Cyclotron harmonic waves in an inhomogeneous plasma. II. Experiment

Abstract

For pt. I see ibid., vol. 14, no. 5, 473. Electrostatic cyclotron harmonic waves were excited in an inhomogeneous plasma column with axial magnetic field by means of an externally applied r.f. capacitor field. Amplitude and phase of the r.f.-potential inside the plasma were measured with a probe. For frequencies just above the harmonics of the electron cyclotron frequency forward waves were found in the low density region of the plasma where the local density omega p2 is less than the hybrid density omega pH2= omega 2- omega c2. For frequencies just below the harmonics backward waves were found in the dense core of the column, where omega p2> omega pH2. In the latter case standing waves (Buchsbaum-Hasegawa-modes) were found which are set up by two oppositely propagating backward waves with group velocities directed towards the dense plasma core. Phase measurements indicate an excitation near the hybrid layer and allow a direct determination of the phase condition for the Buchsbaum-Hasegawa resonances to occur. In applying a short microwave pulse to the capacitor plates short wave trains were excited in the plasma. Time of flight measurements show that both the forward and the backward waves are excited around the hybrid layers. Multiple reflection of backward wave trains at the hybrid layers were observed.