Inertial Fusion Energy Driver Research Articles

Polychromatic drivers for inertial fusion energy

Although tremendous achievements have been made toward inertial confinement fusion, laser plasma instabilities (LPIs) remain to be an inevitable problem for current drive schemes. To mitigate these instabilities, significant efforts have been paid to produce high-power broadband ultraviolet lasers. However, no practical scheme has been demonstrated up to now for efficient triple-frequency conversion of broadband laser. Here we propose the design of polychromatic drivers for the generation of multicolor beams mainly based upon the optical parametric amplification, which can significantly enhance the third-harmonic conversion efficiency. Each polychromatic light has four colors of monochromatic beamlets with a full spectrum width of 3%, and the beamlet colors of any two adjacent flanges are different. The suppression effects of such polychromatic lights have been investigated via large scale particle-in-cell simulations, which indicate that more than 35% of the incident energy can be saved from the LPIs compared with monochromatic lasers for the direct-drive scheme, or high-density filled target for the indirect-drive scheme. The proposed polychromatic drivers are based on the matured technologies, and thus may pave the way towards realization of robust and high-efficiency fusion ignition.

Short-pulse, compressed ion beams at the Neutralized Drift Compression Experiment

We have commenced experiments with intense short pulses of ion beams on the Neutralized Drift Compression Experiment (NDCX-II) at Lawrence Berkeley National Laboratory, with 1-mm beam spot size within 2.5 ns full-width at half maximum. The ion kinetic energy is 1.2 MeV. To enable the short pulse duration and mm-scale focal spot radius, the beam is neutralized in a 1.5-meter-long drift compression section following the last accelerator cell. A short-focal-length solenoid focuses the beam in the presence of the volumetric plasma that is near the target. In the accelerator, the line-charge density increases due to the velocity ramp imparted on the beam bunch. The scientific topics to be explored are warm dense matter, the dynamics of radiation damage in materials, and intense beam and beam-plasma physics including select topics of relevance to the development of heavy-ion drivers for inertial fusion energy. Below the transition to melting, the short beam pulses offer an opportunity to study the multi-scale dynamics of radiation-induced damage in materials with pump-probe experiments, and to stabilize novel metastable phases of materials when short-pulse heating is followed by rapid quenching. First experiments used a lithium ion source; a new plasma-based helium ion source shows much greater charge delivered to the target.

Short intense ion pulses for materials and warm dense matter research

We have commenced experiments with intense short pulses of ion beams on the Neutralized Drift Compression Experiment-II at Lawrence Berkeley National Laboratory, by generating beam spots size with radius r<1mm within 2ns FWHM and approximately 1010 ions/pulse. To enable the short pulse durations and mm-scale focal spot radii, the 1.2MeV Li+ ion beam is neutralized in a 1.6-meter drift compression section located after the last accelerator magnet. An 8-Tesla short focal length solenoid compresses the beam in the presence of the large volume plasma near the end of this section before the target. The scientific topics to be explored are warm dense matter, the dynamics of radiation damage in materials, and intense beam and beam-plasma physics including selected topics of relevance to the development of heavy-ion drivers for inertial fusion energy. Here we describe the accelerator commissioning and time-resolved ionoluminescence measurements of yttrium aluminum perovskite using the fully integrated accelerator and neutralized drift compression components.

Design of kJ-class HiLASE laser as a driver for inertial fusion energy – CORRIGENDUM

Design of kJ-class HiLASE laser as a driver for inertial fusion energy – CORRIGENDUM

Design of a kJ-class HiLASE laser as a driver for inertial fusion energy

Abstract We present the results of performance modeling of a diode-pumped solid-state HiLASE laser designed for use in inertial fusion energy power plants. The main amplifier concept is based on a He-gas-cooled multi-slab architecture similar to that employed in Mercury laser system. Our modeling quantifies the reduction of thermally induced phase aberrations and average depolarization in ${\mathrm{Yb}}^{{3+}}$ :YAG slabs by a combination of helium cryogenic cooling and properly designed (doping/width) cladding materials.

