Mast

Figure 1:Aerial view of Culham (UK)

The History of MAST
MAST Parameters
MAST Milestones
MAST Objectives and Achievements

The history of MAST

In 1996 the EURATOM/UKAEA Fusion Association began construction of a new tokamak at Culham, Oxfordshire (UK). The motivation for building MAST was the outstanding success of its forerunner, START, the world's first "spherical" (low aspect ratio) tokamak (ST) designed to study hot ST plasmas. MAST (Mega Amp Spherical Tokamak) has superseded START (Small Tight Aspect Ratio Tokamak), which completed its experimental programme in March 1998. In a "spherical tokamak" such as MAST the plasma shape, though still toroidal, is much more compact, something like a cored apple. It is almost spherical, hence the name. The MAST experimental programme began in December 1999. 1 MA plasma current was first achieved in MAST in May 2000 and H-mode (higher confinement) operation was first obtained in June 2000 with neutral beam heating.
In addition to the traditional ("outboard") gas puff using piezo valves from the outer walls of the vessel, MAST employs an "inboard" gas puff via a tube located in the centre column armour. This has been found to be very effective in promoting transition to higher (H-mode) confinement and in December 2001 a transition to H-mode occurred in purely Ohmic discharges for the first time in a Spherical Tokamak. This transition has occurred after using inboard gas puffing into a symmetrical Double-Null Diverted (DND) plasma.
First studies of Pellet Injection fuelling were also made in December 2001 using frozen deuterium pellets from an 8-barrel injector donated by the Association EURATOM-FOM and updated by the Association EURATOM-Risø and Culham. A pellet diagnostic programme is currently in development partly in collaboration with the Association EURATOM-ENEA Padua.
From July 2003 to April 2004, major improvements to the MAST central solenoid, divertor (for plasma exhaust) and neutral beam systems were implemented.

Figure 2:MAST seen from above

Figure 3:MAST Inside

MAST Parameters (maximum)

Plasma Major Radius (R)	0.85 m
Plasma Minor Radius (a)	0.65 m
Toroidal Magnetic Field (BT) at R=0.7m)	0.6 T
Vessel volume	50 m3
Plasma Volume	7 m3
Plasma Current (I)	2 MA
Pulse length	5 s
Additional heating power (PAux)	5 MW NBI 1.5 MW ECRH

Figure 4:MAST Machine

MAST Milestones

1991	The START experiment at the Culham Science Center began operation.
Early 1998	New world record in ratio of plasma pressure to magnetic field pressure (40%).
March 1998	START experiment ceased.
Nov. 1998	MAST was fully assembled.
Dec. 1998	First toroidal plasma on MAST.
May 2000	1MA plasma current achieved.
June 2000	First H-mode plasma obtained.
Dec. 2001	H-mode in ohmic plasma achieved.
2002	Effectiveness of Divertor biasing demonstrated.
July 2002	New high time repetition Thomson Scattering diagnostic installed.
Aug. 2002	Electron Bernstein Wave antenna installed.
Feb 2004	Completed installation of new central solenoid and divertor.

Figure 5:MAST Plasma

Physics advances

Key advances in the understanding of spherical tokamak plasmas have been made in the areas of:

• Confinement improvement in H-mode regime
• First observation of neo-classical tearing modes in Spherical Tokamaks
• Understanding the structure of Edge Localised Modes (ELMs)
• Internal transport barrier formation

MAST Objectives and Achievements

Figure 6:Mast from outside

Machines like MAST and START differ from ordinary tokamaks in the shape in which the plasma (a hot ionised gas) is held by magnetic fields. In a conventional tokamak the plasma is held in a toroidal configuration, rather like a car tyre or doughnut. In MAST, the plasma is held in a tighter configuration - more of spherical shape. MAST is already and will continue to add to existing data on conventional tokamaks, so extending the database of results, and providing an increased understanding of the behavior of toroidal plasmas (e.g. effects of plasma shaping and aspect ratio). MAST should also investigate the potential of the spherical tokamak route to fusion power. The ST (Spherical Tokamak) power plant conceptual design has shown that a viable electricity-producing power station could be possible based on a spherical tokamak. A ST could also make a good basis for a volume neutron source, providing a flux of neutrons for testing materials and components being developed for fusion power plants of the future. An outline design for a Component Test Facility has been studied in Culham since 1994.
Much of this research will be of relevance to the Next Step fusion device, ITER, and so increase Europe's contribution to the international partnership.