Publications

Theses [ 2 ]

G. J. Stein. Modeling of Nonlinear Ultrashort Optical Pulse Propagation. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science (S. M. Thesis), 2015.


URL,
I present a numerical package, written in MATLAB, which provides a simplified scripting interface for simulating a host of ultrashort pulse propagation phenomena. With the proliferation of ultrashort laser technologies, the demand for efficient and accurate simulations has grown significantly. Here I introduce a linear-operator-based formalism for nonlinear pulse propagation beyond the slowly-varying-envelope approximation, which includes phenomena such as nonlinear wave mixing, plasma blue-shifting, and high harmonic generation. I also demonstrate the capabilities of our versatile simulation package, which can handle optical pulse propagation through a host of geometries and guiding structures. Finally, the simulation package is used to investigate a number of effects, particularly that of modulational instability in Kagome-type hollow-core photonic crystal fibers.

G. J. Stein. Simulation of a Few-Cycle Nonlinear Laser Pulse Compressor. Cornell University, Department of Applied & Engineering Physics (B. S. Thesis), 2013.



The drive for increasingly high-intensity coherent light sources for probing new physics has led to a demand for ultrashort pulses on the order of just a few optical cycles, approximately 10fs. Pulses of this order serve as valuable research tools, yet the current experimental setup at the Gaeta Group, consisting of a modelocked, titanium-sapphire laser, only allows for pulses on the order of 75fs. This report aims to simulate the propagation of a variable power ultrashort pulse through different pressures of argon gas in the presence of bound nonlinearities and nonlinearities which result from the generation of plasma. The linear step in the simulation is computed by way of a Hankel [Fourier-Bessel] transform, with a novel interpolation based routine introduced and tested by the authors. The new routine is shown to be less efficient than the well known quasi-fast Hankel transform, but its generalizability makes it advantageous for the calculation of other integral transforms, for which a quasi-fast algorithm may not exist. The accuracy of the simulation results will be demonstrated through a comparison to the simulations of Mlejnek et al. The ultimate goal of this report is the simulation of a free-space nonlinear pulse compressor, the specifications of which will be determined through variations over large parameter spaces. With this information, future work can be done to build and test such a device, which could be used for a number of purposes, including nonlinear self-focusing and high harmonic generation.

Journal Articles [ 11 ]

Best Paper Finalist at CoRL 2018;
Best Oral Presentation at CoRL 2018 [talk link] G. J. Stein, C. Bradley, and N. Roy. Learning over Subgoals for Efficient Navigation of Structured, Unknown Environments. Conference on Robot Learning (CoRL), 2018.
Best Paper Finalist at CoRL 2018;
Best Oral Presentation at CoRL 2018 [talk link]

URL, Summary Video,
We propose a novel technique for efficiently navigating unknown environments over long horizons by learning to predict properties of unknown space. We generate a dynamic action set defined by the current map, factor the Bellman Equation in terms of these actions, and estimate terms, such as the probability that navigating beyond a particular subgoal will lead to a dead-end, that are otherwise difficult to compute. Simulated agents navigating with our Learned Subgoal Planner in real-world floor plans demonstrate a 21% expected decrease in cost-to-go compared to standard optimistic planning techniques that rely on Dijkstra’s algorithm, and real-world agents show promising navigation performance as well.

G. J. Stein and N. Roy. GeneSIS-RT: Generating Synthetic Images for training Secondary Real-world Tasks. Proceedings of the International Conference on Robotics and Automation, 2018.


URL, Summary Video,
We propose a novel approach for generating high-quality, synthetic data for domain-specific learning tasks, for which training data may not be readily available. We leverage recent progress in image-to-image translation to bridge the gap between simulated and real images, allowing us to generate realistic training data for real-world tasks using only unlabeled real-world images and a simulation. GeneSIS-RT ameliorates the burden of having to collect labeled real-world images and is a promising candidate for generating high-quality, domain-specific, synthetic data. To show the effectiveness of using GeneSIS-RT to create training data, we study two tasks: semantic segmentation and reactive obstacle avoidance. We demonstrate that learning algorithms trained using data generated by GeneSIS-RT make high-accuracy predictions and outperform systems trained on raw simulated data alone, and as well or better than those trained on real data. Finally, we use our data to train a quadcopter to fly 60 meters at speeds up to 3.4 m/s through a cluttered environment, demonstrating that our GeneSIS-RT images can be used to learn to perform mission-critical tasks.

