Upgrades to the Alcator C-Mod gas system

The C-Mod gas system includes eight piezoelectric valves for putting gas into the torus and instruments for monitoring gas pressure. During plasma operation, the gas system is responsible for initial fueling and real-time control of plasma density. It also handles injection of impurity gases for various diagnostics. Between shots and run days, the gas system is used for Electron Cyclotron Discharge Cleaning (ECDC) and boronization under the control of a programmable logic controller (PLC). Piezoelectric valves exhibit hysteresis and other non-linear behavior. On C-Mod the valve drivers incorporate pulse-start and pulse-width modulation to linearize valve operation. The drivers can be configured for seconds-long operation with realtime control or continuous operation over multiple days as in the case of ECDC. Interlocks for the system are implemented using the PLC while remote control is shared between the PLC and Digital Plasma Control System (DPCS). A Ratiomatic Ionization Gauge Controller has been designed to monitor torus pressure on a millisecond time scale. The nominal measurement range of the controller is 10^-8 to 10^-3 Torr, which covers base torus pressure, initial fueling, and plasma evolution. The design is based on the discontinued Varian 843 Controller and works with a Bayard-Alpert gauge placed inside the vacuum vessel. This design allows remote switching between two filaments and uses a microprocessor to control filament current and a PLC interface for remote control. Data from the gas system is digitized and stored in the C-Mod data system. This paper will discuss the main features and implementation of the gas system along with performance data.