Discussion on application of current hole towards reactor T.Ozeki (JAERI) Current hole plasmas were observed in the large tokamaks of JT-60U and JET. This means that larger toroidal plasmas may cause the current hole naturally. We should consider to use the current hole for reactor positively.

Physics Issues of Current Hole for the Reactor Formation and Sustainment MHD Equilibrium and Beta limits Thermal Transport / Impurity transport Particle Loss / TAE Current Drive

Possibility of current hole formation in power reactors The current hole was produced with the local bootstrap current in the internal transport barrier in JT-60U. Overdrive by bootstrap and non-inductive current, which is important to raise Ip. Current hole is likely to be produced in the core of reactor.

Sustainment of current hole After 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 issues of autonomous property of the advanced tokamak plasma (large BS and high  plasma). Current hole shrinks due to the penetration of inductive current in JT- 60U. [Simulation by N.Hayashi ]

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 linear/non-linear MHD simulation[Huysmans, Stratton, Jardin]. From point of view a reactor, what is a disruptive or beta limiting MHD stability.

Equilibrium of high q 0 /q min plasmas Grad-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) q 0 ~70, q min ~2 Current hole P’=0

Beta limit of high q 0 /q min plasmas Low n ideal MHD stability : ERATO-J Stability boundaries are improved by tailoring the pressure profile and improved more by the wall. q 0 /q min ~12

Beta limits improved by aspect ratio, profile control and wall -It can be expected that  N 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). Analysis of 1.5D transport code TOPICS shows that the transport is the neo-classical level. –It is like a neo classical tokamak However, impurity accumulation and helium exhaust are issues.

Experimental observation in JT-60U is well explained by transport simulation Transport model - Anomalous in positive shear - Neo-classical inside  c Evolution of bootstrap (a) (b)  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.

Particle Loss Current hole anticipated to cause more significant alpha-loss due to ripple transport. Alpha transport due to TAE modes –Strongly reversed shear TAE –RSAE, etc

Typical alpha particle orbits in current hole plasma Analyzed by Orbit-Following Monte Carlo Simply experience vertical drift in current hole

 -particle loss due to the ripple on reversed shear plasmas r/a q a b c d e f a b c def Ripple Loss(Power%) 0.2% 0.6% 0.7% 2.0% 12.6% 16.7% allowable For  hole ≤ ~0.3 (  of q min ≤ ~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 x large 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 A In low-A, TF ripple damps sharply along R for broad S  profile Ripple at surface (%) Alpha loss (Power%) 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 In the reactor, non- inductive current drive is important issue for the current profile control. 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.

J bd (r) driven in the outer of CH In CH plasma, j bd is driven in the outer region, because of particle orbits. r/a J bd (MA/m 2 ) positive shear r/a J bd (MA/m 2 ) current hole Evaluation of beam-driven current by Orbit-following Monte Carlo n b e for passing particles  Include return current  J bd Beam driven current is sensitive to particle orbits.

Possibility of the profile control by j bd (r) J bd driven outer than Jeq move q min outward 2) Control for sustainment of the current hole 1) Improvement of stability Beam driven current

Summary Physics issues on current hole plasmas should be investigated continuously to establish the current hole reactor. Thank you for the discussion.

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. 2 MON Feb. 3 TUE Feb. 4 WED Feb. 5 THU Feb. 6 FRI US/Japan MHD LT W56 Current Hole ITPA M.D.C