Real-time high-field measurements of joint resistance in the alcator C-mod TF coil

The Toroidal Field [TF] coil on Alcator C-Mod is a liquid nitrogen [LN2] cooled copper magnet with 120 turns. It can carry up to 250 kA and generate nominal fields of 8 Tesla. Each turn is made up of four straight segments: outside leg, top arm, inside leg, and bottom arm, 480 segments in all. Sliding felt-metal joints between adjacent segments make it possible to assemble and disassemble the magnet, and allow it to flex during high-field operation. The design of the TF magnet has always included provisions to monitor the voltage drop and resistance along the magnet. Fused voltage taps are installed on each of the outside, top, and bottom segments. One outside leg is split to accommodate the power supply connections. Limited space around the central column precludes the installation of taps on the inside legs. There are, in total, 361 TF voltage taps and 360 measurements. These data can provide valuable information about joint integrity and any local heating or cooling issues, but we have not been able to fully exploit the voltage tap signals. Measuring the TF voltage taps during a magnet pulse is problematic, because the self-inductance of the TF magnet, and coupling to the other C-Mod coils and to the plasma swamps the resistive effects. The high voltage on the TF magnet (on the order of 1 kV) presents isolation issues for the instrumentation as well. Consequently most of the TF voltage tap data has been taken between plasma shots, with a modest 3 kA excitation and a slow reed relay scanner. The new Real-time TF Voltage Tap Monitor must allow simultaneous monitoring of all 360 channels at 12-bit resolution and up to 1 kHz bandwidth during a plasma shot. It should also allow for higher resolution measurements between shots. The large number of channels and modest performance specifications, combined with a 5 kV electrical isolation requirement make this a poor fit for commercial digitizers. A brute force approach, using 360 high-voltage isolation amplifiers, would be expensive and unappealing. This paper describes a novel design using floating V-to-F converters and optical isolators. All the signal processing can be done digitally using counters and registers in a CPLD or FPGA. Finally, there are a number of attractive options for the data interface: inexpensive digital I/O cPCI boards, an Arduino type computer, or even a simple USB connection.