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Our research program studies physics at the intersection of lasers, plasmas, and particle beams. We are primarily focused on the study and development of the plasma wakefield accelerator (PWFA) and closely related topics, such as the underdense plasma lens and the ion channel laser. The heart of our experimental program is carried out at SLAC National Accelerator Laboratory’s FACET-II facility, though we also carry out experiments in our own high-power laser laboratory on CU’s campus. In addition to our experimental work, we also perform theoretical and computational work to inform the planning of our experimental campaigns and to aid in the interpretation of our experimental results.

Plasma Wakefield Accelerator (PWFA) Research

Plasma wakefield accelerators (PWFAs) have been shown to provide high-gradient acceleration of electrons with high energy transfer efficiency. Preservation of the beam emittance (i.e. quality) represents the next significant step toward advancing PWFA research to becoming an application-ready technology. This is important for allowing the beam to lase in a free electron laser (FEL) or to maximize the luminosity of a high energy particle collider.

Emittance preservation is achieved through the matching of the beam’s divergence to the radial Coulomb force experienced within the PWFA such that the beam envelope does not undergo oscillations. This generally requires focusing of the electron beam to an extremely tiny size as it enters the PWFA plasma source. One way to achieve this is to create a gradual density ramp at the entrance to the plasma source in order to adiabatically focus the beam to the desired size.

PWFA Plasma Source Development

Diagnosis of the PWFA plasma source is non-trivial due to its narrow diameter and rapid degeneration. The plasma filament decays via hydrodynamic expansion and electron-ion recombination in only tens of nanoseconds, far faster than the typical electronic diagnostic response time. Our group has developed a method of accessing the nanosecond dynamics via imagery of the plasma glow utilizing inexpensive cameras with ten-microsecond integration time, providing a robust means of diagnostic access to the PWFA plasma source.

Advanced Electron Beam Diagnostic Development

The PWFA utilizes two electron beams, or bunches: one bunch in front that is used to drive the plasma, generating strong accelerating electromagnetic fields in its wake; and one trailing bunch that is accelerated by the plasma wakefields. The relative separation and transverse alignment of these two bunches is paramount to the performance of the PWFA, but is impossible to measure with conventional particle beam diagnostics without destroying the beam itself. To solve this problem, our group developed a diagnostic called the Electro-Optic Sampling Beam Position Monitor that leverages ultrafast optical signals to reconstruct the 3D profile of the two electron bunches.

Underdense Plasma Lens

Waves in plasmas can provide not only strong acceleration of electron beams, but also very strong focusing forces. The underdense plasma lens resembles a short (sub-millimeter) PWFA that imparts an intense focusing impulse on an electron beam with negligible energy modulation. Like the PWFA is to conventional particle accelerators, the underdense plasma lens is to its conventional counterpart, the electromagnet, providing orders of magnitude stronger focusing in an ultra-compact footprint.

Ion Channel Laser

The X-ray FEL revolutionized the capability of light sources, providing X-ray laser pulses with a brightness many orders of magnitude greater than the synchrotron light sources that came before it. However, these devices require a kilometer-scale electron accelerator to achieve the particle beam energies required to generate X-ray pulses. Plasma-based particle accelerators are on the precipice of being able to replace these large-scale accelerators, and have even demonstrated lasing in the ultraviolet regime, but they still struggle to produce electron beams with sufficiently low energy spread to reach the X-ray regime.

The ion channel laser is an alternative approach the reach the same ends by replacing the FEL’s magnetic undulator with an ion channel. In theory, it has the advantage of accommodating beams that can be generated in plasma-based accelerators today, but it has never been demonstrated. We aim to perform the first ion channel laser experiments at the FACET-II facility with the hope of opening the door to compact high-brightness X-ray laser sources.