius, there is little

allowed for the direct measurement of fluctuations insome of these quantities and is providing a moredetailed understanding of the nature and behavior of
turbulence in magnetically confined plasmas. Resultspresented here incorporate measurements obtained withthe Beam Emission Spectroscopy (BES) diagnostic on
the DIII-D tokamak [5]. BES observes density
fluctuations in the wavenumber range ft ( 3 cm-i wheremodes that drive most of the anomalous ion transportchange in fluctuation level. The integrated broad-band

density fluctuation amplitude (fi/n) is plotted here. Thissuggests that the transport barrier is very localizedradially since the spatial region graphed covers just theradial range from p = 0.63-0.76. Inside of about p =0.65, the fluctuations drop by a factor of two to three
after formation of the internal transport barrier. The

density is gradually drifting upwards over this timeframe and changes by less than 30Vo over the time range
observed, so the reduction results primarily from a drop
in fr. The emission intensity fluctuation spectrum isshown in Fig. 2 before and after barrier formation in aare believed to be dominant.

The development of advanced operational regimes

in tokamaks has been shown to dramatically alter thesimilar discharge. It is seen that the overall level

underlying plasma turbulence and thereby affect andoften reduce the transport in these plasmas. Plasmas
with negative central magnetic shear on DIII-D, TFTR,
JT-60U, JET and other tokamaks exhibit regions of(integrated power) of the fluctuations are significantly
reduced after barrier formation. In addition, the averagefrequency of the fluctuations increases from 135 KHz tosignificantly reduced or completely eliminatedturbulence with the resulting transport in the ion channel

at or near the neoclassical level, the irreducible
minimum of collisionally induced transport. Electrontransport is typically still anomalously large though-some discharges have been produced in JT-60U [6] and: :'- '::--"-'-;l:s

DIII-D [7] with reduced electron transport as well.

rc

2. Transport Barrier Turbulence Dynamics
Internal transport barriers are reproducibly
generated in L-mode plasmas in DIII-D using theCore Banior Fomation

12

technique of early neutral beam injection to increase theelectron temperature, slowing penetration of ohmic flux.
This produces an inverted safety factor, q, profile with

1.3'iir" (ri'r'617Fig. 1 Fluctuation amplitude at several spatial locations
before and after the formation of a core transportbarrier showing rapid drop in turbulence inside ofnegative central magnetic shear which facilitates
production of an internal transport barrier [8,9]. The
mechanism is believed to be that magnetic shear

p = 0.65. Outside of the region, there is littlechange in the fluctuation amplitude. Broadband
density fluctuations are integrated over the range

stabilizes Kelvin-Helmholtz instabilities which in turnallows for poloidal rotational shear to develop and80 to 350 KHz.

86

Mckee G.R. et al., Expeimental Investigations of Plasma Turbulence Suppression in the DIII-D Tokamak
2.5x10-7NI!

;xo^:o;
66YL

3oorl-lsharp Fluctuationl |I Suppression l*--\\4oorr"qu"lnTy 1rn.y

1.0

1.1

200

Fig.2 Broadband density fluctuation spectrum before
and after the formation of an internal transportEm.L(!
150 globarrier showing the reduction in amplitude due tocog

100 3do!t.9

the barrier formation and increase in the averagefrequency from the increasing doppler shift.

ooq

50-oover 200 KHz owing to an increase in the plasma
rotation velocity and the resulting doppler shift as the1.0

1.1transport barrier evolves.

The fluctuation amplitude is compared in two

to four sources. Fluctuations are most dramaticallysuppressed in the higher power discharge, shown in Fig.
3(a), consistent with ideas of shear stabilization. Thehigher power discharge transiently exhibits almostcomplete suppression of turbulence at the observed

01.5

3. lmpurity Effects on Turbulence
Recent experiments have suggested that impurities
may aid achieving improved confinement in tokamak
plasmas. In particular, TEXTOR-94 [12], TFTR [13]and DIII-D [14] have demonstrated improved energy
confinement in plasmas seeded with small quantities ofimpurities, e.9., neon, argon, krypton or xenon. The
physical mechanism is not well understood but, basedon observations from recent experiments and turbulent

again as the pressure gradients increase owing to thereduced transport. This can be explained by a larger
E x B shear in the higher beam power plasma. The force
balance from which the radial electric field is derived,n,Z,e

1.4

suppress turbulence.

radius ofp = 0.6. Subsequently, the fluctuations increase-vP',=---=- +ve,iBo -ve.iBo,