Krypton Fluoride (KrF) Laser Driver for Inertial Fusion Energy

The United States Naval Research Laboratory (NRL) is developing the krypton fluoride (KrF) laser technology for a direct drive laser inertial fusion energy (IFE) power plant. The overall projected wall plug efficiency for KrF laser system is ~7%, including thermal management and optical losses. There are two KrF lasers at NRL. The first, Nike, provides up to 3 kJ of laser light per shot for experimental research in KrF laser-target interactions. The Electra Laser at NRL is a repetitively pulsed electron beam pumped 700 Joule KrF laser facility. The objective with Electra is to develop technologies to meet the IFE requirements for repetition rate, efficiency, and durability. Electra produces over 750 Joules in oscillator mode. Based on experiments, there is expected to be virtually no degradation in the laser focal profile, even at 5 Hz, high efficiency operation. Progress in durability has lead to achievement of KrF laser runs for 10 continuous hours at 2.5 Hz (90,000 shots) and 100 minutes at 5 Hz (over 30,000 shots). The main impediment to achieving long duration runs is the present pulsed power system that is based on spark gap switches. NRL has developed a new all solid state system that has operated for 11 million pulses continuously at 10 Hz and is based on components attaining 300 million pulses. These studies show an electron beam pumped KrF laser should be a viable approach for a laser fusion energy driver.

Plasma source development for the NDCX-I and NDCX-II neutralized drift compression experiments

AbstractCompressed ion beams are being studied as a driver for inertial confinement fusion energy and for the creation of matter in the high-energy-density regime. In order to facilitate compression of a positive ion charge bunch longitudinally and transversely beyond the limit determined by the space-charge field of the bunch, a source of charge-neutralizing electrons must be provided. Plasma sources have been developed for the NDCX-I and NDCX-II experimental facilities, both for the 2-m-long, field-free drift regions, and for the small-diameter interior of the multi-Tesla final focus solenoid. Barium titanate based cylinders with a high dielectric coefficient are used to line the wall of the 2-m-long drift region and by applying a 9 kV pulse between the inner and outer surfaces of the cylinders, plasma with a density in the 1010 cm−3 range is formed. Results are presented from experiments using this plasma source on NDCX-I. A compact plasma source 5.1 cm long and 3.8 cm in diameter, also made using the barium titanate based material, has been developed for use in the bore of the final focus solenoid. Plasma generated near the wall of the plasma source will follow the fringing magnetic field lines of the solenoid and help to fill the bore of the magnet with plasma. Improved designs for the barium titanate plasma sources are being considered that use different inner-surface electrode materials and structures, and also use a modified electrical driver employing a spark gap crowbar switch. In addition, plasma source designs using so-called flashboard technology have been developed. In the flashboard plasma source, high density plasma is formed when the applied high voltage pulse causes a series of breakdowns between isolated copper patches aligned in rows along the surface of the 0.2 mm thick flashboard.

Determination of the thermo-optic coefficient dn/dT of ytterbium doped ceramics (Sc2O3, Y2O3, Lu2O3, YAG), crystals (YAG, CaF2) and neodymium doped phosphate glass at cryogenic temperature

In this paper, we report the measurements of the thermal expansion coefficient and the thermo-optic coefficient dn/dT for the ytterbium doped cubic sesquioxides (Sc2O3, Y2O3, Lu2O3) at cryogenic temperature. These materials appear to have very interesting properties for setting up high average power laser chains useful for plasma physics and for inertial fusion energy drivers. Measurements have also been done on YAG ceramic and crystal, CaF2 crystal, and neodymium phosphate glass (Hoya, LHG-8).

Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator

The optical properties, Faraday effect and Verdet constant of ceramic terbium gallium garnet (TGG) have been measured at 1064 nm, and were found to be similar to those of single crystal TGG at room temperature. Observed optical characteristics, laser induced bulk-damage threshold and optical scattering properties of ceramic TGG were compared with those of single crystal TGG. Ceramic TGG is a promising Faraday material for high-average-power YAG lasers, Yb fiber lasers and high-peak power glass lasers for inertial fusion energy drivers.

High-Current Linear Transformer Driver Development at Sandia National Laboratories

Most of the modern high-current high-voltage pulsed power generators require several stages of pulse conditioning (pulse forming) to convert the multimicrosecond pulses of the Marx generator output to the 40-300-ns pulses required by a number of applications including X-ray radiography, pulsed high-current linear accelerators, Z-pinch, isentropic compression, and inertial fusion energy drivers. This makes the devices large, cumbersome to operate, and expensive. Sandia, in collaboration with a number of other institutions, is developing a new paradigm in pulsed power technology: the linear transformer driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high-current and high-voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. Trains of a large number of high-current pulses can be produced with variable interpulse separation from nanoseconds to milliseconds. Most importantly, these devices can be rep-rated to frequencies only limited by the capacitor specifications (usually 10 Hz). Their footprint, as compared with current day pulsed power accelerators, is considerably smaller since LTD do not require large oil and deionized water tanks. This makes them ideally suited for applications that require portability. In this paper, we present Sandia National Laboratories' broad spectrum of developmental effort to design construct and extensively validate the LTD pulsed power technology.

