In quiescent runaway electron plasmas in the DIII-D tokamak, whistler waves with frequencies
between 90 and 200 MHz are driven unstable in plasmas with appreciable hard x-ray
and nonthermal electron cyclotron emission (ECE). Narrow (delta f < 50 kHz) discrete
modes with erratically spaced frequencies are observed. Unstable modes often extend
over a range Delta f similar or equal to 50 MHz but lower frequency unstable modes
are usually most intense. The dependency of the frequency on field and density implies
a wavenumber k similar or equal to 150 m(-1) with parallel wavenumber k parallel to
<< k. Reducing the gap between the plasma and the wall increases the number of detected
modes. Lowering the magnetic field promotes instability. Nonlinear limit-cycle-like
oscillations in the whistler amplitude occur on a 10 ms timescale. The ECE signals
often jump at whistler bursts, suggesting that the modes pitch-angle scatter the runaways.
Sawteeth cause transient stabilization of the whistlers.