In moderate-pulse reflectometry, either a swept frequency source or a number of fixed frequency sources are gated on with the instantaneous output frequency reasonably well defined (Df much less than fo). In this technique, a wavepacket with a given frequency and moderate time duration propagates into the plasma from a transmitting horn, reflects, and then returns to be collected by a receiving horn. Through double-pass time-of-flight measurements of the reflected wave, the group delay is obtained and hence the distance to the reflecting layer can be computed. To employ the moderate pulse reflectomtric diagnostic, one requires a low loss, broadband millimeter wave switch will high contrast ratio. This has motivated the development of beam control arrays. The figure below shows the arrangement employed by the Investigator's group to test this approach in the laboratory where a full mock-up of a DIII-D vacuum port was utilized. As can be seen below, extremely good spatial resolution is possible (less than 3 mm).

Schematic illustration of the moderate-pulse reflectometry laboratory proof-of-principle test setup.

Laboratory test results of a moderate-pulse reflectometer system.

Comparison of beam switching performance using monolithic beam control arrays and monolithic PIN diode arrays.

	Monolithic Beam Control Array	Monolithic PIN Diode Array
Possible Diode Density	large	small
Loss	low, ~2.2 dB (single grid) ~4.5 dB (grid pair)	high, ~6 dB
ON/OFF Contrast Ratio	large, ~24 dB (single grid) 40 dB (grid pair)	~24 dB
Switching Speed	5 GHz	1 GHz