# Presentation: 535288

## Compact  Stellarator Approach  to  DEMO

**J.F. Lyon for the US stellarator community**
**FESAC Subcommittee     Aug. 7, 2007**

**Notes:**
The design is being done as a collaboration

## Compact stellarators address DEMO issues 

- Compact stellarators  confinement physics as in tokamaks
- Crucial advantages for steady-state reactors
- quiescent, steady-state, high-, disruption-free
- no power input to sustain current or rotation  true ignition
- no profile control or close fitting walls
- high density limited only by power density
-      * reduced  slowing-down time  reduced  instability drive
-      * less energetic particle fluxes to wall
- 3-D shaping of plasma edge
- optimal control of distribution of particle fluxes, radiation losses
- Features shown in high-R/a, non-symmetric stellarators
- Develops important tools for 3-D control of tokamaks
- ELMs, RWMs, disruptions, plasma-wall interactions

## Quasi-symmetry  key to compact stellarator

- Quasi-symmetry **** minimize variation of |***B***| in symmetry direction in straight field line coordinates
- toroidal, poloidal or helical quasi-symmetry
- conserved canonical momentum as in axisymmetric system
**   ****** good orbit confinement
- reduced effective field ripple along ***B***
**	****** reduced neoclassical transport (depends only on |***B***|)
- 	**** allows strong rotational transform at lower R/a
- reduced viscous damping in the symmetry direction
**	****** promotes large ***E*** x ***B*** flow shear  reduced anom. xport
- Exploits physics commonality with tokamaks
- Reduced viscosity, neoclass. xport demonstrated in HSX

## Quasi-symmetry  confinement improvement

- Quasi-symmetry: small |***B***| variation in a symmetry direction

-  lower viscosity  lower flow damping in sym. direction
-  large flow shear and breakup of turbulent eddies

## Stellarator confinement similar to tokamak

- Comparable plasma for same volume, field & power

- Very low effective ripple
-     (eff) in compact stellarator  enhance confinement ?

## ARIES-CS reactors  competitive with tokamak

- Costing approach, algorithms as in other ARIES; updated mat’l costs
- CoE similar to that for ARIES-AT & ARIES-RS
- Main issues: coil complexity & optimizing divertor geometry
- High density operation reduces  losses, reduces divertor load

## Issues to be addressed before CS DEMO

**Physics issues include**
- size scaling at a/i relevant to DEMO
- 	 adequate thermal confinement and  confinement
- workable steady-state divertor
- simpler coil design, cheaper construction
**How can issues be addressed?**
- build on results from ITER, other tokamaks: overseas stellarators; and materials & component development programs
- results of US compact stellarator program: NCSX, QPS, HSX, CTH
- results from large, next-generation compact stellarator
-  extend parameters to fill gaps
-  D-T operation needed . . . or simulate ’s with tail ion heating ?
-  superconducting vs extrapolation from LHD and W 7-X?
-  experience constructing superconducting stellarators & ITER
-    sufficient to develop reliable cost estimates for a CS DEMO?

## Start now on definition of Next Generation Compact Stellarator (NGCS)

- US compact stellarators + foreign stellarators (LHD, W 7-X)
- 	 development of NGCS to overlap with ITER
- integrate burning plasma experience into a compact stellarator configuration better suited for a DEMO
- Study can begin now to
- assess options for NGCS based on
-  NCSX and QPS physics principles
-  3-D plasma theory & simulation
- explore tradeoffs, sensitivity; costing models
- optimized configurations with simpler coils, enhanced flows, improved confinement, robust flux surfaces & high ** limits
- integrate full 3-D plasma, RF heating, divertor & boundary
-     physics in assessing NGCS performance

## Conclusions

- Compact stellarators offer:
- tokamak-like confinement physics
- ignited, sustained plasma operation without external power input
- passive safety to MHD instabilities, including disruptions
- reduced energy and controlled distribution of particle flux to walls
- reactor size & CoE comparable to advanced tokamak
- 3-D coils vs 50-200 MW of CW current drive + feedback systems
- DEMO concept evolution  clear opportunity for US leadership
- compact stellarator = US innovation
- aimed at convergence with tokamak/ITER program
- well differentiated from overseas stellarator & tokamak programs
- coordinated multi-inst. program: exp’ts, theory/comp., engineering
- explore elements of improved toroidal reactor
- define performance extension device to overlap with ITER

## |B| variation for various stellarators

- Rewoldt, Ku, & Tang, PPPL-4082  (2005)