K. Murari, G. J. Stein, H. Cankaya, B. Debord, F. Gérôme, G. Cirmi, O. D. Mücke, P. Li, A. Ruehl, I. Hartl, K.-H. Hong, F. Benabid, and F. X. Kärtner. Kagome-fiber-based pulse compression of mid-infrared picosecond pulses from a Ho:YLF amplifier. Optica, 3(8), 2016.


URL, DOI,
Over the last decade, the development of ultrafast laser pulses in the mid-infrared (MIR) region has led to important breakthroughs in attosecond science and strong-field physics. However, as most such broadband MIR laser sources are near-IR pumped, the generation of high-intensity, long-wavelength MIR pulses is still a challenge, especially starting from picosecond pulses. Here we report, both experimentally and numerically, nonlinear pulse compression of sub-millijoule picosecond pulses down to sub-300 fs at 2050 nm wavelength in gas-filled Kagome-type hollow-core photonic crystal fibers for driving MIR optical parametric amplifiers. The pump laser is comprised of a compact fiber laser-seeded 2 $\mu$m chirped pulse amplification system based on a Ho:YLF crystal at 1 kHz repetition rate. Spectral broadening is studied for different experimental conditions with variations of gas pressure and incident pulse energies. The spectrally broadened 1.8 ps pulses with a Fourier-limited duration of 250 fs are compressed using an external prism-based compressor down to 285 fs and output energy of 125 $\mu$J.

G. J. Stein, P. D. Keathley, P. Krogen, H. Liang, J. P. Siqueira, C.-L. Chang, C.-J. Lai, K.-H. Hong, G. M. Laurent, and F. X. Kärtner. Water-window soft x-ray high-harmonic generation up to the nitrogen K-edge driven by a kHz, 2.1 $\mu$m OPCPA source. Journal of Physics B: Atomic, Molecular and Optical Physics, 49(15), 2016.


URL,
We report the generation of coherent water-window soft x-ray harmonics in a neon-filled semi-infinite gas cell driven by a femtosecond multi-mJ mid-infrared optical parametric chirped-pulse amplification (OPCPA) system at a 1 kHz repetition rate. The cutoff energy was extended to ~450 eV with a 2.1 μ m driver wavelength and a photon flux of $\sim 1.5\times 10^6$ photons/s/1% bandwidth was obtained at 350 eV. A comparable photon flux of $\sim 1.0\times 10^6$ photons/s/1% bandwidth was observed at the nitrogen K-edge of 410 eV. This is the first demonstration of water-window harmonic generation up to the nitrogen K-edge from a kHz OPCPA system. Finally, this system is suitable for time-resolved soft x-ray near-edge absorption spectroscopy. Further scaling of the driving pulse's energy and repetition rate is feasible due to the availability of high-power picosecond Yb-doped pump laser technologies, thereby enabling ultrafast, tabletop water-window x-ray imaging.

C. Jin, G. J. Stein, K.-H. Hong, and C. D. Lin. Generation of Bright, Spatially Coherent Soft X-Ray High Harmonics in a Hollow Waveguide Using Two-Color Synthesized Laser Pulses. Phys. Rev. Lett., 115, 2015.


URL, DOI,
We investigate the efficient generation of low-divergence high-order harmonics driven by waveform-optimized laser pulses in a gas-filled hollow waveguide. The drive waveform is obtained by synthesizing two-color laser pulses, optimized such that highest harmonic yields are emitted from each atom. Optimization of the gas pressure and waveguide configuration has enabled us to produce bright and spatially coherent harmonics extending from the extreme ultraviolet to soft x rays. Our study on the interplay among waveguide mode, atomic dispersion, and plasma effect uncovers how dynamic phase matching is accomplished and how an optimized waveform is maintained when optimal waveguide parameters (radius and length) and gas pressure are identified. Our analysis should help laboratory development in the generation of high-flux bright coherent soft x rays as tabletop light sources for applications.

