Energetic wave-trapped electrons of $\sim 20\mathrm{eV}$, moving at the 13.56 MHz helicon wave phase velocity, are directly measured with a 3 ns response-time retarding-potential analyzer. The rf axial wavelength is measured with $B$-dot probes scanned axially. Changes in the axial magnetic field or the rf power cause order of magnitude changes in the energetic electron current. The current correlates with either a transition from partially propagating to nearly pure standing waves or quantized jumps in the number of half wavelengths between the two azimuthal antenna straps.

• Received 7 November 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.233