Data acquisition

The picture at left illustrates the LAPD experimental cycle. We start with the detection of some physical quantity, here the magnetic field of a wave measured with an induction coil. The loop is inside the machine and is driven in 3D by computer controlled stepping motors. We then digitize the signal and store the resulting time series on local disk. The probe is them moved to a new location and the process is repeated until a one, two, or three-dimensional dataset is acquired. The data are then moved to workstations which process the raw data and generate anything from line plots to stereo-pair, three-dimensional movies of complicated plasma processes.

The LAPD-U uses a data acquisition system developed at BAPSF using LabView. It enables users to control the position of the probes throughout the volume of the machine and to digitize signals applied or received by probes. The present system has twelve high-speed analog-to-digital (A/D) channels, which can operate at up to 5 GHz each at 8 bit vertical resolution. The digitizers reside in a VXI crate and are manufactured by Tektronix.

A second set of 64, slower (100 MHz, 14 bit) digitizers reside in the main lab area and are connected to the data acquisition system by a fiber optic link. A 4-channel, 40GHz digital scope has also been incorporated into the acquisition system. As need arises, more digitizers will be acquired. Automated data acquisition for overnight runs are typically 30GB in size when the data are not averaged prior to storage. The data acquisition system also supports control of a wide variety of equipment (arbitrary waveform generators, digital delay generators, programmable power supplies, tunable dye lasers) via RS232, GPIB, or TCP/IP/SCPI.

The image at right shows the data acquisition and control room. The data acquisition control center has a large monitor (which displays incoming data and machine state information) and smaller monitors to interact with the data acquisition system. Several other computers control the stepping motors, the lasers and document the operational parameters of the LAPD chamber and plasma.