C.-L. Chang, P. Krogen, K.-H. Hong, L. E. Zapata, J. Moses, A.-L. Calendron, H. Liang, C.-J. Lai, G. J. Stein, P. D. Keathley, G. Laurent, and F. X. Kärtner. High-energy, kHz, picosecond hybrid Yb-doped chirped-pulse amplifier. Optics Express, 23(8), 2015.


URL,
We report on a diode-pumped, hybrid Yb-doped chirped-pulse amplification (CPA) laser system with a compact pulse stretcher and compressor, consisting of Yb-doped fiber preamplifiers, a room-temperature Yb:KYW regenerative amplifier (RGA), and cryogenic Yb:YAG multi-pass amplifiers. The RGA provides a relatively broad amplification bandwidth and thereby a long pulse duration to mitigate B-integral in the CPA chain. The ~1030-nm laser pulses are amplified up to 70 mJ at 1-kHz repetition rate, currently limited by available optics apertures, and then compressed to ~6 ps with high efficiency. The near-diffraction-limited beam focusing quality is demonstrated with M$_x^2$ = 1.1 and M$_y^2$ = 1.2. The shot-to-shot energy fluctuation is as low as ~1% (rms), and the long-term energy drift and beam pointing stability for over 8 hours measurement are ~3.5% and <6 $\mu$rad (rms), respectively. To the best of our knowledge, this hybrid laser system produces the most energetic picosecond pulses at kHz repetition rates among rod-type laser amplifiers. With an optically synchronized Ti:sapphire seed laser, it provides a versatile platform optimized for pumping optical parametric chirped-pulse amplification systems as well as driving inverse Compton scattered X-rays.

H. Liang, P. Krogen, R. Grynko, O. Novak, C.-L. Chang, G. J. Stein, D. Weerawarne, B. Shim, F. X. Kärtner, and K.-H. Hong. Three-octave-spanning supercontinuum generation and sub-two-cycle self-compression of mid-infrared filaments in dielectrics. Optics Letters, 40(6), 2015.


URL,
We experimentally and numerically investigate the spectral and temporal structure of mid-infrared (mid-IR) filaments in bulk dielectrics with normal and anomalous group velocity dispersion (GVD) pumped by a 2.1 $\mu$m optical parametric chirped-pulse amplifier (OPCPA). The formation of stable and robust filaments with several microjoules of pulse energy is observed. We demonstrate a supercontinuum that spans more than three octaves from ZnS in the normal GVD regime and self-compression of the mid-IR pulse to sub-two-cycle duration in CaF2 in the anomalous GVD regime. The experimental observations quantitatively agree well with the numerical simulations based on a three-dimensional nonlinear wave equation that reveals the detailed spatio-temporal dynamics of mid-IR filaments in dielectrics.

C.-L. Chang, P. Krogen, H. Liang, G. J. Stein, J. Moses, C.-J. Lai, J. P. Siqueira, L. E. Zapata, F. X. Kärtner, and K.-H. Hong. Multi-mJ, kHz, ps deep-ultraviolet source. Optics Letters, 40(4), 2015.


URL,
We demonstrate a 0.56-GW, 1-kHz, 4.2-ps, 2.74-mJ deep-ultraviolet (DUV) laser at ~257.7 nm with a beam propagation factor (M$^2$) of ~2.54 from a frequency-quadrupled cryogenic multi-stage Yb-doped chirped-pulse amplifier. The frequency quadrupling is achieved using LiB$_3$O$_5$ and $\beta$-BaB$_2$O$_4$ crystals for near-infrared (NIR)-to-green and green-to-DUV conversion, respectively. An overall NIR-to-DUV efficiency of ~10% has been achieved, which is currently limited by the thermal-induced phase mismatching and the DUV-induced degradation of transmittance. To the best of our knowledge, this is the highest peak-power picosecond DUV source from a diode-pumped solid-state laser operating at kHz repetition rates.

* Article also featured in the Journal of Optics' LabTalk series. C.-J. Lai, K.-H. Hong, J. P. Siqueira, P. Krogen, C.-L. Chang, G. J. Stein, H. Liang, P. D. Keathley, G. Laurent, J. Moses, L. E. Zapata, and F. X. Kärtner. Multi-mJ mid-infrared kHz OPCPA and Yb-doped pump lasers for tabletop coherent soft x-ray generation. Journal of Optics, 17(9), 2015.
* Article also featured in the Journal of Optics' LabTalk series.

