Optical Visualization of Beam-driven Plasma Wakefield Accelerators

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This research project aims to create and diagnose pliable, microscopic structures made of ionized gas, or "plasma", that are suitable for accelerating, shaping and bunching beams of electrons and their antimatter twins called "positrons" far more compactly and cheaply than conventional accelerators. Nearly all of the world's 30,000 conventional particle accelerators, which serve industry, hospitals and discovery science, accelerate ordinary matter particles, such as electrons and protons. A mere handful of mega-facilities accelerate parallel matter/antimatter particle beams - such as electrons and positrons, or protons and anti-protons. When collided with each other, such twin beams yield a treasure trove of new particles, with exotic names like "W", "Z" and "Higgs" bosons, which have reaped several Nobel prizes. Unfortunately, the technology underlying such past breakthroughs has become too large and expensive to support new breakthroughs. In this project, an energetic electron bunch racing through a pipe filled with lithium gas creates and energizes short-lived plasma filaments only a yard long, and thinner than a human hair, just as a boat racing across a pond creates a wake. This "wake" can accelerate both electrons and positrons to the same energy as a mile-long conventional accelerator as wide as an 18-wheeler. Computer simulations show that the part of the plasma wake 1/1000 inch behind the electron “boat” is a good electron accelerator, whereas the part one inch behind is a good positron accelerator. Flash holograms of these parts of the wake test the simulations. Moreover, carefully timed electron and positron bunches can be surfed on the wake, to test its viability as an accelerator directly. Success could lead not only to compact, affordable technology for future particle physics discoveries, but to new medical and industrial applications of positron imaging that take advantage of the unique intensity, narrowness and energy uniformity of positron beams from accelerators.The 3-year project carries out approved experiment E-324 "Optical visualization of beam-driven plasma wakefield accelerators" at the 2nd generation SLAC Facility for Advanced Accelerator Science and Experimental Tests (FACET-II). FACET-II's 10 GeV electron bunches drive strongly nonlinear plasma wakes in a meter-long lithium plasma of density 10^17 electrons per cm^3. A 100 fs, near-infrared, near-co-propagating optical probe pulse, synchronized with the electron bunch, impinges on the bunch's path at grazing angle at time delays ranging from 0 to 50 ps, and diffract from the wake. Downstream detectors record the probe's diffraction pattern, from which the wake's evolving electron density profile can be reconstructed and compared to predictions of computer simulations. Scientific goals are: (1) to observe a sharp on-axis ion density peak that computer simulations predict to form at ~50 ps; (2) to excite and diagnose an electron wake in this ion density structure with a secondary electron bunch, and to test its predicted suitability for stable positron acceleration; (3) to observe the bubble-shaped electron wake at delays below 1 ps behind the primary electron bunch, which is well suited for accelerating electrons, for the first time. The intellectual merit consists in observing particle-beam-driven plasma wakes for the first time, and in creating and identifying complementary plasma structures suitable for electron and positron acceleration, thereby paving the way for a dual plasma-based electron-positron accelerator. The broader impacts include development of compact, affordable accelerator technology and training of the next generation workforce, including graduate and undergraduate students with diverse backgrounds.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.