Broadband amplifiers for the active MHD diagnostic on alcator C-Mod

An Active MHD Diagnostic has been installed and operated on Alcator C-Mod. The diagnostic consists of two antennas and two broadband amplifiers. It can excite stable modes in the Toroidal Alfvén Eigenmode (TAE) frequency range. The plasma response to the excitation is measured by existing magnetic fluctuation diagnostics. The Active MHD amplifiers are designed to drive ±20 A sinusoidal currents in the antennas at frequencies from a few kHz to 1 MHz. The amplifiers are essentially digitally programmable, bipolar current sources. They have a relatively low output impedance of approximately 5.5 ohms. At low frequencies (up to 50 kHz) the amplifiers can drive the antenna directly. At higher frequencies, a tuning network consisting of a single series capacitance is required to cancel the antenna inductance. The amplifier output impedance is in series with the antenna, reducing the overall efficiency, but increasing the effective bandwidth. The design of the Active MHD Amplifiers is novel, inherently stable, and inexpensive. The two amplifiers share a ±125-volt, 20-amp DC power supply. Each amplifier uses eight N-channel MOSFET's configured as optically isolated switches. Resistors in series with each MOSFET set the switch current and output impedance. Four MOSFET's and resistors are connected between the positive bus and the output. The other four are connected between the negative bus and the output Output waveforms are programmed by turning the MOSFET gates on and off in sequence. A 256K PROM can hold 32 × (1K × 8 bit) waveforms. The response time for the switches is on the order of 100 ns. This allows for 16 samples per cycle at 1 MHz, and 1024 samples per cycle at 2 kHz.