S6-3
JT-60UのType-I	 ELMy	 Hモードにおけるプラズマ周辺部の径電場の時空間構造	 
Spatio-temporal structure of the edge radial electric field in JT-60U Type-I
ELMy H-mode plasmas
神谷	  健作
Kensaku KAMIYA
日本原子力研究開発機構 那珂核融合研究所
Japan Atomic Energy Agency, Naka Fusion InstituteIntroduction
The radial electric field and its associated shear
play a key role for plasma stability and enhanced
plasma confinement [1]. The phenomenon of
spontaneous generation of the sheared ExB flow isobserved in nearly all tokamaks at the plasma
peripheral region of just inside the separatrix after theL-H transition, in which there is a strong (or complex)
relationship with the plasma turbulence and transport.
Observations from small/medium sized devices
suggest that the causality of the L-H transition could
be related to the bifurcation of the edge radial electricfield as predicted by a theory [2, 3], which waspartially supported by experiments with H-modes
produced by direct biasing of plasma at just inside theseparatrix. In addition, Er-shear stabilization effects
have also been observed in many deices. However theexact causality seems to be still unclear, including theorigin of the various parameter dependences of thethreshold power for the L-H transition in terms of theEr bifurcation at the plasma edge region. Furthermore,
the effect of Er-curvature on the turbulence
suppression, including its dependence on the sign, is
important for considering the nonlinear effects
resulting in a radial squeezing or broadening of theturbulent eddies [4]. It needs a more detailed
experimental and theoretical verification based on ahigh-resolution
measurement
withabetter
signal-to-noise ratio.
On JT-60U, we measured the radial profiles forthe density, temperature, and poloidal/torodal plasma
flows for the fully stripped carbon impurity ions using
CXRS diagnostic with fast time resolution (up to 400Hz) at 59 spatial points (23 toroidal and 36 poloidal
viewing chords). The radial electric field, pressure
gradient and plasma velocity perpendicular to themagnetic field are governed by the following radial
force balance equation:
𝐸! = ∇𝑝! (𝑍! 𝑒𝑛! ) − 𝑉!,! 𝐵! + 𝑉!,! 𝐵!
(1)
With regard to determining the edge radial
electric field, by means of conditional averaging
method as defined by the time of the measurement
relative to the ELM event, an improved
signal-to-noise ratio (S/N) was achieved (in particular,pre- and post-ELM phases).

Experimental results
We performed the following H-mode experiments
having Type-I ELMs (fELM~30-50 Hz), comparing theexternal momentum input directions between co(discharge E049228) and counter- (dischargeE049229) cases as shown in Fig. 1.

Fig. 1 Temporal evolutions of (a) 𝐷! and Wdia, (b) 𝑇! and, (c)𝐸! , comparing between co- (left column) and counter-NBI
discharges (right column) as function of the time relative to theELM-even, 𝛿𝑡!"# . The plasma current, IP, was 1.6 MA, andthe toroidal magnetic field, BT, was 3.9 T. The corresponding
q95, was thus 4.2. The elongation, κ, and triangularity, δ, were1.47 and 0.36, respectively.

As illustrated in Figs 2(a) and (b), the location ofthe Ti,ped at the ELM-onset shifts inwards when Vφ,pedis enhanced in the co-direction. On the other hand, theEr-well bottom value becomes large when Vφ,ped isenhanced in the counter-direction as shown in Fig.
2(c). The Er-shear values for both cases are almost
zero at which the normalized Ti gradient (as expressedby the inverse scale length of the ion temperature,LTi-1) has a local maximum value, R_LTi-1, while
R_LTi-1 can be seen near the location at which Er
and/or its curvature (Er”) have local peak values asshown in figure 1 (c) and (d). It should be noted that
this trend described above is confirmed by a variouscases, including post-ELM phases.