URL,
We present our recent progress on the development of a mid-infrared (mid-IR), multi-mJ, kHz optical parametric chirped-pulse amplification (OPCPA) system, pumped by a homebuilt picosecond cryogenic Yb:YAG chirped-pulse amplifier, and its application to soft x-ray high-order harmonic generation. The cryogenic Yb:YAG laser operating at 1 kHz repetition rate delivers 42 mJ, 17 ps, 1.03 $\mu$m pulses to pump the OPCPA system. Efficient second and fourth harmonic generations from the Yb:YAG system are demonstrated, which provide the pumping capability for OPCPA at various wavelengths. The mid-IR OPCPA system produces 2.6 mJ, 39 fs, 2.1 $\mu$m pulses with good beam quality ( $M^2 ~=1.5$ ) at 1 kHz repetition rate. The output pulses of the OPCPA are used to generate high-order harmonics in both gas cell and hollow-core fiber targets. A photon flux of $~10^6$ photon/s/1% bandwidth at 160 eV in Ar is measured while the cutoff is 190 eV. The direct measurements of the photon flux from x-ray photodiodes have confirmed the generation of water-window soft x-ray photons with a flux $~10^6$ photon/s/1% bandwidth at 330 eV in Ne. The demonstrated OPCPA and Yb:YAG pump laser technologies provide an excellent platform of energy and power scalable few-cycle mid-IR sources that are suitable for high-flux tabletop coherent soft x-ray generation.

A. J. Lohn, B. L. Doyle, G. J. Stein, P. R. Mickel, J. E. Stevens, and M. J. Marinella. Rutherford forward scattering and elastic recoil detection (RFSERD) as a method for characterizing ultra-thin films. Nuclear Inst. and Methods in Physics Research, B, 332, 2014.


URL,
We present a novel ion beam analysis technique combining Rutherford forward scattering and elastic recoil detection (RFSERD) and demonstrate its ability to increase efficiency in determining stoichiometry in ultrathin (5–50 nm) films as compared to Rutherford backscattering. In the conventional forward geometries, scattering from the substrate overwhelms the signal from light atoms but in RFSERD, scattered ions from the substrate are ranged out while forward scattered ions and recoiled atoms from the thin film are simultaneously detected in a single detector. The technique is applied to tantalum oxide memristors but can be extended to a wide range of materials systems.

K.-H. Hong, C.-J. Lai, J. P. Siqueira, P. R. Krogen, J. Moses, C.-L. Chang, G. J. Stein, L. E. Zapata, and F. X. Kärtner. Multi-mJ, kHz, 2.1  $\mu$m optical parametric chirped-pulse amplifier and high-flux soft x-ray high-harmonic generation. Optics Letters, 39(11), 2014.


URL,
We report on a multi-mJ 2.1 $\mu$m optical parametric chirped-pulse amplification (OPCPA) system operating at 1 kHz repetition rate, pumped by a picosecond cryogenic Yb:YAG laser, and the demonstration of soft x-ray high-harmonic generation (HHG) with a flux of ∼2×10^8  photon/s/1% bandwidth at 160 eV in Ar. The 1 kHz cryogenic Yb:YAG pump laser amplifies pulses up to 56 mJ and delivers compressed 42 mJ, 17 ps pulses to the 2.1 $\mu$m OPCPA system. In the three-stage OPCPA chain, we have obtained up to 2.6 mJ of output energies at 2.1 $\mu$m and pulses compressed to 40 fs with good beam quality. Finally, we show cut-off extension of HHG driven by this 2.1 $\mu$m source in Ar and N2 gas cells to 190 eV with high photon flux. Our 3D propagation simulation confirms the generation of soft x-ray attosecond pulses from the experiment with Ar.

Conference [ 15 ]

K. Murari, G. J. Stein, H. Cankaya, B. Debord, F. Gerome, G. Cirmi, O. D. Muecke, P. Li, A. Ruehl, I. Hartl, K.-H. Hong, F. Benabid, and F. X. Kärtner. Kagome fiber based nonlinear pulse compression of 1.8 ps to 250 fs at 2.05 $\mu$m from Ho:YLF amplifier. Conference on Lasers and Electro-Optics, 2016.


