A novel HPM array system based on mode locking multi-frequency

Induction machines have the unique capability of delivering very high current and peak power in a pulsed mode. It was the need for high power for fusion that led Nicholas Christofilus in the 1950’s to invent and build Astron, the first induction accelerator [1]. Since then, various high power applications have led to the construction of the Electron Ring Accelerator (ERA) [2], Experimental Test Accelerator (ETA) [3], Advanced Test Accelerator (ATA) [4], the flash radiography machine FXR and the high repetition rate machine, ETA II [5], which was designed to deliver high average power as well as high peak power. The largest of these machines is ATA with design goals of 50 MeV and 10 kA. All of the high current electron machines since ERA are “short-pulse” devices ( 50 ns). Meanwhile, the invention of heavy ion fusion in the 70’s led to the development of induction machines for ion acceleration [6]. For this application, the devices tend to have long pulses (>1 μs). The induction machines for ions and electrons operate on the same principle. The pulse length difference between electron and ion machines is historical and incidental, although long-pulse devices lead to different approaches to magnetic material and pulse power technology than short-pulse machines. While some of the past applications, particularly those related to the Strategic Defense Initiative, have come and gone, the need for high power continues to exist in energy, environment, national defense, and basic sciences. In this paper, we will review three ongoing applications, one in fusion energy, one in defense, and one in high energy physics. Induction machines for heavy ion fusion [7], for radiography in hydrodynamic tests [8], and for relativistic-klystron twobeam-accelerators [9] are three areas of active research. Induction machines have also been considered for other applications such as treatment of nuclear wastes [10], neutron spallation sources [11], and μμ collider components [12]. These applications will not be reviewed here because of space limitations. In addition, the inductive voltage adder (IVA) technologies are described in a separate paper in these proceedings [13]. The three applications we will review were chosen to demonstrate that with vastly different goals, different machine parameters, and very different architectures, a similar set of performance objectives has led to technological advances along closely parallel paths. In any large machine, the economic issues of cost and efficiency, and the technological issues of machine and beam performance are equally important considerations. We hope to show how these factors have affected the development paths in these three areas. We will first summarize recent activities in each one of these fields, and then proceed to describe issues and advances in the control of beam energy flatness, emittance preservation, and beam instability suppression. While these issues are common to all accelerators, the fact that we are working with very intense beams and long pulses makes the challenges of induction machines unique. For the purpose of this review, we will broadly include the induction accelerators proper, as well as their injectors.

For achieving highest peak powers in a solid state laser (SSL) system, significant energy output and short pulses are necessary. For mode-locked lasers, it is well-known from the Fourier theorem that the largest gain bandwidths produce the narrowest pulse-widths; thus are transform limited. For an inhomogeneously broadened line width of a laser medium, if the intensity of pulses follow a Gaussian function, then the resulting mode-locked pulse will have a Gaussian shape with the emission bandwidth/pulse duration relationship of pulse ≥ 0.4402/c. Thus, for high peak power SSL systems, laser designers incorporate gain materials capable of broad emission bandwidths. Available energy outputs from a phosphate glass host doped with rare-earth ions are unparalleled. Unfortunately, the emission bandwidths achievable from glass based gain materials are typically many factors smaller when compared to the Ti:Sapphire crystal. In order to overcome this limitation, a hybrid "mixed" laser glass amplifier – OPCPA approach was developed. The Texas petawatt laser that is currently in operation at the University of Texas-Austin and producing high peak powers uses this hybrid architecture. In this mixed-glass laser design, a phosphate and a silicate glass is used in series to achieve a broader bandwidth required before compression. Though proven, this technology is still insufficient for the future compact petawatt and exawatt systems capable of producing high energies and shorter pulse durations. New glasses with bandwidths that are two and three times larger than what is now available from glass hosts is needed if there is to be an alternative to Ti:Sapphire for laser designers. In this paper, we present new materials that may meet the necessary characteristics and demonstrate the laser and emission characteristics these through the internal and external studies.

The quest for higher performance optical parametric oscillators (OPOs) has been dominated largely by the pump laser requirements. Pump lasers with moderate average power, high peak power, good beam quality and low cost have not been readily available. Therefore, little data has been accumulated on the effects of average power operation in an OPO. A pump laser was constructed that has a high pulse repetition frequency (PRF), 1 Hz to 20 KHz, a modelocked/Q-switched temporal format, and an average power of greater than 40 Watts. Using this source, several types of synchronously pumped OPOs were tested for average power output and thermal effects. The highest performance to date was achieved with a LiNbO3 SRO in a ring resonator. LiNbO3 was the crystal chosen for its reasonable deff, thermal properties, and ease of obtaining large, good quality crystals.

