Donald Umstadter Publications

Laser-Wakefield Accelerators: Glass-guiding benefits

Donald P. Umstadter — Wed, 28 Nov 2012 13:34:54 PST

A main attraction of laser-driven electron accelerators is their absence of cavity walls, which can break down in the presence of intense electric fields. Now it seems that the inclusion of a hollow glass fibre cavity could lead to more efficient acceleration at lower laser intensities. ... Further research will reveal which of the above methods for guiding light will ultimately prove best for future accelerator designs. In any case, although it is perhaps ironic that the relatively old technology of glass waveguides may benefit next-generation accelerators, it is nonetheless satisfying to see such a classic photonic solution come to the aid of a current research problem.

Submillimeter-resolution radiography of shielded structures with laser-accelerated electron beams

Viswanathan Ramanathan et al. — Fri, 19 Oct 2012 09:46:55 PDT

We investigate the use of energetic electron beams for high-resolution radiography of flaws embedded in thick solid objects. A bright, monoenergetic electron beam (with energy >100 MeV) was generated by the process of laser-wakefield acceleration through the interaction of 50-TW, 30-fs laser pulses with a supersonic helium jet. The high energy, low divergence, and small source size of these beams make them ideal for high-resolution radiographic studies of cracks or voids embedded in dense materials that are placed at a large distance from the source. We report radiographic imaging of steel with submillimeter resolution.

Generation of tunable, 100–800 MeV quasi-monoenergetic electron beams from a laser-wakefield accelerator in the blowout regime

Sudeep Banerjee et al. — Fri, 19 Oct 2012 09:43:34 PDT

In this paper, we present results on a scalable high-energy electron source based on laser wakefield acceleration. The electron accelerator using 30–80 TW, 30 fs laser pulses, operates in the blowout regime, and produces high-quality, quasi-monoenergetic electron beams in the range 100–800 MeV. These beams have angular divergence of 1–4 mrad, and 5%–25% energy spread, with a resulting brightness 10¹¹ electrons mm^-2 MeV^-1 mrad^-2. The beam parameters can be tuned by varying the laser and plasma conditions. The use of a high-quality laser pulse and appropriate target conditions enables optimization of beam quality, concentrating a significant fraction of the accelerated charge into the quasi-monoenergetic component.

Electron self-injection into an evolving plasma bubble: Quasi-monoenergetic laser-plasma acceleration in the blowout regime

S. Y. Kalmykov et al. — Fri, 19 Oct 2012 09:39:41 PDT

An electron density bubble driven in a rarefied uniform plasma by a slowly evolving laser pulse goes through periods of adiabatically slow expansions and contractions. Bubble expansion causes robust self-injection of initially quiescent plasma electrons, whereas stabilization and contraction terminate self-injection thus limiting injected charge; concomitant phase space rotation reduces the bunch energy spread. In regimes relevant to experiments with hundred terawatt- to petawatt-class lasers, bubble dynamics and, hence, the self-injection process are governed primarily by the driver evolution. Collective transverse fields of the trapped electron bunch reduce the accelerating gradient and slow down phase space rotation. Bubble expansion followed by stabilization and contraction suppresses the low-energy background and creates a collimated quasi-monoenergetic electron bunch long before dephasing. Nonlinear evolution of the laser pulse (spot size oscillations, self-compression, and front steepening) can also cause continuous self-injection, resulting in a large dark current, degrading the electron beam quality.

Background-Free, Quasi-Monoenergetic Electron Beams from a Self-Injected Laser Wakefield Accelerator

Sudeep Banerjee et al. — Fri, 19 Oct 2012 09:36:26 PDT

Stable 200–400-MeV quasi-monoenergetic electron bunches (ΔE/E<10%), ~ 10-pC charge, and no dark-current are produced when a self-injected laser plasma accelerator is optimized. PIC simulations demonstrate these beams are produced near the threshold for selfinjection.

High-energy Laser-accelerated Electron Beans for Long-range Interrogation

Nathaniel Cunningham et al. — Fri, 19 Oct 2012 09:30:14 PDT

We are studying the use of 0.1 - 1.0 GeV laser-accelerated electron beams as active interrogation probes for long-standoff radiography or nuclear activation of concealed special nuclear material. Use of beams in this energy range is largely unexplored, but such beams could provide notable advantages over lower-energy beams and x-rays. High-energy laser-accelerated electrons exhibit large penetration range through air and solids, and low beam divergence for both direct beams and secondary Bremsstrahlung x-rays. We present laboratory measurements of radiography and activation using the high power Diocles laser system at the University of Nebraska, as well as MCNP and GEANT Monte Carlo simulation results used to aid experiment design and interpretation.

