High density conics in a magnetically expanding helicon plasma

A two-dimensional mapping of ion density and plasma potential in a diverging magnetized low pressure (0.4 mTorr) carbon dioxide helicon plasma containing a double layer reveals the presence of high density conics (∼7x109 cm-3) along the most diverging magnetic field lines exiting the helicon source and connecting with the grounded expansion chamber. The density in the conic is about 30% greater than the density at the double layer and this results from local ionization associated with the presence of a high energy tail in the electron energy probability function. The plasma potential along the conic is constant at about 30 V.