URL, DOI,
We present Ar-filled Kagome fiber based nonlinear pulse compression of 225 $\mu$J, 1.8 ps pulses from a Ho:YLF amplifier to 285 fs at 125 $\mu$J, supporting a FL duration of 250 fs at 2.05 $\mu$m.

P. D. Keathley, G. J. Stein, P. Krogen, H. K. Liang, J. Siqueira, C.-L. Chang, C.-J. Lai, K.-H. Hong, G. M. Laurent, and F. X. Kaertner. Water-Window Soft X-ray High-Harmonic Generation up to the Nitrogen K-edge Driven by a kHz, 2.1 $\mu$m OPCPA Source. High-Brightness Sources and Light-Driven Interactions, 2016.


URL, DOI,
Water-window soft-X-ray harmonics up to $\approx$450 eV are generated with a kHz, multi-mJ mid-infrared optical parametric chirped-pulse amplifier (OPCPA), in a neon-filled gas cell. The photon flux at the nitrogen K-edge is $\approx 1\times10^6$ photons/s/1% bandwidth.

K.-H. Hong, C.-L. Chang, P. Krogen, H. Liang, G. J. Stein, J. Moses, C.-J. Lai, and F. X. Kaertner. Multi-mJ, kHz picosecond deep UV source based on a frequency-quadrupled cryogenic Yb:YAG laser. Proceedings of SPIE, 9513, 2015.


URL,
We report on the development of a 2.74-mJ, ~4.2 ps, ~258 nm deep-ultraviolet (DUV) source at 1 kHz based on frequency quadrupling of ~32 mJ, 8.4 ps, ~1030 nm near-infrared (NIR) laser pulses with an excellent beam profile, generated from a diode-pumped, ultrafast hybrid Yb-doped chirped-pulse amplification laser system. We have used a two-stage second harmonic generation scheme at LBO (NIR-to-green) and BBO crystals (green-to-DUV), respectively, to achieve the fourth-harmonic generation (FHG). The NIR-to-DUV conversion efficiency of ~10% in the FHG is obtained. The peak power of the produced DUV laser pulses is as high as 0.56 GW. The beam profiles at near-field and far-field are found to be excellent and the M2 value is measured as ~2.6. We also present the systematic parameter study on the optimization of DUV generation. To our best knowledge, this is the most energetic DUV generation from a diodepumped solid-state laser at kHz repetition rates.

H. K. Liang, P. R. Krogen, R. Grynko, O. Novak, C.-L. L. Chang, G. J. Stein, D. Weerawarne, B. Shim, F. X. Kaernter, and K.-H. Hong. Mid-IR Filamentation in Dielectrics: 3-octave-spanning Supercontinuum Generation and Sub-2-cycle Self-compression. CLEO: 2015, 2015.


URL, DOI,
We report on 3-octave supercontinuum generation from ZnS in the normal dispersion regime and self-compression of mid-IR pulses to sub-2-cycle in CaF2 in the anomalous dispersion regime. Temporal characterization shows good agreement with 3D simulations.

C.-L. L. Chang, P. R. Krogen, H. Liang, G. J. Stein, J. Moses, C.-J. Lai, J. P. Siqueira, L. E. Zapata, F. X. Kaertner, and K.-H. Hong. Multi-mJ, kHz, intense picosecond deep-ultraviolet source. CLEO: 2015, 2015.


URL, DOI,
We demonstrate a 2.74 mJ, 1 kHz, ~4.2 ps DUV laser at ~257.7 nm with M$^2$~2.54 from a frequency-quadrupled hybrid Yb-doped chirped-pulse amplifier. An infrared-to-DUV conversion efficiency of ~10% is achieved.

G. J. Stein, C.-J. Lai, P. D. Keathley, P. R. Krogen, H. Liang, C.-L. L. Chang, K.-H. Hong, G. Laurent, and F. X. Kaertner. Versatile Simulation Package for Ultrafast Pulse Propagation and High Harmonic Generation. CLEO: 2015, 2015.


URL,
A simulation package for 3D pulse propagation and HHG is reported. Our package is capable of emulating pulse propagation for a host of geometries and nonlinear effects, and has been employed to reproduce experimental HHG spectra.