Development of a Source of Quasi-Monochromatic MeV Energy Photons

Donald Umstadter et al. — Fri, 19 Oct 2012 09:14:06 PDT

We report current progress on a project to develop an all-optically-driven x-ray photon source. A laser pulse with 40-50 TW of peak power is focused on a supersonic helium nozzle to drive a relativistic plasma wave. Electron beams with energies of 320 MeV (+/- 28 MeV) are accelerated by means of laser wakefield acceleration. Remarkably, the acceleration region is only 3 mm in length. This accelerator is currently being employed to demonstrate the generation of MeV- energy x-ray by means of all-optical Thomson scattering. By this mechanism, a lower power, laser pulse (from the same laser system) is focused onto the above laser-driven electron beam, 1-eV energy photons are Doppler-shifted in energy to > 1 MeV.

Ultrashort Ultraviolet Free-Electron Lasers

Donald Umstadter et al. — Fri, 16 Dec 2011 11:42:24 PST

In this work we combine elements of chirped pulse amplification (CPA) techniques, now familiar in solid-state lasers, with an amplifier based upon a seeded free-electron laser (FEL), The resulting device would produce amplified pulses of unprecedented brevity at wavelengths shorter than can be currently obtained by any tunable laser system. We use a subharmonically seeded FEL to illustrate the concept. Radiation from a Ti:sapphire laser is frequency-tripled and stretched optically to provide a coherent seed pulse for the FEL. When coupled to an electron beam inside a magnetic wiggler, the seed radiation introduces an additional energy modulation on the electron bunch, which has been prepared with an energy chirp to match the chirp in the optical pulse. The energy modulated electrons are then spatially bunched in a dispersion magnet and introduced to a wiggler configured to be resonant to a harmonic of the seed laser, providing additional frequency multiplication. The coherent radiation produced by these electrons is amplified as it traverses the wiggler and recompressed optically. The preservation of phase coherence provided by this scheme results in a device which can yield 4-fs pulses with 0.3 mJ at a central wavelength of ca. 88 Am, easily the shortest duration amplified pulses produced by any laser. In this paper, we discuss various aspects of the concept, including the generation of short pulses, tempOral stretching and compression, and potential applications of the device. The phase distortion during the wide bandwidth FEL amplification is discussed in detail, and is shown to be within the bounds required to produce a 4-fs pulse upon compression.

High-energy ion generation in interaction of short laser pulse with high-density plasma

Y. Sentoku et al. — Thu, 24 Jan 2008 08:47:41 PST

Multi-MeV ion production from the interaction of a short laser pulse with a high-density plasma, accompanied by an underdense preplasma, has been studied with a particle-in- cell simulation and good agreement is found with experiment. The mechanism primarily responsible for the acceleration of ions is identified. Comparison with experiments sheds light on the ion-energy dependence on laser intensity, preplasma scale length, and relative ion energies for a multi-species plasma. Two regimes of maximum ion-energy dependence on laser intensity, I, have been identified: subrelativistic, μ I ; and relativistic, μ √I. Simulations show that the energy of the accelerated ions versus the preplasma scale length increases linearly and then saturates. In contrast, the ion energy decreases with the thickness of the solid-density plasma.

Experimental observation of relativistic nonlinear Thomson scattering

Szu-yuan Chen et al. — Thu, 24 Jan 2008 08:36:44 PST

Classical Thomson scattering—the scattering of low-intensity light by electrons—is a linear process, in that it does not change the frequency of the radiation; moreover, the magnetic-field component of light is not involved. But if the light intensity is extremely high (~10¹⁸ Wcm^–2), the electrons oscillate during the scattering process with velocities approaching the speed of light. In this relativistic regime, the effect of the magnetic and electric fields on the electron motion should become comparable, and the effective electron mass will increase. Consequently, electrons in such high fields are predicted to quiver nonlinearly, moving in figure-eight patterns rather than in straight lines. Scattered photons should therefore be radiated at harmonics of the frequency of the incident light, with each harmonic having its own unique angular distribution. Ultrahigh- peak-power lasers offer a means of creating the huge photon densities required to study relativistic, or “nonlinear” (ref. 6), Thomson scattering. Here we use such an approach to obtain direct experimental confirmation of the theoretical predictions of relativistic Thomson scattering. In the future, it may be possible to achieve coherent generation of the harmonics, a process that could be potentially utilized for “table-top” X-ray sources.