The authors describe CLIA (Computer Linear Induction Accelerator), a 750 kV, 10 kA pulse power generator using magnetic switching to produce 100-ns-long pulses at 200 Hz (150 kW average power) for longer than 1 second. An L-band magnetron, an S-band frequency agile klystron, and an L-band high-perveance klystron have been successfully operated in bursts with peak powers exceeding 1 GW and average powers approaching 10 kW. After about 1 year of operating magnetron and klystron loads on CLIA, no unanticipated limits to repetitive microwave source operation at high peak power were discovered. In 1 s, 100 Hz bursts there is no problem of neutral gas build-up that produces impedance collapse or in other ways interferes with the magnetron resonance or the klystron beam bunching or output cavity interaction. There is no diminution of cold cathode emission after several thousand discharges, and no significant erosion in the magnetron anode vanes or the klystron anodes is observed. >

Good range resolution can be achieved with a shorter pulse. But on the other hand, shorter pulses need more peak power. The shorter the pulse gets, the more should its peak power be increased so that enough energy is packed into the pulse. High Peak Power makes the design of transmitters and receivers more difficult since the components used to construct these must be able to withstand the peak power. One way to overcome the problem of high peak power is to convert the short pulse into a long one and using some form of modulation to increase the bandwidth of the long pulse so that the range resolution is not compromised. This technique is called Pulse Compression and is used widely in Radar applications where high peak power is undesirable. This paper introduces a new radar pulse compression system, using a type of Linear Frequency Modulation (LFM) signal which lacks components around zero frequency. A core structure of a pulse processor is similar to a standard matched filter, modified to suit the application. Flexibility in the design permits the use of multiple code types and lengths needed, which eliminate ambiguities, caused by high pulse repetition frequencies. The device used in this application is a field programmable gate array (FPGA) which has distinct advantages compared to other application specific integrated circuits (ASIC) for the purposes of this work. The FPGA provides flexibility, for example, full reconfiguration in milli-seconds and permits a complete single chip solution.

This paper discusses latest results obtained in high-pulse repetition frequency micro hole percussion drilling of metal foils ranging between 25 μm and 100 μm thickness. In the investigations a high-repetition frequency, high average power ultrashort pulse laser source was applied, providing a near-infrared laser beam of 1.03 μm wavelength, 87.5 W maximum average laser power (at the processing plane), 51.48 MHz maximum pulse repetition frequency, 650 fs pulse duration, and 30 μm focal spot size. In the experiments, the process parameters pulse energy, pulse repetition frequency, and the pulse number were varied thus in order to evaluate their influence on both micro hole geometry (hole diameter, roundness), and drilling quality (thermal modification, melting residues). The minimum pulse number required to drill through the metal foils was detected by an in-house developed pulse counter. A significant lower pulse number to drill through the material was observed when irradiating ultrashort pulses of megahertz pulse repetition rates. On the one hand, this effect might be induced by thermal accumulation thus increasing the substrate temperature which, in turn, enhances the laser beam absorption or rather lowers the energy consumption needed for evaporation. On the other hand, pulse interactions with the still existing plasma at these very short pulse intervals cannot be neglected, so a higher amount of material might be ejected assisted by the higher plasma pressure. As another detrimental effect for megahertz pulses, a significantly lower processing quality was obtained that was due to intensive melting affected by heat accumulation, as the melt re-solidified within and/or surrounding the drilling holes. Surprisingly, by irradiating pulses at 20 MHz and above, the melt was completely ejected out from the drilling holes as the drilling efficiency was even the highest. Finally, based on the findings of this study, optimum parameter settings are presented with regard to highest machining quality and drilling throughput.

The appeal of picosecond or shorter pulses, high peak power and wavelength selection are widely reported for laser systems. The high cost of experimental facilities and their relative inflexibility once put in place have proven a potent barrier to the participation of most research groups. The same issues are even more problematical for deployment into manufacturing. A user facility began operation three years ago at Jefferson Lab to provide tunable, ultra-fast light in much the same manner as the synchrotron light sources at the DoE National Laboratories. Results from this 1 kW average power machine point to significant opportunities. Upon completion of the current upgrade by the end of this year, the laser will initially deliver at least 10 kW in the 1.5 to 15 µm range, and later at least 1 kW to below 250 nm, with 100 µJ pulses of 200 fs – 2 ps native length. This work was supported by U.S. DOE Contract No. DE-AC05-84-ER40150, the Office of Naval Research, the Air Force Research Laboratory, the Commonwealth of Virginia and the Laser Processing Consortium.The appeal of picosecond or shorter pulses, high peak power and wavelength selection are widely reported for laser systems. The high cost of experimental facilities and their relative inflexibility once put in place have proven a potent barrier to the participation of most research groups. The same issues are even more problematical for deployment into manufacturing. A user facility began operation three years ago at Jefferson Lab to provide tunable, ultra-fast light in much the same manner as the synchrotron light sources at the DoE National Laboratories. Results from this 1 kW average power machine point to significant opportunities. Upon completion of the current upgrade by the end of this year, the laser will initially deliver at least 10 kW in the 1.5 to 15 µm range, and later at least 1 kW to below 250 nm, with 100 µJ pulses of 200 fs – 2 ps native length. This work was supported by U.S. DOE Contract No. DE-AC05-84-ER40150, the Office of Naval Research, the Air Force Research Laboratory, the Commonw...

