Large Plasma Device

W. Gekelman, P. Pribyl, Z. Lucky, M. Drandell, D. Leneman, J. Maggs, S. Vincena, B. Van Compernolle, S. K. P. Tripathi, G. Morales, T. A. Carter, Y. Wang, and T. DeHaas, The upgraded Large Plasma Device, a machine for studying frontier basic plasma physics, Rev. Sci. Instrum., 87, 025105 (2016) http://dx.doi.org/10.1063/1.4941079.

Diagnostic equipment: Fifteen Digital Oscilloscopes ranging from 2 channel-175 MHz/channel to 4 channel-10 GHz/channel, 6 Stanford digital delay generators (1 ps accuracy), 2 BNC 8 channel pulse generators ( 1 ns accuracy), 1 LeCroy arbitrary waveform generator (10 MHz), 2 Agilent arbitrary waveform generators (80 MHz), HP 8568B spectrum analyzer, 3 LeCroy 1820A Differential Amplifiers/Filters, Agilent Network Analyzser (to 180 MHz), 2 four channel Tektronix-Sony 100 MHz optical isolators, 2 microscopes for probe construction (one with micro-manipulators, 300V/2A Agilent programmable power supply. An array of 7 microwave interferometers provides chord-averaged, axial electron density A wide variety of diagnostic probes are available and users are free to construct their own. Available are probes to measure time-varying magnetic and electric fields, electron density, plasma potential, floating potential, plasma flow, ion temperature, particle flux, and vorticity.

Computing and networking: The communications network in the STRB uses a dedicated 10/100/1000 Mbit switch with a direct fiber optic link to the facility office space next door. The STRB itself is connected to the main UCLA campus via a 10 Gbit uplink. Wireless (G/N) network connectivity is also provided to the users in both the STRB and offices. While at UCLA, experimenters can use the data analysis and visualization systems developed by the local group, as well as networked workstations, disk farms and the equipment and software needed to make computer-generated videos and CD's from data acquired on the LAPD-U.

The available resources include over 12 terabytes of centralized RAID6 data storage, hosted by a dual-64 bit Linux server. Disk-to-disk backups of this array are performed nightly. An 8-core server with 64GB RAM is available for data processing. Three Linux workstations with 1GB RAM and high-end 3D graphics cards are dedicated to facility users during their stay, 8 Windows XP systems (for experiment data acquisition, automated probe motion, machine state monitoring and control), 7 Mac computers (one configured for making videos with 8GB ram, 2TB disk, DVD burner, Adobe Premier software, PhotoShop, Illustrator, Maya, and other graphics software). A 3D HDTV with stereo shutter glasses is available to view rendered images and movies of complex plasma phenomena. Workstations have IDL and AVS for scientific visualization as well as several language compilers (C, C+, and Fortran), Xerox color laser printer, slide scanner, and flatbed scanner.

Amplifiers and sources for launching waves:1 custom built 30 kW amplifier (may be tuned for 200 kHz-5 MHz operation), 1 Velonix 360 high voltage pulser (up to 30 kV pulses, 100 ns rise-time), 1 AR 2500L, 10 kHz-220 MHz, 2.5 kW broadband amplifier 1 AR 2000L, 10 kHz-220 MHz, 2 kW broadband amplifier, 1 AR 200L 1 MHz-200 MHz, 200 W broadband amplifier, 1-250 kW ( 2.5 ms pulse) 9 GHz source.

Lasers: One Nd-YAG pumped laser with 7 ns, 150 MW pulse (up to 10 pps) with a 532-nm Gaussian beam. One tunable dye laser, 1-Watt CW (Coherent 899 driven by an INNOVA Argon ion laser) mounted on an air-isolated optical bench (Spectra Physics Pro 290). Can operate in the blue to far red depending upon choice of dye, bandwidth 1 MHz, and free spectral range 30 GHz. Equipment associated with this laser is: a confocal spectrum analyzer, New-Focus 7711 Fizeau Wavelength meter, iodine cell, opto-acoustic light chopper and light detection phototubes, amplifiers and power supplies. The newest addition is a pulsed dye laser (10 ns pulse and 100 ns pulse stretcher) 2-12 MW programmable, spectral output from 270-700 nm for LIF photography. This is coupled with a Cooke Gen III , high speed ( tmin = 3 ns), (1024X1280 pixel CCD), computer controlled camera with averaging and background subtraction capabilities. This camera is for the imaging of planar LIF signals. The laser system is shown in Fig 6. Both lasers are interfaced to computers and can be driven by the data acquisition system or used independently. Pictured is the 50 MW frequency doubled YAG pump laser and SIRAH dye laser. The pulse stretcher is sown on the left. The laser is focused into an optical fiber and delivered to the machine.