Relativistic laser–plasma interactions

Donald Umstadter — Thu, 24 Jan 2008 08:33:52 PST

By focusing petawatt peak power laser light to intensities up to 10²¹ Wcm⁻², highly relativistic plasmas can now be studied. The force exerted by light pulses with this extreme intensity has been used to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. This acceleration gradient is a thousand times greater than in radio-frequency-based accelerators. Such novel compact laser-based radiation sources have been demonstrated to have parameters that are useful for research in medicine, physics and engineering. They might also someday be used to ignite controlled thermonuclear fusion. Ultrashort pulse duration particles and x-rays that are produced can resolve chemical, biological or physical reactions on ultrafast (femtosecond) time scales and on atomic spatial scales. These energetic beams have produced an array of nuclear reactions, resulting in neutrons, positrons and radioactive isotopes. As laser intensities increase further and laser-accelerated protons become relativistic, exotic plasmas, such as dense electron–positron plasmas, which are of astrophysical interest, can be created in the laboratory. This paper reviews many of the recent advances in relativistic laser–plasma interactions.

Plasma density gratings induced by intersecting laser pulses in underdense plasmas

Z.-M. Sheng et al. — Thu, 24 Jan 2008 08:31:06 PST

Electron and ion density gratings induced by two intersecting ultrashort laser pulses at intensities of 10¹⁶ W/cm² or lower are investigated. The ponderomotive force generated by the inhomogeneous intensity distribution in the intersecting region of the interfering pulses produces deep electron and ion density modulations at a wavelength less than a laser wavelength in vacuum. Dependence of the density modulation on the plasma densities, temperatures, and the ion mass, as well as the laser pulse parameters are studied analytically and by particle-in-cell simulations. It is found that the density peaks of such gratings can be a few times that of the initial plasma density and last as long as a few picoseconds. It is also demonstrated that the scattering of signal pulses by such a bulk density grating results in high-harmonic generation. The density gratings may be incorporated into ion-ripple lasers [K.R. Chen and J.M. Dawson, Phys. Rev. Lett. 68, 29 (1992)] to generate ultrashort X-ray pulses of a few angstroms by using electron beams at only a few tens of MeV only.

An All Optical Laser Wakefield Electron Injector

Donald Umstadter — Thu, 24 Jan 2008 08:26:21 PST

The personnel who were supported by the grant included the P.I. (Prof. Umstadter), several research scientists (A. Maksimchuk and V. Yanovsky), a postdoc (P. Zhang) and several graduate and undergraduate students.
Although there were several setbacks in developing the novel laser technology required to produce a monoenergetic beam of electrons from an all-optical accelerator, several important steps were taken towards reaching that ultimate goal. The most important outcome of this project was that we demonstrated the principle of optical control of laser accelerators, namely, that one laser pulse could modify the properties (e.g., emittance and electron number) of an electron beam accelerated by a separate but synchronized laser pulse. Another recent highlight was that, using our new 304s 10-TW laser system, we accelerated with a laser accelerator an electron beam with a record low divergence (0.2 degrees). This is more than 100 times lower than the 30-degree divergence that was reported recently by a French group using a laser with similar parameters [V. Malka et al., Science, 298, 1596 (2002)]. A detailed discussion of the results of the project are presented below.

Study of Laser Plasma Interactions in the Relativistic Regime

Donald Umstadter — Thu, 24 Jan 2008 08:22:16 PST

We discuss the first experimental demonstration of electron acceleration by a laser wakefield over distances greater than a Rayleigh range (or the distance a laser normally propagates in vacuum). A self-modulated laser wakefield plasma wave is shown to have a field gradient that exceeds that of an RF Iinac by four orders of magnitude (E ≥ 200 GV/m) and accelerates electrons with over l-nC of charge per bunch in a beam with space-charge-limited emittance (1 mm-mrad). Above a laser power threshold, a plasma channel, created by the intense ultrashort laser pulse (I~ 4 x 10¹⁸ W/cm², λ = 1 μm, τ = 400 fs), was found to increase the laser propagation distante, decrease the electron beam divergence, and increase the electron energy. The plasma wave, directly measured with coherent Thomson scattering is shown to damp—due to beam loading-in a duration of 1.5 ps or ~ 100 plasma periods. These results may have important implications for the proposed fast ignitor concept.

