Keith Jobe

Bio: Keith Jobe is an academic researcher from SLAC National Accelerator Laboratory. The author has contributed to research in topics: Particle accelerator & Laser. The author has an hindex of 17, co-authored 45 publications receiving 1126 citations. Previous affiliations of Keith Jobe include University College London & Stanford University.

High-Power Multimode X-Band RF Pulse Compression System for Future Linear Colliders

Abstract: We present a multimode $X$-band rf pulse compression system suitable for a TeV-scale electron-positron linear collider such as the Next Linear Collider (NLC). The NLC main linac operating frequency is 11.424 GHz. A single NLC rf unit is required to produce 400 ns pulses with 475 MW of peak power. Each rf unit should power approximately 5 m of accelerator structures. The rf unit design consists of two 75 MW klystrons and a dual-moded resonant-delay-line pulse compression system that produces a flat output pulse. The pulse compression system components are all overmoded, and most components are designed to operate with two modes. This approach allows high-power-handling capability while maintaining a compact, inexpensive system. We detail the design of this system and present experimental cold test results. We describe the design and performance of various components. The high-power testing of the system is verified using four 50 MW solenoid-focused klystrons run off a common 400 kV solid-state modulator. The system has produced 400 ns rf pulses of greater than 500 MW. We present the layout of our system, which includes a dual-moded transmission waveguide system and a dual-moded resonant line (SLED-II) pulse compression system. We also present data on the processing and operation of this system, which has set high-power records in coherent and phase controlled pulsed rf.

Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction

Abstract: Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions These findings benchmark and validate ab initio nonadiabatic dynamics calculations

Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules.

Abstract: Observing the motion of the nuclear wave packets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wave packet in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 A and temporal resolution of 230 fs full width at half maximum. The method is not only sensitive to the position but also the shape of the nuclear wave packet.

Demonstration of the Echo-Enabled Harmonic Generation Technique for Short-Wavelength Seeded Free Electron Lasers

Abstract: We report the first experimental demonstration of the echo-enabled harmonic generation technique, which holds great promise for generation of high-power, fully coherent short-wavelength radiation. In this experiment, coherent radiation at the 3rd and 4th harmonics of the second seed laser is generated from the so-called beam echo effect. The experiment confirms the physics behind this technique and paves the way for applying the echo-enabled harmonic generation technique for seeded x-ray free electron lasers.

Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses

Abstract: Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angstrom spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 A) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

Handbook of Chemistry and Physics

Abstract: There is a special reason for reviewing this book at this time: it is the 50th edition of a compendium that is known and used frequently in most chemical and physical laboratories in many parts of the world. Surely, a publication that has been published for 56 years, withstanding the vagaries of science in this century, must have had something to offer. There is another reason: while the book is a standard fixture in most chemical and physical laboratories, including those in medical centers, it is not as frequently seen in the laboratories of physician's offices (those either in solo or group practice). I believe that the Handbook can be useful in those laboratories. One of the reasons, among others, is that the various basic items of information it offers may be helpful in new tests, either physical or chemical, which are continuously being published. The basic information may relate

Theory and Design of Charged Particle Beams

Abstract: Review of Charged Particle Dynamics. Beam Optics and Focusing Systems Without Space Charge. Linear Beam Optics with Space Charge. Self-Consistent Theory of Beams. Emittance Variation. Beam Physics Research from 1993 to 2007. Appendices. List of Frequently Used Symbols. Bibliography. Index.

Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet

Abstract: Researchers demonstrate the FERMI free-electron laser operating in the high-gain harmonic generation regime, allowing high stability, transverse and longitudinal coherence and polarization control.

X-Rays and Extreme Ultraviolet Radiation: Principles and Applications

Abstract: This self-contained, comprehensive book describes the fundamental properties of soft x-rays and extreme ultraviolet (EUV) radiation and discusses their applications in a wide variety of fields, including EUV lithography for semiconductor chip manufacture and soft x-ray biomicroscopy. The author begins by presenting the relevant basic principles such as radiation and scattering, wave propagation, diffraction, and coherence. He then goes on to examine a broad range of phenomena and applications. The topics covered include EUV lithography, biomicroscopy, spectromicroscopy, EUV astronomy, synchrotron radiation, and soft x-ray lasers. He also provides a great deal of useful reference material such as electron binding energies, characteristic emission lines and photo-absorption cross-sections. The book will be of great interest to graduate students and researchers in engineering, physics, chemistry, and the life sciences. It will also appeal to practicing engineers involved in semiconductor fabrication and materials science.

X-ray free-electron lasers

Abstract: With intensities 108–1010 times greater than other laboratory sources, X-ray free-electron lasers are currently opening up new frontiers across many areas of science. In this Review we describe how these unconventional lasers work, discuss the range of new sources being developed worldwide, and consider how such X-ray sources may develop over the coming years.