HAPLS is designed to be capable of generating peak powers greater than one petawatt (1 quadrillion watts, or 10^15) at a repetition rate of 10 Hertz, with each pulse lasting 30 femtoseconds (30 quadrillionths of a second). This very high repetition rate will be a major advancement over current petawatt system technologies, which rely on flashlamps as the primary pump source and can fire a maximum of once per second. In HAPLS, the diode arrays fire 10 times per second, delivering kilojoule laser pulses to the final power amplifier. The HAPLS is being built and commissioned at LLNL and then installed and integrated into the ELI Beamlines facility starting in 2017.
The high repetition rate is possible because, unlike existing petawatt lasers, which are flashlamp-pumped, HAPLS is pumped by diode arrays capable of delivering kilojoule pulses at high repetition rates to the final power amplifier.
Each laser-diode array supplied by Lasertel supplied contains multiple 888 nm laser-diode bars mounted on water-cooled stacks (see figure). The array operates at a brightness of 10 kW/cm2, which Lasertel notes is a world record, at a repetition frequency of 10 Hz. Each array operates at a total peak power of 800 kW, with four such arrays combined and used as the primary pump sources for the HAPLS laser. More than 500,000 combined laser diode emitters combine to produce the total diode optical input power of 3.2 MW.
“The laser-diode arrays were custom-designed to LLNL requirements,” says Robert Walker, Lasertel’s director of sales and marketing. “The primary parameter that drove the need to customize is the peak power density of the array. LLNL’s laser requires an 800 kW laser-diode array with an emission area of less than 95 cm2. This ultimately translates to a requirement for 500 W peak-power bars in an array of 1600 bars with a bar-to-bar spacing of less than 350 μm. 500 W peak-power bars had been previously demonstrated on a single-bar level, but never in a multibar array. The team at Lasertel developed an entirely new custom laser design that moved the technology from a single-bar demonstration to a 1600-bar array with a peak power of 800 kW at close to 60% electrical-to-optical efficiency in a period of less than 12 months.”
Walker adds that the array assembly includes a bulk optical element that serves as a protective window and several monolithic multibar fast-axis collimation optics.
Pulser power system
The power system, which was developed at LLNL, creates extremely high-current, precisely shaped electrical pulses to drive the diode arrays; each power supply is capable of driving 40,000 A. LLNL holds a patent on this technology.
The power-conditioning system is modular, with a dedicated current driver for each diode stack (one “channel”), explains Walker. “Any channel can be populated (or not) to optimize price/power,” he notes. “Channel waveforms can be individually adjusted to optimize array uniformity. Each driver can deliver over a kiloamp at 100 V compliance, and drivers have been operated up to 20 Hz with the Lasertel arrays and 120 Hz on other systems. It provides 5 μs rise/fall times over 4 m cables.
To drive the diode arrays, LLNL needed to develop a completely new type of pulsed-power system, which supplies the arrays with electrical power by drawing energy from the grid and converting it to extremely high-current, precisely-shaped electrical pulses.Photos by Damien Jemison.
SOURCE - Laserfocus world, Lawrence Livermore National Laboratory