Experimental observation of nonlinear Thomson scattering

Szu-yuan Chen et al. — Thu, 24 Jan 2008 08:15:30 PST

A century ago, J. J. Thomson showed that the scattering of low-intensity light by electrons was a linear process (i.e., the scattered light frequency was identical to that of the incident light) and that light’s magnetic field played no role. To- day, with the recent invention of ultra-high-peak- power lasers it is now possible to create a sufficient photon density to study Thomson scattering in the relativistic regime. With increasing light intensity, electrons quiver during the scattering process with increasing velocity, approaching the speed of light when the laser intensity approaches 10¹⁸ W/cm². In this limit, the effect of light’s magnetic field on electron motion should become comparable to that of its electric field, and the electron mass should increase because of the relativistic correction. Consequently, electrons in such high fields are predicted to quiver nonlinearly, moving in figure-eight patterns, rather than in straight lines, and thus to radiate photons at harmonics of the frequency of the incident laser light, with each harmonic having its own unique angular distribution. In this letter, we report the first ever direct experimental confirmation of these predictions, a topic that has previously been referred to as nonlinear Thomson scattering. Extension of these results to coherent relativistic harmonic generation may eventually lead to novel table-top x-ray sources.

Observation of relativistic cross-phase modulation in high-intensity laser-plasma interactions

Shouyuan Chen et al. — Thu, 05 Apr 2007 13:07:07 PDT

A nonlinear optical phenomenon, relativistic cross-phase modulation, is reported. A relativistically intense light beam (I=1.3×10¹⁸ W cm^-2, λ =1.05 μm) is experimentally observed to cause phase modulation of a lower intensity, copropagating light beam in a plasma. The latter beam is generated when the former undergoes the stimulated Raman forward scattering instability. The bandwidth of the Raman satellite is found to be broadened from 3.8–100 nm when the pump laser power is increased from 0.45–2.4 TW. A signature of relativistic cross-phase modulation, namely, asymmetric spectral broadening of the Raman signal, is observed at a pump power of 2.4 TW. The experimental cross-phase modulated spectra compared well with theoretical calculations. Applications to generation of high-power single-cycle pulses are also discussed.

TABLETOP LASER ACCELERATORS ARE ON THE WAY

Phillip F. Schewe et al. — Mon, 20 Nov 2006 12:15:22 PST

The goal here is to use high electric fields in plasmas to accelerate electrons to 100-GeV energies over distances of meters rather than kilometers. This should promote the development of new particle colliders and x-ray sources. The predicted high acceleration gradients in plasmas have been achieved in recent years, but could only be used with external electron injection from a conventional source. Now scientists at the University of Michigan (Donald Umstadter, 313-764-2284) have made progress in eliminating conventional electron sources altogether. In a preliminary experiment, by simply focusing a laser into a plasma, the Michigan scientists have extracted a collimated electron beam with multi-MeV energies and hope to have a beam of GeV electrons within a year. They also have a way of creating ultrashort bunches of electrons to make the highest quality electron beams (with much lower energy spread). First, they send a 100-fsec laser pulse into a gas, ionizing the gas and setting up a plasma wave. A second laser pulse, directed at right angles to the first, then induces nearby electrons to catch the plasma wave and ride with it synchronously to high energies.

Tabletop Accelerators are Brighter and Faster

Phil Schewe et al. — Mon, 20 Nov 2006 12:13:01 PST

At last week's APS plasma physics meeting, Donald Umstadter of the University of Michigan's Center for Ultrafast Optical Science (734-764-2284, dpu@umich.edu) reported on advances at his lab and elsewhere in tabletop laser accelerators, devices that use light to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. This acceleration rate or "gradient" is up to a thousand times larger than in conventional accelerators because the tabletop laser light can now exert pressures of gigabars, the highest ever achieved, and approaching the pressure of light near the Sun. Not only that, but Umstadter's lab has just shown that the brightness of a tabletop particle beam is roughly ten times higher than that produced by conventional accelerator technology. This is because laser accelerators can generate very narrow particle beams by focusing light on an extremely tiny spot in a gas target.

Study of Energetic Ion Generation from High-Intensity-Laser Dense-Plasma Interactions

K. Flippo et al. — Mon, 20 Nov 2006 12:09:52 PST

We report on the characteristics of an ultrafast-laser driven proton beam from thinfilm targets. The difference in proton beam profiles, beam energies, and laser induced back ablation plumes between a dielectric (Mylar) and a conductor (aluminum) are discussed. Evidence for front-side acceleration and a method for beam manipulation are also presented.

Status of the LILAC Experiment

N. Saleh et al. — Mon, 20 Nov 2006 12:03:44 PST

We present the status of the LILAC experiment [1], including results on the propagation of 30-fs duration laser pulses in plasmas of the requisite density, and measurements of the dark current [2]. We also discuss the status of a laser upgrade, an electron beam line and plans for the future.