Magnetic systems

Toroidal field winding
During normal operation the TF Winding current must produce a maximum toroidal bias field Bo = 0.7 T (at the beginning of the plasma pulse) and a reversed field at the wall Bw = 0.44 T (during the plasma current flat-top).


Field shaping winding
The FS Winding (often referred to as Primary Winding) controls the plasma equilibrium, together with the Stabilizing Shell. More precisely it is required to provide three functions: (i) in the early phases of the plasma pulse it controls the field on the shell outer surface in order to avoid error fields at the shell gaps and holes; (ii) on the longer times, when the shell is no more effective, it produces an accurately shaped equilibrium field; (iii) in both previous conditions it presents a magnetomotive force equal to the plasma current (up to 2 MAturns) in order to reduce the flux swing requirements.


Main parameters and technical data of the toroidal field winding
Number of coils
Number of turns per coil
Coil mean diameter
1.24 m
Peak toroidal field
0.7 T
Peak current
18.3 kA
Total magnetomotive force
7 MA
Thermal parameter i2t per pulse
300 MA2s
Mean thermal power
30 kW
Voltage to earth
3.5 kV
Main parameters and technical data of the field shaping winding
Number of coils
Number of turns per coil
Coil mean diameter
Up to 5.5 m
Total magnetomotive force
2 MA turns
Average current per turn
5.2 kA
Voltage per turn
730 V
Voltage between terminals
17.5 kV
Voltage to earth
17.5 kV
Tensile stress in the insulation
30 MPa
Shear stress in the insulation
5 MPa


















Ohmic heating winding

RFX requires a poloidal flux swing of 15 Wb and a peak loop voltage of 730 V, while the plasma current rise occurs in some tens of milliseconds. Such requirements demand a large flux consumption during setting up making the use of a bipolar flux swing operation almost useless. Thus 15 Wb also represents the flux to be stored before start-up to allow an unipolar flux swing.

However, would the experimental evidence require long lasting plasma currents, it will be easy to increase the flux swing by implementing a bipolar operation with a slow flux inversion. As a further requirement the stray field intensity in the plasma volume must never exceed 5 mT.

No iron core has been adopted for the poloidal flux because, even if it would have reduced dramatically the coil dimensions, power demand and costs, nevertheless it would have caused field asymmetries, plasma instability, non-linear behaviour and reduced accessibility.

The chosen air core transformer requires a magnetomotive force of about 10 MAt to create the 15 Wb flux, with a top field of about 4.5 T in the central bore. It also requires the ampereturns to be suitably located to limit the stray field in the plasma region.


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