Analog computation system for plasma position and shape control

Part of the DIII-D upgrade of the Doublet III. Fusion experiment involves incorporation of more extensive feedback control of various plasma parameters. More than 200 sensors and measuring instruments generate the primary analog signals during the experiment. These signals are conditioned and transformed to standard voltages by amplifiers, integrators, and sometimes more complicated circuits. To obtain suitable feedback control variables, subsets of primary signals are combined in the form of mathematical functions. A five-input summing amplifier, a two-input divider/two-input multiplier, and a three-input summing amplifier with a two-input multiplier are the standard module functions. In order to eliminate stability problems in feedback loops, the transfer function of the computing circuits must introduce minimal time delay in the required frequency band, even if multiple cascading is used. Long term stability and accuracy is also required, along with high reliability, to save limited maintenance resources and minimize shutdown time of the experiment. During the initial stage of implementation, high flexibility in the circuit configuration and programming was required because of the empirical nature of many of the parameters. Digital and analog solutions were considered before a modular analog system was selected. The VME mechanical standard was chosen mainly because of the high density backplane connectors and availability of shielded backplanes and flat cables, which provide easy and inexpensive signal routing to any module in the system. All inputs are single-ended. Low output noise levels permit daisy chaining of modules which allows implementation of complicated functions. All inputs and outputs are also accessible via miniature coaxial connectors on the front panel, together with polarity and range switches and ten turn precision potentiometers for constant factor selection. 3 figs