Current Hole and VECTOR Approach to High Beta Steady State Operation

Presented by T.Ozeki, JAERIThank to K.Tobita, S.Nishio,

Y.Nakamura, N.Hayashi, T.Fujita

IEA Large Tokamak W55/DOE-JAERI Technical Planning of Tokamak Experiments Joint Workshop

JAERI Naka, November 24, 2003

Introduction• Current hole plasmas were observed in JT-

60U, JET, AUG and so on. Recent investigations show that the evolution of the spatially localized current is the key of the formation of current hole.

• Therefore, the current hole plasma could be significant issues for the large tokamak, ITER and the fusion tokamak reactor, since the width of non-inductive current is relatively smaller than the minor radius.

• Here, we consider current hole in VECTOR which is the low aspect ratio tokamak using the superconductor, proposed in JAERI.

Concepts of VECTOR• Low aspect ratio tokamak with superconducting TF coil - Superconductor reduces circulation power (for remove of jo

ule loss of the center post in ST ) - Low-A reduces the magnetic energy of TF coils, reducing we

ight of coil support (for low cost)

18.2m

VECTOR

Aspect ratio : A=2-2.3Plasma Major Radius : RP = 3.2-3.8 mPlasma Minor Radius : aP = 1.4-1.9 mPlasma Ellipticity : = 2.3Plasma Current : IP = 14-18 MANormalized Beta : N = ~5Fusion Power : PF = 2-2.5 GWMaximum Field : BMAX = 19 TField on axis : B0 = 3.1-5 T

0 300000

ARIES-ST

ARIES-RSA-SSTR2

SSTRARIES-I

DREAM

CREST

ITER

VECTOR

JT-60

2000010000

100

200

300

Machine Weight (ton)Po

wer

Den

s. /

Wei

gh

t (k

WF/t

on

)

Low cost, low waste

Eco

nom

ical

Physics Issues of Current Hole for the Reactor,

VECTOR

• Formation of Current Hole• MHD Equilibrium and Stability• Thermal Transport• Particle Loss• Current Drive

Formation of Current Hole• The current hole was obtained as the extreme case of

reversed shear discharges in JT-60U. [2000]

Poloidal magnetic field Bp was observed to be close to zero by MSE measurements.

• The current hole was produced with the local bootstrap current in the internal transport barrier in JT-60U.

The negative E was induced by the local bootstrap current and E propagates radially.

Possibility of current hole formation in power reactors

• Overdrive by bootstrap and non-inductive current, which is important to raise Ip

• Current hole is likely to be produced in the core of VECTOR

MHD Equilibrium and Stability

• Physics issues of the equilibrium and stability– Equilibrium with the infinite q-value, where the zero

poloidal field exists in the hole region.– Equilibrium with multi-axes: possibility with multi n

=0 islands structure [ATMI model, Takizuka].– Instability of n=0 mode was estimated by the linea

r/non-linear MHD simulation[Huysmans, Stratton, Jardin].

• However, no significant MHD instability in JT-60U and the hole was sustained for a few second. Here, we consider the standard MHD equilibrium and investigate the beta limit.

Equilibrium of high q0/qmin plasmasGrad-Shafranov equation was solved assuming• dp/dp=0, in the hole (r<0.4) and• Extremely small but positive j//A-SSTR2 (p=0.9)

q0~70, qmin~2

Current hole

P’=0

Beta limit of high q0/qmin plasmasLow n ideal MHD stability : ERATO-J• Stability boundaries are improved by

tailoring the pressure profile and improved more by the wall.

q0/qmin~12

Beta limits improved by aspect ratio, profile control and wall

- It can be expected thatN increases ~ 30-40% with the decrease of aspect ratio from 3.3 to 2.2

(A of VECTOR is ~2.2)

ISSUES for high beta:- Profile control and- Stabilization of conducting wall : Control of RWM

Improvement by the aspect ratio for the negative shear plasma ( not optimized profiles )

Aspect ratio (R/a)

Thermal Transport

• Current hole plasma in JT-60U has high confinement performance due to the strong internal transport barrier (ITB).

• To clarify mechanism of formation and to sustain the current hole are important issues.