High-repetition-rate ultrafast laser oscillators with high average power and short pulse duration provide excellent sources for generating optical frequency combs. Here we report a Kerr-lens mode locked Yb:YAG thin-disk oscillator delivering 203-MHz pulses at an average power of 9.4 W. A single additional nonlinear plate was inserted inside the cavity to enhance the Kerr lens effect, which leads to a substantial broadening of the mode-locked spectrum. The resultant pulse duration is 55 fs. The demonstrated oscillator combines a high repetition rate, a high average power and short pulse duration within one resonator, offering an ideal prerequisite for the optical-frequency metrology and frequency-comb spectroscopy with high signal-to-noise ratio.

We demonstrate a robust, compact, low-cost, pulsed, linearly polarized, 1064 nm, Yb:fiber laser system capable of generating ~100 kW peak power pulses and >17 W average power at repetition rates of 80 – 285 kHz. The system employs a configurable microchip seed laser that provides nanosecond (~1.0 – 1.5 ns) pulse durations. The seed pulses are amplified in an all-fiber, polarization maintaining, large mode area (LMA) fiber amplifier optimized for high peak power operation. The LMA Yb:fiber amplifier enables near diffraction limited beam quality at 100 kW peak power. The seed laser, fiber amplifier, and beam delivery optics are packaged into an air-cooled laser head of 152×330×87 mm3 with pump power provided from a separate air-cooled laser controller. Due to the high peak power, high beam quality, spectral purity, and linearly polarized nature of the output beam, the laser is readily frequency doubled to 532 nm. Average 532 nm powers up to 7 W and peak powers exceeding 40 kW have been demonstrated. Potential for scaling to higher peak and average powers in both the green and infrared (IR) will be discussed. This laser system has been field tested and demonstrated in numerous materials processing applications in both the IR and green, including scribing and marking. We discuss recent results that demonstrate success in processing a diverse array of representative industrial samples.

There has been remarkable progress in the development of high peak-power ultrafast lasers in recent years. Lasers capable of generating terawatt peak powers with unprecedented short pulse durations can now be built on a single optical table in a small laboratory. The rapid technological progress has made possible a host of new scientific advances in high-field science, such as the generation of coherent femtosecond X-ray pulses, and the generation of MeV-energy electron beams and high-energy ions. In this paper, we review progress in the development and design of ultrafast highpower lasers based on Ti:sapphire, including the ultrafast laser oscillators that are a very important enabling technology for high-power ultrafast systems, and ultrafast amplified laser systems that generate 20 fs duration pulses with several watts average power at kilohertz repetition-rates. Ultrafast waveform measurements of these pulses demonstrate that such short pulses can be generated with high fidelity. Finally, we discuss applications of ultrafast high-power pulses, including the generation of femtosecond to attosecond X-ray pulses. The 1990s have seen rapid progress in ultrafast laser technology [1, 2]. Ultrafast light sources today are “turn-key” devices, producing peak output powers on the order of a megawatt with pulse durations under 10 fs, directly from a simple laser [3–8]. Since an optical cycle period in the visible and near-infrared is 2–3 fs, this pulse duration approaches fundamental limits for devices operating in this wavelength range. Important new measurement techniques [9] have also been developed, which can extract the complete waveform of an optical pulse of only a few cycles in duration [10, 11]. Such precise measurement tools are essential to properly characterize and utilize extremely short optical pulses. A major application of ultrafast lasers is to study the interaction of atoms, molecules, and plasmas with intense light. These high-field science applications require peak power in the range of 1010–1015 W. Such high-power short pulses are generated using laser amplifier systems. The technology for generating high-power amplified ultrafast pulses has progressed very rapidly over the past decade [12–22]. Amplification of the energy in an ultrashort pulse by factors of 107 or more is now routinely achieved, resulting in a peak power of in the terawatt range from small-scale lasers, and of up to a petawatt from larger systems. By focusing these high-power optical pulses, light intensities of greater than 1020 W cm−2 can be generated, which correspond to an intensity greater than that which would be obtained by focusing the entire solar flux incident on the earth onto a pin-head. Because the size of a laser depends primarily on its pulse energy, by reducing the duration of the pulse being amplified, we can achieve extremely-high power densities using lasers of a realistic and compact scale. As a result, by reducing the pulse duration to 10–20 fs, even very small-scale laboratory lasers operating at kilohertz repetition rates [23, 24] are now easily capable of generating light intensities corresponding to an electric field in excess of that binding a valence electron to an atom. Remarkable progress in high-field optical science has followed as a direct result of this technological progress. It is now possible to experimentally investigate highly-nonlinear processes in atomic, molecular, plasma, and solid-state physics, and to access previously-unexplored states of matter. Ultrashort-pulses at ultraviolet and soft X-ray wavelengths can be generated through harmonic upconversion [25–28] and also at hard and soft X-ray wavelengths through the creation of an ultrafast laser-produced plasma [29] or by scattering off electron beams [30–32]. Such ultrafast softand hard-X-ray pulses can be used to directly probe both longand short-range atomic dynamics and to monitor the evolution of highly-excited systems [33]. In other work, the use of extremely short-pulse high-intensity lasers may make it possible to generate coherent X-rays with pulse duration as short as 10−16 seconds (or 100 attoseconds) [34–36]. 1 Ultrashort-pulse sources The technology of femtosecond lasers changed dramatically with the demonstration in 1990 of the self-modelocked Ti:sapphire laser by Sibbett and his group [1]. Titaniumdoped sapphire is a solid-state laser material with extreme-

