Controlling the World’s Most Powerful Laser with NI LabVIEW and PXI Instruments

Gallery

Using LabVIEW as our universal communication language, we can acquire data from numerous instruments and perform various control tasks. (Photo courtesy of The University of Texas at Austin)

Author(s):
Dr. Erhard Gaul - University of Texas at Austin

The Texas Petawatt Project is an ongoing venture of the High Intensity Laser Science Group at The University of Texas at Austin. During the final development stages, the laser already proved to be the most powerful, fully operational laser in the world with its final power output documented to reach 1.1 PW (one quadrillion watts), which is about 2,000 times greater than all U.S. power-generation plants combined. Few other laser projects, including the decommissioned Lawrence Livermore Petawatt laser, have achieved or exceeded petawatt power. In addition, we expect the Texas Petawatt Project laser to be able to exceed 1.25 PW of power.

However, a significant factor that differentiates the Texas Petawatt Project from other similar lasers under development is that our laser delivers substantially shorter pulses, as short as 130 fs, which is about one-tenth of one-trillionth of a second, and enables many unique applications. Because our laser can deliver extremely short pulses of petawatt power, it facilitates high-energy density physics experiments with principles such as particle fusion for alternative energy research and other applications of controlled and extremely compacted energy. For example, in addition to effectively studying the possibilities for future fuel sources, on a small scale, we can create the conditions of supernovas including the plasma associated with various astrophysical phenomena.

How It Works

The process for delivering high energy includes six major phases: charging, generating the short-pulse laser, stretching, amplifying, compressing, and final targeting. Before initiating the firing sequence, we charge  large capacitors for a few minutes to provide the laser with ample firing energy.

After charging, we fire the beam, which begins as a seed laser – a small, full-spectrum beam of light pulses generated by an average-sized laboratory laser. We then “stretch” that light pulse, which carries only one-billionth of a Joule of energy, through a series of mirrors and lenses that lengthen the pulse duration and sequentially separate the light into its different frequencies. This helps us to lower the intensity of the light pulse and amplify it without damaging the laser. Next, we propagate the beam of light through a series of amplifiers, which consists of lenses, laser crystals, and mirrors of sequentially increasing size. The amplifiers create what becomes, before compression, a very large laser beam that is approximately 12 in. in diameter and carries 250 J of energy.

Then, we direct the stretched and amplified pulse into a compressor tank that concentrates it back into a shorter pulse without losing much of its energy. When we compress the stretched pulse to a short pulse, its peak power increases greatly to within the petawatt range. Finally, we use mirrors to direct the compressed beam to a final firing chamber where it is focused from 12 in. in diameter into its final, concentrated size that is narrower than a human hair.

For many experiments, we fire the beam into a steel sphere into which we deliver a mist of special gas. The laser ignites the gas particles in a powerful burst that lasts about 150 fs, depending on the experiment. We acquire and interpret the ignition data from numerous sensors using equations. By observing the results of these experiments, we gain knowledge on a variety of principles from the nature of the beginnings of our universe to the application of fusion for alternative energy.

After final targeting, the laser has to cool down for approximately one hour, which limits the number of firings to one per hour.

Laser Configuration

The laser system occupies several rooms in the basement of our building. One large room is filled with capacitors that are charged up to 20,000 V.A second area houses the seed laser, mirrors, stretchers, and amplifiers that comprise the main laser chain. A third room serves as the target area and houses the laser compression chamber and target sphere. This area is insulated with very dense concrete blocks to help prevent radiation from escaping the target site. Lastly, we analyze and control all system operations from a CPU located in a separate area, although we can remotely access the central control system while performing tasks in places throughout the laser chain.

Control System

Our diagnostics and controls system manages all aspects of laser operations including the pre-shot routine, firing sequence, and post-shot data acquisition. The system also checks for human and machine safety by verifying human safety before it initiates laser firing. At the core of the control system is a CPU running LabVIEW. We connected the CPU to a data logging and supervisory control (DSC) engine to interface with field PCs that run the appropriate software procedures. Using LabVIEW as the common communication software, these field PCs and various PXI controllers interface to all of our instruments. During operation, we can use one or several of the field PCs to interface remotely with the central processor.

We chose to use LabVIEW for several reasons, but primarily because it offers commercial-off-the-shelf (COTS) instrument drivers for the majority of the instruments in our laser system. Also, we are pleased that NI offers the LabVIEW Datalogging and Supervisory Control (DSC) Module as well as LabVIEW data server and Web capabilities, all of which facilitate remote observation and control through an easy-to-program graphical user interface. Finally, the LabVIEW Real-Time Module is critical to help us control the laser pulses at high speed rates.

Because of its power, flexibility, and ease of use, we use LabVIEW with PXI controllers to manage several hundred control points and different instruments within the laser system including motion and vision devices, switches, limiters, digital and analog data traces, triggers, and other equipment shown in Table 1. Using NI products, we successfully developed a control system that can precisely control the charging, firing, amplification, and targeting of the world’s most powerful operating laser.

Table 1. We control numerous instruments using LabVIEW, the LabVIEW Real-Time Module, and PXI controllers.

Author Information:
Dr. Erhard Gaul
University of Texas at Austin
University of Texas at Austin
Austin, TX 78712
United States
Tel: (512) 471-3434
gaul@physics.utexas.edu