Hahm transport by L5RNy4

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									Outstanding Issues in Transport Physics for DEMO

T.S. Hahm

Princeton University, Plasma Physics Laboratory

In this short essay, a few outstanding transport issues for DEMO are discussed, and
possible contributions from present day and near term future tokamak, ST, and stellarator
experiments are listed. Topics are chosen based on the author’s experience and interest
which are limited in scope. While operational scenarios for DEMO can be explored
further, I discuss only two options in a broad sense. The first one is non-inductive steady
state operation with reversed shear, and the second one is hybrid plasmas with an H-
mode edge. Since DEMO will need higher Pfusion, higher Q, and higher operational
reliability compared to ITER, simultaneous achievement and sustainment of improved
confinement and macroscopic stability in a reactor-like condition are required.

One approach for steady-state operation of a reactor is the “advanced tokamak scenario”
with reversed magnetic shear (RS) and an internal transport barrier (ITB), simultaneously
producing both improved confinement over standard H-mode regimes and allowing
access to high values of N via active control of resistive wall modes (RWM). A
significant challenge for the applicability to DEMO is that the best performance was
obtained in a regime which is not reactor compatible. The relevant regime is
characterized by Te = Ti with low external torque and particle input at high density with
self-heating from alpha particles. From present day tokamak experiments, ion thermal
ITBs are mostly formed via NBI heating, while electron ITBs are formed via localized
electron heating on RS plasmas in most cases. Significant progress has been made in
forming ITBs for both the ion and electron channels for Ti= Te. However, most results to
date indicate that either confinement enhancement was modest with an ITB at small
radius [Guenter, 2000] or ITB plasmas with box-type temperature profiles [Ide, 2004]
were formed. These box-type ITB plasmas usually occur with strongly RS, and a sharp
localized pressure gradient at the ITB location is unfavorable to MHD stability. The
reduction in electron thermal diffusivity is radially narrow and localized at the ITB
location. Another concern regarding strongly RS plasmas is that energetic particle driven
mode (EPM) simulations based on the gyrokinetic energetic particle-bulk MHD hybrid
model [Park 1992] has exhibited an avalanche of alpha particle radial transport in the RS
region [Zonca 2006a]. Obviously, for successful steady state (SS) operation of RS
plasmas with ITBs in DEMO, one should find a way to form an ITB which is sufficiently
large in both location and width with a broader radial region of transport reduction and
favorable MHD stability properties. The following experiment can provide crucial
knowledge which will help us in planning desirable operational scenarios for DEMO.

NSTX is an ideal device to explore electron thermal ITBs which are DEMO compatible.
It’s not only equipped with reliable diagnostics for q, radial electric field, rotation (MSE,
CHERS) and for microturbulence (high-k tangential scattering, reflectometry), high
harmonic fast wave (HHFW) heating can produce an isotropic distribution of high energy
electrons which can be used to study the behavior of alpha particles expected in DEMO
relevant conditions. Note that both alpha particles in DEMO and energetic electrons
produce by HHFWs in NSTX are characterized by isotropic distribution functions and
very small * values. In other words, nonlocality arising from a high value of * for
energetic ions in present day machines will be much reduced in DEMO due to its large
size. A similar argument for energetic electron transport studies has been made
previously in relation to the electron fishbone instability in FTU [Zonca 2006b].

Another operational scenario for DEMO is the “hybrid discharge” which is characterized
by an H-mode edge and weak magnetic shear at the core with q(0) ~1 which is sustained
via MHD activity localized at the core, or non-inductive current. While this regime has
achieved higher performance in terms of H N/q952, and has a potential for achieving
higher N without active RWM control and high confinement at densities approaching
nGW [Campbell 2006], H-mode access should be guaranteed since core confinement is
sensitive to the H-mode pedestal temperature. Given the lack of theoretical understanding
of the H-mode power threshold and of pedestal width [Diamond 2007], the margin for H-
mode operation in DEMO seems tight, and more research is urgently needed to refine
scenarios for H-mode transition and sustainment.

For instance, the possibility of an entry to the H-mode state at low density with a lower
power threshold, followed by sustainment at higher reactor relevant density by taking
advantage of hysteresis, should be further tested in present day devices. Flux-gradient
characterization from C-Mod data analysis [Hubbard 2002] narrowed the gap between
operational knowledge and experience and the theoretical paradigm in the context of
transport bifurcation as illustrated by a simple S curve. This line of research should be
extended to exhibit explicitly the degree of hysteresis in the H to L back transition.
Other factors which can affect the H-mode transition, but are not explicitly used in
empirical scalings, include the isotopic dependence of the power threshold. It improved
from H to D in many tokamaks including ASDEX and DIII-D [Wagner 2006], but stayed
basically unchanged from D-D to D-T in TFTR, although the edge localized mode (ELM)
behavior changed [Bush 1995]. In stellarators, isotopic dependence of the H-mode power
threshold was not observed [Wagner 2006]. This can be an outstanding DEMO relevant
topic for research in quasi-axisymmetric stellarators such as NCSX.

Study of spontaneous rotation with low external torque and its extrapolation to DEMO is
also a crucial issue which deserves a long dedicated article regarding RWM control and
transport reduction [Diamond 2005], but will not be discussed in this short essay.
Obviously, some means of external flow drive such as Ion Bernstein Wave (IBW)
[Craddock 1991] should be in preparation for DEMO in case the spontaneous rotation
does not scale favorably to reactor-like conditions. IBW was already in the operational
plan of the EAST tokamak and was strongly recommended to the KSTAR team by the
International Advisory Committee in the fall of 2006. Strong collaboration with and
technical support for EAST and KSTAR on this topic is well justified.
The author acknowledges useful discussions with P.H. Diamond on related subjects. This

work was supported by the U.S. Department of Energy.



References:
[Bush 1995] C. E. Bush et al., 1995 Phys. Plasmas 2, 2366

[Campbell 2006] D.J. Campbell et al., 2006 IAEA-FEC FT/1-1 (Chengdu, China)

[Craddock 1991] G.G. Cradock and P.H. Diamond , 1991 Phys. Rev. Lett. 67, 1535


[Diamond 2005] P.H. Diamond, K. Itoh, S.-I. Itoh, and T.S. Hahm, 2005,Plasma Phys.
Control. Fusion 47, R35-R161

[Diamond 2007] P.H. Diamond and T.S. Hahm, 2007 Plasma Science and Technology 9,
320-325

[Guenter 2000] S. Guenter et al., 2000 Phys. Rev. Lett. 84, 3097

[Hubbard 2002] A. Hubbard et al., 2002 Plasma Phys. Control. Fusion 5, A359

[Ide 2004] S. Ide et al., 2004 Nucl. Fusion 44, 87

[Park 1992] W. Park et al., 1992 Phys. Fluids B 4, 2033

[Wagner 2006] F. Wagner et al., 2006 Plasma Phys. Control. Fusion 48, A217

[Zonca 2006a] F. Zonca et al., 2006 Plasma Phys. Control. Fusion 48, B15

[Zonca 2006b] F. Zonca et al., 2006 IAEA-FEC TH/3-2 (Chengdu, China)

								
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