Magnetic field mapping system on the W-l heliaca)

The H-l heliac recently brought into operation is a medium-sized 3-field-period heliac with major radius Ru=l m, plasma mean minor radius (a)<0.2 m and a wide range of rotational transforms 0.6 G+(O) G 2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150-200 toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas @-low4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured P profiles show very good agreement with the computer model results. 0. 1995 American Institute of Physics.