K. Murari, H. Cankaya, B. Debord, P. Li, G. Cirmi, S. Fang, G. M. Rossi, O. D. Muecke, P. Kroetz, G. J. Stein, A. Ruehl, I. Hartl, F. Gérôme, F. Benabid, and F. X. Kaertner. Sub-300 fs, 0.5 mJ Pulse at 1kHz from Ho:YLF Amplifier and Kagome Pulse Compression. CLEO: 2015, 2015.


URL,
We demonstrate a compact 290 fs, 0.5 mJ laser source at 2-mm wavelength generated from mJ-level 3.4-ps pulses from a fiber laser seeded Ho:YLF regenerative amplifier system via pulse compression in a gas-filled Kagome type HC-PCF.

H. Liang, P. R. Krogen, R. Grynko, O. Novak, C.-L. L. Chang, G. J. Stein, D. Weerawarne, B. Shim, F. X. Kärtner, and K.-H. Hong. 3-octave Supercontinuum Generation and Sub-2-cycle Self-compression of Mid-IR Filaments in Dielectrics. Advanced Solid State Lasers, 2014.


URL,
We demonstrate more than 3-octave supercontinuum for the first time from ZnS in the normal dispersion regime and self-compression of the mid-IR pulse to sub-2-cycle for the first time in CaF2 in the anomalous dispersion regime.

G. J. Stein, K. H. Hong, C.-J. Lai, J. Siqueira, P. Krogen, J. Moses, C.-L. Chang, and F. X. Kärtner. High-flux soft X-ray high-harmonic generation driven by a multi-mJ, kHz, 2.1 $\mu$m optical parametric chirped-pulse amplifier. Gordon Research Conference on Multiphoton Processes, 2014.



We experimentally demonstrate soft x-ray, high harmonic generation (HHG) in Ar and N2, driven by a multi-mJ 2.1 $\mu$m optical chirped-pulse amplification (OPCPA) system. Pumped by a picosecond cryogenic Yb:YAG laser, which delivers compressed 42 mJ, 17 ps pulses, the OPCPA system has produced up to 2.6 mJ of output energies at 2.1 µm with kHz repetition rates and pulses compressed down to 40 fs with good beam quality. With such a system, we show a cutoff extension up to 190 eV in Ar and N2 filled gas cells with high photon flux, reaching ~$2×10^8$ photons/s/1% bandwidth at 160 eV in Ar. Finally, our 3D propagation simulation is in good agreement with experimental results and confirms the generation of a soft x-ray attosecond pulse train from the experiment with Ar.

P. R. Krogen, H. Suchowski, G. J. Stein, F. X. Kärtner, and J. Moses. Tunable Few-Cycle Mid-IR Pulses towards Single-Cycle Duration by Adiabatic Frequency Conversion. 19th International Conference on Ultrafast Phenomena, 2014.


URL,
Using adiabatic difference frequency generation, we generate Fourier-limited, few-cycle, tunable 2-4 $\mu$m mid-IR pulses at $\mu$J-level, with controllable amplitude and phase by shaping before conversion from the near-IR, paving the way for arbitrary single-cycle mid-IR waveforms.

P. Krogen, H. Suchowski, G. J. Stein, F. Kärtner, and J. Moses. Tunable and Near-Fourier-limited Few-Cycle Mid-IR Pulses via an Adiabatically Chirped Difference Frequency Grating. Conference on Lasers and Electro-Optics: 2014, Optical Society of America, 2014.


URL,
A $\mu$J-level source of few-cycle, mid-IR pulses tunable over 2-4 microns is demonstrated based on adiabatic difference frequency generation. This opens up the possibility for single-cycle mid-IR pulses controllable by a near-IR phase shaper.

C.-L. L. Chang, K.-H. Hong, P. R. Krogen, H. Liang, G. J. Stein, J. M. Moses, C.-J. Lai, J. P. Siqueira, L. E. Zapata, and F. X. Kärtner. Multi-mJ, kHz intense picosecond deep ultraviolet source based on a frequency-quadrupled cryogenic Yb:YAG laser. Advanced Solid State Lasers, 2014.


URL,
We report on a 183-MW, 1-kHz, 9-ps, 1.65-mJ deep ultraviolet laser at 257.25 nm based on a frequency-quadrupled, cryogenic Yb:YAG chirped-pulse amplifier and an overall nonlinear conversion efficiency of ~5%, scalable to ~25%.

