WENDELSTEIN 7-AS

Figure 1:Max-Planck Institut in Garching (Germany)

The history of WENDELSTEIN 7-AS
WENDELSTEIN 7-AS Parameters
WENDELSTEIN 7-AS Milestones
WENDELSTEIN 7-AS Objectives

The history of WENDELSTEIN 7-AS

As an alternative to the tokamak, IPP also conducts fusion experiments with the WENDELSTEIN stellerator devices. As opposed to tokamaks, stellarators confine the plasma without enlisting the magnetic field generated by the plasma current, i.e. they use a field generated solely by external coils. This makes stellarators basically suitable for steady-state operation. Tokamaks, on the other hand, can only work in pulsed mode unless provided with auxiliary systems.
The WENDELSTEIN 7-AS stellarator experiment, which went into operation at Garching in 1988, belongs to the further developed generation of "Advanced Stellarators": WENDELSTEIN 7-AS is distinguished from conventional stellarators by its re-computed, physically improved magnetic field generated by innovative, 3D-shaped coils. The experiment was closed of 31st July 2002 in order to concentrate budget and manpower on the forthcoming Wendelstein 7-X in Greifswald (Germany).

Figure 2:Wendelstein 7 AS

WENDELSTEIN 7-AS Parameters

Radius of the device (overall)	3.6 m
Height (overall)	4 m
Weight	250 t
Plasma major radius	2 m
Plasma minor radius	0.18 m
Plasma volume	1 m3
Number of coils	45
Magnetic field	Up to 2.5 Tesla
Pulse length	Up to 3 s
Electron Cyclotron heating	2.1 MW
Ion Cyclotron heating	0.5 MW
Neutral Beam Injection	2.6 MW
Plasma temperature	16 - 70 Million degrees
Plasma density	Up to 4 x 1020 particles/m3

Figure 3:Wendelstein 7-AS Outside

Figure 4:Wendelstein 7-AS Plasma

WENDELSTEIN 7-AS Milestones

1988	Wendelstein 7-AS stellarator experiment went into operation, mainly with four 70 GHz gyrotrons for plasma heating.
1990	Wall conditioning by boronization: first successful neutral beam heated discharges.
1992	First 140 GHz gyrotron for plasma heating at higher electron densities, discovery of H-mode discharges on stellarators.
1995	Upgrade of the neutral beam heating system from 4 to 8 sources average beta values up to 2% achieved.
1998	Installation of 10 control coils to vary the size of boundary magnetic islands (preparation of island divertor experiments).
1999 / 2000	Installation of 10 divertor modules.
2001	First successful island divertor experiments: detached plasmas with densities up to 4 x 1020 particles/m3, discovery of the HDH-mode, average beta values raised to 3% without signs of instabilities.
2002	The experiment was closed on July 31.

Figure 5:Plasma (red) in the magnetic
coil (blue)

WENDELSTEIN 7-AS Objectives

The experiment has demonstrated that the innovative modular coil system can be manufactured to the exact specifications required. With plasma temperatures of 70 million degrees for the electrons and 16 million degrees for the ions, energy confinement times of up to 60 milliseconds and reactor-grade plasma densities, WENDELSTEIN 7-AS has broken all stellarator records in its size group.
The optimization criteria used have also been confirmed. The troublesome displacement of the plasma column in the vessel as the plasma pressure increases - as compared with a conventional stellarator - is appreciably reduced in WENDELSTEIN 7-AS. Its successor now being built at the Greifswald Branch Institute of IPP, the completely optimised WENDELSTEIN 7-X device, is intended to demonstrate the reactor relevance of the new stellarators.
WENDELSTEIN 7-AS is the first stellarator to be equipped with a so-called island divertor. Whereas previous devices restricted the outward extent of the plasma column by means of material limiters, WENDELSTEIN 7-AS has done this in a contact-free way since 2000 by means of magnetic island field lines. Here, the plasma boundary splits - in keeping with the symmetry of the magnetic field - into individual offshoots through which energy and particles move to limited areas of the vessel wall, just like the divertor plasma in a tokamak. These areas of the wall are protected by ten special collector plates positioned along the plasma column. Here the incident particles - together with the undesirable impurities from the plasma - can be neutralised and pumped away. This greatly improves impurity and density control and the plasma power can be more uniformly distributed on the vessel walls by radiation. During the divertor experiments the "high density H-mode" (HDH-mode) was discovered. It combines good energy confinement with very low impurity confinement thus allowing stationary discharges with high heating power at very high densities.
The device thus has provided advanced information for WENDELSTEIN 7-X, which will be fitted with superconducting coils and a similar divertor.