The enhancement of the efficiency of CO2 slab waveguide lasers for micro-machining applications by the reduction of the pulse width and an increase in the peak power has been demonstrated by two different approaches. The first uses short high RF peak power radiofrequency discharge excitation pulses, coupled with high pressure operation to produce peak laser powers to 3kW, with pulses durations down to around 10μs. Further reductions in pulse duration are obtained by acousto-optic pulse slicing. The second approach uses an acousto-optic modulator to reduce the ∼300 W peak power long square pulses (>100μSec) from an oscillator to durations in the range 1-10 μs. These pulses are then amplified in a multi-pass planar waveguide amplifier section, where pulse characteristics of 14kW peak power and 1μsec duration are obtained. Preliminary experiments on the use of these short, high peak power laser pulses in micro-machining glass, quartz and FR4 resin are discussed.

inside view

Sonoluminescence characterization of inertial cavitation inside a BSA phantom treated by pulsed HIFU

We demonstrate an efficient wavelength-selectable output in the attractive deep-red spectral region from an intracavity frequency converted Nd:YLF/KGW Raman laser. Driven by an acousto-optic Q-switched 1314 nm Nd:YLF laser, two first-Stokes waves at 1461 and 1490 nm were generated owing to the bi-axial properties of KGW crystal. By incorporating intracavity sum-frequency generation and second-harmonic generation with an angle-tuned bismuth borate (BIBO) crystal, four discrete deep-red laser emission lines were yielded at the wavelengths of 692, 698, 731, and 745 nm. Under the incident pump power of 50 W and the repetition rate of 4 kHz, the maximum average output powers of 2.4, 2.7, 3.3, and 3.6 W were attained with the pulse durations of 3.4, 3.2, 4.3, and 3.7 ns, respectively, corresponding to the peak powers up to 177, 209, 190, and 245 kW. The results indicate that the Nd:YLF/KGW Raman laser combined with an angle-adjusted BIBO crystal provides a reliable and convenient approach to achieve the selectable multi-wavelength deep-red laser with short pulse duration and high peak power.

Ultrafast, optically pumped, passively modelocked vertical external -cavity surface-emitting lasers (VECSELs) are excellent sources for industrial and scientific applications that benefit from compact semiconductor based high -power ultrafast lasers with gigahertz repetition rates and excellent beam quality. Applications such as self-referenced frequency combs and multi -photon imaging require sub -200-fs pulse duration combined with high pulse peak power. Here, we present a semiconductor saturable absorber mirror (SESAM) modelocked VECSEL with a pulse duration of 1 47 fs and 328 W of pulse peak power. The average output power was 100 mW with a repetition rate of 1.8 2 GHz at a cent er wavelength of 103 4 nm. The laser has optimal beam quality operating in a fundamental transverse mode with a M 2 value of <1.05 in both orthogonal directions. The VECSEL was grown by metal -organic vapor phase epitaxy (MOVPE) with five pairs of strain-compensated InGaAs quantum wells (QW s). The QWs are placed symmetrical around the antinodes of the standing electric field at a reduced average field enhancement in the QWs of ! 0.5 (normalized to 4 outside the structure). The modelocking results overcome the trade -off between pulse duration and peak power of the state -of-the-art threshold values of 4.3 5 kW peak power for a pulse d uration of 400 fs and 3.3 W peak power for a pulse duration of 107 fs. Keywords: Mode -locked lasers, Semiconductor lasers, Ultrafast lasers, Vertical emitting lasers.