Driver Transition Geometries and Inductance Considerations Leading to Design Guidelines for a Z-IFE Power Plant

A 70-MA, 7-MV, ∼100-ns driver for a Z-pinch Inertial Fusion Energy (Z-IFE) power plant has been proposed. In this summary we address the transition region between the 70 Linear Transformer Driver (LTD) modules and the center Recyclable Transmission Line (RTL) load section, which convolves from the coaxial vacuum Magnetically Insulated Transmission Lines (MITL) to a parallel tri-plate and then a bi-plate disk feed. An inductive annular chamber terminates one side of the tri-plate in a manner that preserves vacuum and electrical circuit integrity without significant energy losses. The simplicity is offset by the disadvantage of the chamber size, which is proportional to the driver impedance and decreases with the addition of more parallel modules. Inductive isolation chamber sizes are estimated in this paper, based on an optimized LTD equivalent circuit simulation source driving a matched load using transmission line models. We consider the trade-offs between acceptable energy loss and the size of the inductive isolation chamber; accepting a 6% energy loss would only require a 60-nH chamber.

Electron cloud measurements in heavy-ion driver for HEDP and inertial fusion energy

The high-current experiment (HCX) at LBNL is a driver scale single beam injector that provides a 1MeV K+ ion beam current of 0.18A for 5μs. It transports high-current beams with large fill factor (ratio of the maximum beam envelope radius to the beam pipe radius) and low emittance growth that are required to keep the cost of the power plant competitive and to satisfy the target requirements of focusing ion beams to high-power density. Beam interaction with the background gas and walls desorbs electrons that can multiply and accumulate, creating an electron cloud. This ubiquitous effect grows at higher fill factors and degrades the quality of the beam. We review simulations and diagnostics tools used to measure electron production, accumulation and its properties.

Recent US advances in ion-beam-driven high energy density physics and heavy ion fusion

During the past two years, significant experimental and theoretical progress has been made in the US heavy ion fusion science program in longitudinal beam compression, ion-beam-driven warm dense matter, beam acceleration, high brightness beam transport, and advanced theory and numerical simulations. Innovations in longitudinal compression of intense ion beams by > 50 X propagating through background plasma enable initial beam target experiments in warm dense matter to begin within the next two years. We are assessing how these new techniques might apply to heavy ion fusion drivers for inertial fusion energy.

The HALNA project: Diode-pumped solid-state laser for inertial fusion energy

High-enery, rep.-rated, diode-pumped solid-state laser (DPSSL) is one of leading candidates for inertial fusion energy driver (IFE) and related laser-driven high-field applications. The project for the development of IFE laser driver in Japan, HALNA (High Average-power Laser for Nuclear Fusion Application) at ILE, Osaka University, aims to demonstrate 100-J pulse energy at 10 Hz rep. rate with 5 times diffraction limited beam quality. In this article, the advanced solid-state laser technologies for one half scale of HALNA (50J, 10 Hz) are presented including thermally managed slab amplifier of Nd:phosphate glass and zig-zag optical geometry, and uniform, large-area diode-pumping.

Physical and technological issues of KrF laser drivers for inertial fusion energy

Physics and technology of Krypton Fluoride (KrF) laser have been studied experimentally and theoretically to improve its efficiency and to increase a lifetime, and thus to verify the challenge of KrF laser for Inertial Fusion Energy (IFE). Experiments were performed with e-beam-pumped multistage 100-J output energy GARPUN KrF laser facility and 200-A/cm 2 current density EL-1 electron gun, both operating at P.N. Lebedev Physical Institute. They formed the database for verification of numerical codes capable to predict IFE-scale KrF drivers. Monte Carlo code was developed to calculate e-beam energy deposition inside GARPUN laser chamber while a quasistationary numerical KrF laser code based on generalized forward-back multi-direction approximation for radiation transfer equation was used to describe amplification of nanosecond pulses and amplified spontaneous emission (ASE). Long-lived absorption in UV optical materials induced by fast electrons and bremsstrahlung X-ray radiation was measured at EL-1 electron gun with total fluence of ionizing radiation up to 20.6 kJ/cm 2 . Using these data together with measurements and scaling of bremsstrahlung X-ray yield, we can predict that the most stable windows of IFE-scale KrF laser driver would be able to withstand no less than 2 × 10 6 shots. Fluorine-resistant coatings onto fused silica windows of KrF laser were developed and demonstrated damage thresholds as high as 29 J/cm 2 in test experiments with large 13 x 13-mm uniformly irradiated spot.

