Correlated beam distribution functions at the transition 1st-2nd regime in a beam-plasma experiment

In a beam-plasma experiment, with typical conditions V b =1500 V, i b approximately=10 mA, p approximately=5*10 -4 torr, the 1st-2nd regime transition is studied as a function of the axial magnetic field. The beam-plasma system is characterized by time resolved measurements of the excited frequency spectrum, plasma density and beam axial and transverse energy spreads. These parameters show strong fluctuations with a typical repetition frequency of about 30 kHz. Correlation measurements permitted us to identify in the plasma, despite the d.c. character of the external parameters, two completely different states, observed respectively when the plasma and cyclotron instabilities are present. These two instabilities alternate in time Special attention is paid to the beam velocity distribution function which always shows a plateau, as a result of the excitation of beam-plasma instabilities.