The Compass-D Project

The Compass device at Culham is not currently operating. The last experimental campaign ended before the commissioning of MAST. The following information was prepared when Compass-D was still fully operational.

Role

Collaborative exchanges and attachments are an important feature of the work on Compass-D, as in all the UK Fusion Programme, serving to broaden the range of skills and experience in our research activity.

The programme of experimental work on Compass-D is divided into related topic areas:

• Control of instabilities: this is vital to tokamak development, as instabilities reduce the confinement properties of the plasma and/or create disruptive plasma behaviour.
• Impurity control: low impurity levels and control of plasma exhaust are critical prerequisites for efficient tokamak operation.
• "H-mode" (i.e. High energy confinement mode) operation: the characterisation and control of Edge Localised Modes (ELMs), which are a feature of H-mode operation, are important for power exhaust and impurity control optimisation.

Facilities

Compass-D is a highly flexible, medium-sized tokamak that has been specifically designed to address the key physics issues associated with magnetic confinement fusion.

The extensive facilities on Compass-D include:

• Magnetic perturbation coils for correcting internal instabilities and for modifying the plasma edge region.
• 2MW 60GHz microwave system for precise, localised heating and current drive, for bulk heating and instability control.
• 1.3GHz Lower Hybrid system for bulk current drive and current profile control.
• Flexible plasma shaping and control systems and special diagnostics.
• Real-time electronic neural network for feedback control of the plasma equilibrium position and shape.
• The same magnetic geometry as JET (Joint European Torus) and the planned Next Step device, ITER (International Tokamak Experimental Reactor).
• Compact and flexible design, readily modified.

Technical Specification

Vacuum Vessel

Major radius (R)                                0.557 m
Minor semi-axis, horizontal (a)                 0.232 m
Minor semi-axis, vertical                       0.385 m
Aspect ratio, R/a                               2.53
Material                                        Inconel 625
Baking temperature                              150 deg C

Magnetic Perturbation Coils

Helical mode numbers (m,n)                      4,1 3,1 2,1 1,1
                                                4,2 3,2 2,2 1,2

Toroidal Field

Maximum toroidal field on axis                  2.1 T
Rise time                                       approx 1 s
Flat top duration at max field                  1 s
Decay time                                      approx 1 s
Number of field coils                           16
Field ripple on axis                            < 0.006%

Magnetising Field System

Plasma current                                  < 400 kA
Plasma duration (without current drive)         approx 600 ms
                (with RF current drive)         approx 2 s
Plasma rise time                                < 80 ms
Flux swing:     rise                            0.52 Vsec
                flat top                        0.30 Vsec

Vertical Field System

Maximum field on axis                           200 mT

Microwave Heating Systems

Electron Cyclotron Resonance                    2 MW at 60 GHz
Lower Hybrid Wave                               600 kW at 1.3 GHz

Toroidal Field Power Supply

24-phase thyristor convertor                    40 MVA

Main Poloidal Field Power Supplies

Capacitor banks (various functions)             3.6 MJ total
12-phase thyristor convertors:
                Magnetising field               9.7 MVA
                Main vertical field             4.2 MVA
                Shaping field                   5.4 MVA

Auxiliary Supplies for Fast Positional Feedback Coils and Resonant Magnetic Perturbation Coils

Transistor amplifiers                           4 x 275 kVA
6-phase thyristor convertor                     0.8 MVA
Fast 3-rail chopper                             16 MVA

Fast Positional Feedback

Maximum radial and vertical field on axis       < 10 mT
Feedback response time                          < 100 microsec