G. J. Stein, J. E. Stevens, M. Marinella, and B. L. Doyle. Optimizing the Composition Measurement of Ultra-Thin Metal Oxide Films Through the Combination of Recoil and Forward Scattering. 22nd Conference on the Application of Accelerators in Research and Industry, 2012.


URL,
Ion Beam Analysis (IBA) has routinely been used to characterize this composition of thin films because of its broad elemental sensitivity and its quantitative approach. Sandia has recently developed a new IBA technique especially well suited for the composition measurement of metal-oxide memristive materials, particularly Ta2O5-x that combines simultaneous Rutherford Forward Scattering (RFS) and Elastic Recoil Detection (ERD), called RFSERD. This new technique utilizes high energy heavy ions from tandem-style accelerators, and a standard ERD geometry where both the heavy ions that are scattered by the sample atoms, and the atoms recoiled by the Si beam are energy analyzed using a single surface barrier detector mounted in the forward direction. Before detection, all of the scattered heavy ions and target recoils pass through a Mylar range foil. This talk will describe a theoretical treatment of the RFSERD ion-atom collision physics that has led to the optimal selection, for a 6MV tandem accelerator, of the incident beam ion (Si or Cl), it's energy (~46 MeV), range foil thickness (~13 microns) and scatter/recoil angle (30 degrees). This treatment has also indicated that the reproducibility of the scattering angle dominates the propagation of error in the measurement of stoichiometry, and suggests the need for control of this angle to ~0.1 degree or standards when the RFSERD technique is used. We will also use this ion-atom collision theory to suggest optimum beam and detector geometries for a wide range of other particle accelerators typically used in IBA labs. * Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

J. Yoo, M. Yamada, G. J. Stein, H. Ji, and T. Tharp. Measurement of the in-plane ion flow and space potential profiles in the diffusion region of magnetic reconnection. American Geophysical Union 2011 Fall Meeting, 2011.


URL,
The Magnetic Reconnection Experiment (MRX) is a toroidal device dedicated to the study of magnetic reconnection in the laboratory. Here, we have modified the standard operation of MRX to include additional internal Shaping Field coils which control the current sheet motion. Controlled sweeping of the current sheet enables us to measure radial profiles of various physical quantities across the diffusion region with probes that contain only a single measurement point. With the help of this current sheet jogging, in-plane ion velocity and electric field profiles are measured using 5-tip Langmuir-Mach probe assemblies. The ion outflow is measured to reach speeds up to 40 km/s, about the half of the Alfven velocity. The electric field is simultaneously measured and points downstream (in the outflow direction) with a magnitude of 300-700 V/m. Radial floating potential profiles are measured using a floating potential array and additional Langmuir probes. Similar to those seen in numerical simulations and space observations, potential wells develop across the diffusion region. The potential well becomes deeper and broader further downstream in the outflow direction: the magnitude of the potential well is about 10-15V at the center and 15-35V downstream. The magnitude of this potential well is related to the dip in the sum of magnetic pressure and electron pressure, indicating ions are heated by the potential well.

G. J. Stein, J. Yoo, M. Yamada, H. Ji, S. Dorfman, E. Lawrence, C. Myers, and T. Tharp. Measurement of ion velocity profiles in a magnetic reconnection layer via current sheet jogging. 53rd Annual Meeting of the APS Division of Plasma Physics, 2011.


URL,
In many laboratory plasmas, constructing stationary Langmuir and Mach probe arrays with resolution on the order of electron skin depth is technically difficult, and can introduce significant plasma perturbations. However, complete two- dimensional profiles of plasma density, electron temperature, and ion flow are important for studying the transfer of energy from magnetic fields to particles during magnetic reconnection. Through the use of extra ``Shaping Field'' coils in the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physics Laboratory, the inward motion of the current sheet in the reconnection layer can be accelerated, or ``jogged,'' allowing the measurement of different points across the sheet with stationary probes. By acquiring data from Langmuir probes and Mach probes at different locations in the MRX with respect to the current sheet center, profiles of electron density and temperature and a vector plot of two-dimensional ion velocity in the plane of reconnection are created. Results from probe measurements will be presented and compared to profiles generated from computer simulation.