• Here, these issues are investigated by 1.5D transport code TOPICS, using the model of current limit inside the current hole based on the ATMI equilibrium model.

Experimental observation in JT-60U is well explained by transport

simulationTransport model- Anomalous in positive shear- Neo-classical inside c

Evolution of bootstrap

(a)

(b)

€

∂ E

∂ t=

η

μ0

∇2

E − η∂ j

BS

∂ t

(1)(2)

neo+ano f()

c is inside the q-min

Formation of negative E

Good agreement with experiments

Evolution of BS is faster than current diffusion, negative E is formed as a reaction.

Sustainment of current holeAfter the formation of current hole, CH was sustained for some seconds. But it gradually shrinks according to the shrinkage of radii at the current peak and the ITB, because of no profile control.

The current profile control is important.The sustainment is the common issue in the steady state advanced tokamak plasma (large BS and high plasma).

Current hole shrinks due to the penetration of inductive current in JT-60U.

Particle Loss

• Current hole anticipated to cause more significant alpha-loss due to ripple transport.

• Analyzed by Orbit-Following Monte Carlo – Following the guiding center orbit– Employs Monte Carlo to simulate

Coulomb collisions

Typical alpha particle orbits in current hole plasma

Simply experience vertical drift in current hole

-particle loss due to the ripple on reversed shear plasmas

r/a

q

a

b

c

de

f

a b c d e f

Rip

ple

Lo

ss

(Po

we

r%)

0.2% 0.6% 0.7%2.0%

12.6%

16.7%

allowable

For hole ≤ ~0.3 ( of qmin ≤ ~0.8), -particle loss can be less than 2%, acceptable in the light of heat load on the wall.

Ripple at surface: 0.5%

Characteristics of low aspect tokamak, VECTOR

xlarge contr.

small contr.

In low-A, B produced by the assembly of coil current of TFC inner legs

In low-A, TF ripple damps sharply along R

-particle loss in low AIn low-A, TF ripple damps sharply along R

for broad Sprofile

Ripple at surface (%)A

lph

a lo

ss (

Po

wer

%)

• Comparing at realistic ripple (≥0.5%), low-A has a significant advantage over -particle confinement• For conventional A, low TF ripple (~0.2%) required to confine

-particles• Better -particle confinement can be expected in VECTOR.

Current Drive• Non-inductive current drive is

important issue for the high beta, steady-state plasma, especially, in the light of plasma current profile control.

• NBCD is employed as the current drive device in VECTOR. Beam driven current is sensitive to particle orbits.

• In JT-60U, no observable current is driven in current hole.

• No need to drive the seed (central) current is advantage over NBCD in CH, in that ‘lower energy’ NBI is likely to be usable.

EDDC experiment on current hole in JT-60U.

Jbd(r) driven in the outer of CH

In CH plasma, jbd is driven in the outer region, because of particle orbits.

r/a

J bd (M

A/m

2 )

positive shear

r/a

J bd (M

A/m

2 )

current hole

Evaluation of beam-driven current by Orbit-following Monte Carlo

nbe<v//B> for passing particles Include return current Jbs

Uniform ne profile assumed

Possibility of the profile control by jbd

(r)Jbd driven outer than Jeq move qmin outward

2) Control for sustainment of the current hole

1) Improvement of stability

Beam driven current

Summary• Physics issues on current hole plasmas are inve

stigated for the low aspect tokamak with the superconductor, VECTOR.

• Current hole plasma potentially has – high beta by profile control of p’ and j and RWM– high confinement by transport of the neo-classical lev

el and strong ITB – significant -particles loss for a wide hole radius, whi

ch can be reduced in low aspect ratio.– possibility of profile control for the improvement of st

ability and the sustainment of the hole.

Announcement• IEA Large Tokamak W56 "Physics of Current Hole”,

which will be planed in Naka-JAERI, February 3-4, 2004,with US/Japan MHD workshop and ITPA meeting of MHD, Disruption and Control.

• Key-person: T.Taylor(GA), F. Crisanti(Frascati), T.Ozeki(JAERI)

Feb. 2MON

Feb. 3TUE

Feb. 4WED

Feb. 5THU

Feb. 6FRI

US/Japan MHD

LT W56

Current Hole

ITPA M.D.C