  A novel quadrature Doppler Backscattering (DBS) system has been developed and
optimized for the E-band (60-90GHz) frequency range using either O-mode or
X-mode polarization in DIII-D plasmas. In general, DBS measures the amplitude
of density fluctuations and their velocity in the lab frame. The system can
simultaneously monitor both low-frequency turbulence (f < 10MHz) and
radiofrequency plasma density fluctuations over a selectable frequency range
(20-500 MHz). Detection of high-frequency fluctuations has been demonstrated
for low harmonics of the ion cyclotron frequency (e.g., 2fci~23MHz) and
externally driven high-frequency helicon waves (f = 476MHz) using an adjustable
frequency down conversion system. Importantly, this extends the application of
DBS to a high-frequency spectral domain while maintaining important turbulence
and flow measurement capabilities. This unique system has low phase noise, good
temporal resolution (sub-millisecond) and excellent wavenumber coverage
(k_{\theta} ~ 1-20cm^{-1} and k_r ~ 20-30cm^{-1}). As a demonstration,
localized internal DIII-D plasma measurements are presented from turbulence (f
<= 5MHz), Alfvenic waves (f~6.5MHz), ion cyclotron waves (f >= 20MHz) as well
as fluctuations around 476MHz driven by an external high-power 476 MHz helicon
wave antenna. In the future, helicon measurements will be used to validate
GENRAY and AORSA modeling tools for prediction of helicon wave propagation,
absorption and current drive location for the newly installed helicon current
drive system on DIII-D.