The LArge Plasma Device (LAPD): The LAPD-Upgrade plasma column has a maximum length of 18 meters and a 75-centimeter diameter. Plasmas of varying length can be explored by segmenting the device (i.e., inserting a terminating copper end plate at various axial locations). The confining magnetic field can achieve a steady-state value of up to 3.5 kG. Ordinarily the magnetic field is uniform, but it be can varied to generate multiple or single mirror geometries, magnetic cusps, axial field gradients and other configurations of scientific interest. The device currently has one cathode and will soon have two independent cathodes (each in a two meter long, 1.6 m diameter source chamber) which permits the creation of controlled plasma flows. Each cathode is driven by a 4-Farad capacitor bank that can supply a total discharge current of 32 kA. The electron sources are switched by banks of 2.5 kA, 1.5 kV transistors. The cathodes may be synchronized or operated independently. A pulsed gas feed system at each end of the device permits the formation of uniform or non-uniform axial neutral gas profiles that permit the study of effects associated with ion mass gradients. The equipment and software needed to make computer-generated videos and CD's from data acquired on the LAPD.

The length of the plasma column and the radial density profiles can be tailored to study various physical processes that depend on transverse or axial gradients in density and temperature. Plasmas will be routinely available with density in excess of 5x10^12/cm^3 and electron temperatures in the 10 eV range. The ion temperature will be varied through the application of 100kW of ICRF power. Smaller, lanthanum-hexaboride (LaB6) cathodes, up to 20cm diameter can be used to created higher density (10^13/cm^3) enhancements in the main plasma column.

The upgraded LAPD has unprecedented access. It has eight, 12 cm diameter access ports between magnets (424 in total) including eight, unique "octo-ports". The octo-ports have eight, 10 x 40 cm, rectangular openings, which allow for a nearly unimpeded 360o view of the plasma column. These ports can be used for visible and microwave tomography, laser fluorescence, or the insertion of various large antennas or plasma terminating end plates.

It will be possible to connect several (up to four), computer-controlled, probe drive systems at once through any one of 50 adjacent ports on either side along the machine axis. If the need arises, simultaneously activated probes could acquire data at several spatial locations in the plasma volume at once. Probes may be introduced during machine operation through vacuum interlocks and portable pump-down stations without disturbing the vacuum system or plasma conditions. Several pump-down stations comprising a small 200-400 l/s turbo and mechanical pump, ionization gauges and vacuum fittings enable users to move instrumentation in and out of the device. The main vacuum system relies on four turbo pumps with a combined pumping speed of 8000 l/s. Two Stanford residual gas analyzers with computer interfaces monitor the vacuum and gas evolution during cathode conditioning. Arbitrary concentrations of neutral gases (H2, He, Ne, Ar) can be created using 4 independent mass flow controllers.

The magnetic coil system consists of 90 pancake magnets placed at 6-inch intervals along the machine length. The magnets are controlled by ten separate power supplies, specifically designed for this system. The power supplies can be controlled manually or by computer, and allow operation of up to 4.0 kG axial magnetic field with 0.1% current ripple. The magnet power supplies (4 supplies: 9.6 kA, 40 V; 6 supplies : 3600 A, 84 V) are fed by a 4.0 megawatt substation acquired for this project. An additional megawatt of power is available in the laboratory for general use. The building is unique in its electrical capabilities; it is designed to accommodate large experiments that require high power levels. Presently there is 15 MW available in the building switchyard, and this can be doubled if necessary.