Design for a diode-pumped 1-kJ zig-zag slab laser with cryogenically cooled ceramic Yb:YAG

With the scope of future inertial fusion energy (IFE) driver, design of a diode-end-pumped 1-kJ zig-zag slab laser with cryogenically cooled ceramic Yb:YAG is presented. Pulse repetition rate is 16 Hz, and optical-optical efficiency is 34 % at 140 K operation. Gradient of Yb 3+ ion density increasing toward the slab center successfully suppresses the parasitic on the slab ends.

Beam combined IFE driver using phase controlled stimulated Brillouin scattering phase conjugation mirrors

For inertial fusion energy (IFE) generation, the giant laser facilities have been constructed in the several countries. The recent laser driver technology can achieve the output energy more than 1 M-J. In addition, the recent works shows the fast ignition can reduce the required energy for the laser fusion. This seems to realize the laser fusion, but the repetition rate is not so high for obtaining the practical laser fusion energy. The beam combination laser is a promising candidate as the laser fusion driver because it can operate at the high repetition rate and it is easily scalable. In this paper, we introduce the beam combination technique using stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs), which was already proposed and shows the recent theoretical and experimental results about the phase controlling which is essentially required for the beam combination laser using SBS-PCMs.

Development of diode‐pumped solid‐state laser HALNA for fusion reactor driver

AbstractThe diode‐pumped slab laser for inertial fusion energy driver has been demonstrated, which produces the 1053‐nm output energy of 10 J at 10 Hz. The glass slab laser amplifier has been pumped by quasi‐CW 290‐kW AlGaAs laser‐diode arrays at 803 nm. The optical system can compensate for thermal effects by use of zigzag optical propagation, image‐relayed telescope, and 45° Faraday rotator. The output energy of 10.6 J at 1 Hz with the optical‐to‐optical conversion efficiency of 19.9% has been successfully obtained. Also, the 10‐Hz operation has been performed with an output energy of 5.1 J. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(2): 27–35, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20288

Measurements of low-level prepulse on Nike KrF laser

The krypton fluoride (KrF) laser is a leading candidate driver for inertial fusion energy. Some of the current fusion target designs call for targets with thin metallic coatings. These targets could be particularly susceptible to preheat by a low-level laser prepulse. Knowledge of the prepulse can be important in understanding and modeling the behavior of such targets. This paper presents measurements of low-level prepulse on target with the Nike KrF laser. Sources of prepulse are discussed and measurements are performed under several specific laser conditions in order to evaluate the relative contribution of these sources to the overall prepulse. Prepulse is found to be ∼2×10−7 from peak intensity for approximately 120ns prior to the main laser pulse. Prepulse energy density on target is ∼2J∕cm2. The first laser amplifier in the time- and angle-multiplexed section of the laser is found to be the dominant source of prepulse.

Thermal Loading of a Direct Drive Target Rarefied Gas

In an inertial fusion energy (IFE) power plant, each fusion micro-explosion (~10 Hz) causes thermal and structural loads on the IFE reactor wall and driver optics. The loading on the wall must remain sufficiently low to ensure that economic and safety constraints are met.One proposed method for decreasing the intensity of the wall loading is to fill the reaction chamber with a gas, such as Xe, at low density. The gas will absorb much of the radiation and ion energy from the fusion event, and then slowly release it to the chamber wall. Unfortunately the protective gas introduces major heat loads on the direct drive target. The thermal loading of a target, during injection, largely determines the viability of that target upon reaching chamber center. Thus, the density of the gas must be carefully selected to ensure that a target will survive injection.The objective of this work is to quantify and characterize the heat flux resulting from the interaction of the target and the protective gas. The loading of the target is modeled using DS2V, a commercial DSMC (Direct Simulation Monte Carlo) program. Using DS2V, this work explores the effect of the protective gas density, temperature, sticking (condensation) and accommodation coefficients on the heat flux to the target.