Ti [keV](a) Ion temperature vs. Inverse scale-length 60
40220

1

LTi-1 [1/m]30

Er [V/m]

4 104
0-4 104-8 104Er' [V/m2]

6 106

Er" [V/m3](b) Radial electric field, Er(c) Er-shear

3 106
0-3 106-6 106
1.5 109
1 109
5 108

(d) Er-Curvature
co-NBI (E049228)
ctr-NBI (E049229)

0-5 108
-0.1

-0.05

0R-Rsep [m]where the 𝜒!!" is the ion thermal conductivity and𝜔 !! is the ion transit frequency, 𝜔 !! ≡ 𝐶! 𝑞𝑅, 𝐶! ision sound speed, q is safety factor, and R is major
radius. In deriving the result from Ref. 10, anapproximation that the neoclassical perpendicularviscosity is close to the neoclassical ion thermal
conductivity, and a neoclassical model of 𝐽! 𝐸! isemployed according to Ref. 7. This relation provides
an order of magnitude estimate as
𝐸! 𝐸!" ≈ − 𝑇! /(𝑒𝜌!" ) ! (𝜖𝜌!! )(3)
where the 𝜖 is inverse aspect ratio, and 𝜌! is ion
banana width. If experimental plasma parameters aresubstituted into this relation, the RHS gives an orderof ≈ 10!"~!"   [𝑉 ! 𝑚 ! ]. This value is not far fromexperimental observation as illustrated in Fig. 1(c) and(d). This comparison encourages future experimental
test of the bifurcation model, by comparing the timescale of transition, to the observation in the relevant
tokamak experiments.

Fig. 2 Radial profiles of (a) 𝑇! and 𝐿!!
!" ≡ −∇𝑇! /𝑇! , (b) E! ,(b) E! ! , and (d) E! " , comparing between co- (red line) andcounter-NBI discharges (blue line) as function of the distancefrom separatrix R−RSEP. The data are measured just before a
set of reproducible ELMs (80-90% intervals of the ELM cycle).
Vertical dash dot lines correspond to the locations at which𝐿!!
!" has a local maximum value.

Comparison with theory
An important caveat to this discussion is that theEr-shear (Er’) stabilization effects (∝ Er’Er’) have adouble hump structure, since the sign in the Er’ canchange when Er structure has a local peak value(regardless of Er-well or –hill structures) but we havenever observed a double hump structure in thegradient of the temperature and density (and hence,pressure) profiles in the pedestal region.
Because of the importance of the localized radial
electric field for the transport barrier formation at thepedestal, we have made a comparison between theseexperimental results [5, 6] and theoretical model [7].
In the previous publication [8], it was shown thatthe radial electric field jumps to a larger value during
an ELM-free H-mode phase, while the ion
temperature gradient does not change much
simultaneously. This phenomenon happens in theparameter regime of the absolute value of the radial
electric field, 𝐸! ≈ 𝑇! /(𝑒𝜌!" ) as shown in Fig. 3, at
which Er-bifurcation is associated with n=0 fluctuation[9]. Here, the 𝜌!" is ion poloidal Larmor radius. It
has been pointed out theoretically that the strongly
localized radial electric field structure could be
self-organized owing to the nonlinear dependence ofthe radial current on the radial electric field, 𝐽! 𝐸! ,[7]. According to the model, the radial scale length of
the isolated peak of radial electric field, ℓ𝓁, is given(for the plateau regime) asℓ𝓁! ≈ 𝜒!!" 𝜔 !!
(2)

Fig. 3 Time history for (a) normalized Er and Dα intensity, (b)amplitude for n = 0 component in the magnetic fluctuation, and(c) frequency and time resolved spectrogram for n = 0component in the magnetic fluctuation detected by the sum ofeight channel saddle loop arrays (toroidal).

References
[1]
[2]
[3]
[4]

K. H. Burrell, Phys. Plasmas 4, 1499 (1997).
H. Biglari, et al., Phys. Fluids B2, 1 (1990).
S.-I. Itoh, et al., Phys. Rev. Lett. 72, 1200 (1994).
P. Diamond, et al., Plasma Phys. Control. Fusion 47,R35 (2005).
[5] T. Tokuzawa, et al., Phys. Plasmas 21, 055904 (2014).
[6] K. Kamiya, et al, Phys. Plasmas 21, 122517 (2014).
[7] K. Itoh, et al., Plasma Phys. Control. Fusion 57,075008 (2015).
[8] K. Kamiya, et al., Phys. Rev. Lett. 105, 045004(2010).
[9] K. Kamiya, et al., Contrib. Plasma Phys. 54, 591(2014).
[10] K. Itoh, et al., Phys. Plasmas 5, 4121 (1998).