1.3

Fig. 3 (a) Comparison of the fluctuation amplitude in
two discharges at p = 0.6 with initially similar conditions, one with a two to three source transition,and one with a two to four source transition. (b)Pressure gradient and E x B shearing rate evolution at the same minor radius.

discharges with similar prelude phases (prior to thetransport barrier formation) and different neutral beam
power steps. Both discharges start with two beamsources, with one stepping up to three sources, the otherE.1.2
Time (s)og-c(l)is typically dominaied by the tangential rotation term,
vrBs, drue to the all co-injection neutral beam system on
DIII-D. Thus faster rotation will generally yield a higher
electric field and typically, though not necessarily, thear6r3 shearing rate is also higher. The pressure gradient,transport simulations, may be related to the suppressionof turbulence as a result of the injected impurity. Probe
measurements in TEXTOR-94 have shown reduced
turbulent driven transport in the edge region of a neonseeded discharge [5].

which is a primary turbulence drive mechanism, shownin Fig. 3(b), increases monotonically throughout thisperiod ofthe discharge. The shearing rate, also shown inFig. 3(b) also tends to increase. The pressure gradientitself contributes to reducing the radial electric field, but
in the these discharges has a minor effect compared withthe much larger tangential rotation term. These results,

Initial experiments on DIII-D to examine thisRadiative-Improved (RI) mode have been performed
and fluctuations have been measured with BES. The
effect of neon on integrated density fluctuations isshown in Fig. 4(a). A pair of discharges are comparedwith nearly identical control parameters (beam power,plasma current, density), while neon was injected at 800
ms in one discharge and not in the reference discharge.
The time evolution of the fluctuation amplitude showsin combination with the fluctuation measurements
indicate that at least transiently, the increasing shearing

rate dominates the increasing pressure gradient to87

Mckee G.R. et al., Expeimental Investigations of Plasma Turbulence Suppression in the DIII-D Tokamak

growth rates of the dominant turbulent drift wavethat there is a sharp reduction in turbulence starting
about 100 ms after the neon injection. Prior to neonmodes. Initial results for the pair of shots discussed
above suggest that indeed the presence of neon reduces
the mode growth rate in a similar region of k-space tothe observed fluctuation reduction, namely 0.5 < ft < 3
cm-r that is dominated by ion temperature gradient andtrapped electron modes [7]. It appears that the impurity
density gradient acts to reduce the drive of the main ion

injection, the fluctuations levels are nearly the same, aswould be expected since the discharges are very similar
at this point. At 1.0 s, an additional neutral beam isinjected, reducing the fluctuation ievels, likely forreasons discussed in Section 2. The reduction inturbulence after neon injection is correlated with anincrease in the energy confinement time as well as
central electron pressure and neutron rate [16]. The
stored energy for the two discharges is shown in Fig.

modes through the 4tmp parameter, the ratio of theimpurity density scale length to the temperature scale
length.

The tools and experimental techniques discussed

4(b). The discharge with neon injection exhibits a rapid
increase in radiated power (not shown), to nearly double
the radiated power in the no neon reference discharge,

here have allowed for a greater understanding of plasma

turbulence and in the process provided methods for
influencing and perhaps controlling levels of turbulencein experimental confinement devices and ultimately inreactors. In general, reducing turbulence is desired toreduce cross field anomalous particle and energy
transport while improving confinement. Confinement

while the total stored energy also increases, indicatingthat heat transport is reduced.

Gyrokinetic simulations have been performed tohelp interpret these observations by predicting linear

directly impacts reactor economics as a criticals

parameter in determining the size of a device. Thus,
methods to control turbulence levels are crucial to theBeam Power Step

Eooptimization of reactor scenarios.

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b)

0.5
o.4
0.3
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0.0
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[10] C. Rettig et a/., Rev. Sci. Instrum. 61, 3010 (1990).
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1 .2Time (s)(l996).

Fig. 4 (a) Density fluctuation amplitude (dl/l) in twodischarges, one with neon injection at 0.8 sec, one

t13l K. Hlll et a/., Bull. Am. Phys. Soc. 42, 20O4without neon, demonstrating the suppression of
turbulence resulting from the neon puff. At 1.0sec, the beam power is increased from two to
three sources in both discharges, likely(1997).

t14l G. Jackson et al., J.Nucl. Mat. 266-269, 38O(leee).

accounting for the observed reduction in[15] J. Boedo et al., accepted by Nucl. Fusion (1998).
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fluctuations at that time. (b) Comparison of thestored energy in the two discharges discussed,
one with neon, one without. The stored energy
increases in the neon discharge, indicating a
reduction of transport.
88