I am a high-energy-density physicist. My major research interest is particle sources produced by laser-plasma interaction processes: electrons, photons, neutrons and ions generated by laser wakefield acceleration, laser-driven betatron radiation, and laser-driven fusion. I study them using a mix of experimental observations (especially developing observables and algorithms for laboratory diagnostics), theory modeling and simulations. Applications of these particle sources span upcoming industrial technologies such as semiconductor and medical imaging, medical isotope production, and nuclear waste transmutation, as well as fundamental physics investigations such as plasma properties and fusion cross sections under extreme conditions, nuclear isomer population, and particle radiation in strong fields. I am also interested in developing new methods for predicting the performance of secondary particle sources using machine learning/ artificial intelligence approaches.
Electron beams can be accelerated by laser-excited plasma waves to very high energies (known as laser wakefield acceleration) as well as by charge-separation fields especially at the boundary of high-density plasmas. Although my experimental work mainly focuses on the first acceleration mechanism, I study the general fundamental behaviors of electron beams under strong electromagnetic fields as well.
Wakefield accelerators generate electron beams whose spectra display internal structure and important phase space correlations. I created a statistical method for reconstructing the electron beam transverse phase space using magnetic spectrometers of a design commonly found in laser wakefield experiments.
For laser wakefield accelerators to become a viable alternative to radio-frequency accelerators, we need clearer, more quantitative relationships between the facility input parameters (laser energy, laser power, plasma density, etc.) that determine cost and the accelerator outcomes (especially particle energy and beam quality). I have used the ensemble of past experiments to estimate the performance envelope of a laser wakefield accelerator(paper), that is the maximum achievable outcome for given inputs. I have also extended the definition of collider luminosity to laser-electron collisions, which are hoped to provide a tunable and bright source of high-energy (>MeV) photons for both applications and experimental tests of radiation at ultra-high acceleration. The laser-electron collision luminosity(paper) metric enables design optimization for photon or secondary-event yield.
With collaborators in Brazil, I have proven the equivalence(paper) of accelerated frame and classical electrodynamics predictions for the electron recoil to radiation (as known as radiation reaction) under linear acceleration, identified a realistic observable, and evaluated the leading QED corrections. Wakefield accelerators appear to be best option to perform the relevant experiment, if stabilized and with enough statistics. I have also shown how choice of observables and laser fluctuations impact(paper) efforts to prove even basic theories of particle acceleration in laser fields.
Our experimental group (with simulation support from U. Strathclyde) used the Texas Petawatt laser to demonstrate laser wakefield acceleration of high-charge electron bunches to 10 GeV(paper) by injecting metallic nanoparticles into the plasma. My extended analysis of the data suggests hidden correlations in the electron beam phase space. The technology is now patented and jump-started efforts to commercialize laser wakefield accelerators.
Photons are usually generated in the following few scenarios: high-energy X-ray emission due to relativistic electrons oscillating in a self-generated magnetic field, known as betatron radiation; high-frequency coherent light production due to nonlinear interaction of the laser with the plasma, known as high-harmonic generation; general and continuous X-ray emission due to electron-ion collisions, known as Bremsstrahlung radiation; or quantum electrodynamics gamma emission due to plasma electrons interacting with strong classical electromagnetic fields, known as hard QED gamma rays. I have been focusing on the last, high-energy gamma ray-producing mechanism, aiming to answer the questions: what are the quantum properties of the many-body system under the influence of a strong classic electromagnetic background field, how can it be calculated consistently with the current quantum field theory formalism and what can we learn from it for a strong gravitational field?
Ultra-high intensity lasers irradiating plasma in an optimal density range are predicted to drive jet-like distributions of high-energy (>MeV) photons. With students at UT Austin, Los Alamos National Laboratory and Ludwig-Maxilimians University in Munich, we showed: (1) these jets can be produced with presently-available lasers(paper) and simple, unstructured target plasma, and (2) defining a jet observable(paper) opens the door to better measurements of the phenomenon and how it is controlled by laser and plasma initial conditions. These works have inspired a DOE-grant supported experimental program. I have summarized the challenges and needs(paper) to improve the theoretical description of radiation in strong-field plus plasma regime.
High flux neutrons and ions can be generated by laser-driven fusion processes.
In an experiment with SLAC, we obtained high neutron yields(paper) best explained by deuteron-deuteron fusion processes in a small domain of laser-heated plasma, suggesting peak neutron fluxes >1e22 per cm-squared per second. Such peak fluxes, especially when combined with a dense photon environment, may be reaching a threshold where novel studies of nuclear reaction cross sections become possible.
I used effective field theory methods to study the strong interactions(paper) known as quantum chromodynamics. I also studied nuclear physics phenomenology.