NDLI: Broadband amplifiers for the active MHD diagnostic on Alcator C-Mod

Content Provider	IEEE Xplore Digital Library
Author	Burke, W. Cochran, W. Schimttdiel, D. Snipes, J. Zhong, X.
Copyright Year	2003
Description	Author affiliation: Plasma Sci. & Fusion Center, Massachusetts Inst. of Tecnology, Cambridge, MA, USA (Burke, W.; Cochran, W.; Schimttdiel, D.; Snipes, J.; Zhong, X.)
Abstract	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 Alfven 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 /spl plusmn/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 /spl plusmn/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 256 K PROM can hold 32 /spl times/ (1 K /spl times/ 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.
Starting Page	363
Ending Page	366
File Size	709213
Page Count	4
File Format	PDF
ISBN	078037908X
DOI	10.1109/FUSION.2003.1426659
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2003-10-14
Publisher Place	USA
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Broadband amplifiers Magnetohydrodynamics Optical amplifiers Frequency Impedance Broadband antennas Plasma measurements Optical switches Resistors MOSFET circuits
Content Type	Text
Resource Type	Article

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