Prof. Pisin Chen Wins 2024 Chandrasekhar Prize

Prof. Pishin Chen has been selected by the Association of Asia-Pacific Physical Societies Division of Plasma Physics (AAPPS-DPP) as the 11th Laureate of Chandrasekhar Prize of Plasma Physics. This award was named after Indian American physicist and 1984 Nobel Laureate in Physics Subrahmanyan Chandrasekhar. AAPPS is a coalition of physics societies from 20 countries and regions across the Asia-Pacific, including Australia, New Zealand, and Taiwan. Together with the American Physical Society and the European Physical Society, it is one of the world's most important physics societies. This recent achievement follows Prof. Chen's receipt of the European Physical Society's Hannes Alfvén Prize in 2023. The Chandra Prize is generally awarded to only one recipient per year, making it a highly prestigious international academic honor due to its rigorous selection process.

Similar to his receipt of the Hannes Alfvén Prize in 2023, Prof. Chen was once recognized for the invention of the plasma wakefield accelerator (PWFA) concept in 1985. In this paper, he and co-authors demonstrated that a relativistic charged particle beam traversing a plasma can be equally effective in exciting plasma waves as that by a laser pulse (LWFA), which was proposed by Tajima and Dawson in 1979. By now, LWFA and PWFA have become the two major plasma accelerator schemes under active pursuit worldwide. Historically, the term "plasma wakefield" was born because of the PWFA invention. The concept of PWFA, and its first demonstration, was achieved in the 1980s, explored in the decades since, and today, a vibrant community drives forward their development and exploitation at numerous smaller, medium-sized and large laboratories, including at CERN. The physics of PWFA is analogous to that of ducklings riding on the water wake induced by the mother duck (see Fig.1).

The particle beam driven Plasma Wakefield Accelerator (PWFA) is analogous to what happens in Fig. 1. The mother duck exerts her energy to pump the water wakes, while the ducklings enjoy the free ride by sitting on the crest of the wake. In PWFA, a higher current charged particle beam injected into a plasma would excite the plasma wakefields, while a lower current beam trailing with a proper distance would pick up the energy and be accelerated.

In the following two years since this seminal paper, Prof. Chen published two more seminal papers that have further developed the theoretical foundation of PWFA. In his 1986 paper, he demonstrated mathematically that the energy gain restricted by the Fundamental Theorem of Beam Loading can be ameliorated through the breaking of the head-tail symmetry of the driving beam density profile. He further introduced a new theorem, Theorem of Optimal Transformer Ratio, that provides the optimum beam density profile for the maximum transformer ratio (the ratio of the maximum accelerating wakefield behind the driving beam to the maximum retarding field experienced by the driving beam). He further contributed to the understanding of critical beam dynamics issues in plasma wakefield accelerators, including beam loading, dephasing, phase slippage, pump depletion, etc. Then in 1987, he discovered the relativistic particle beam self-induced plasma focusing effect, and, based on that, proposed a plasma lens for the final focus in future high energy linear colliders. These three papers completed the trilogy of the PWFA principle.

He extended himself from a pure theorist to an experimentalist in the 1990s to lead the SLAC-E150 Plasma Lens Experiment at Stanford University. In 2000, his experiment observed for the first time the plasma lensing of not only high energy electron beams, but also positron beams with a focusing strength more than a thousand times stronger than that of the conventional magnets, as he predicted in 1987. The invention of PWFA was pathbreaking in the energy frontier, while the discovery and application of plasma self-focusing was pathbreaking in the luminosity frontier. Both are equally essential to future high energy physics.

Concurrent with and independent of Prof. Tajima’s publication, Prof. Chen proposed in 1987 the use of conduction-band electrons in solid state material such as metals to excite plasma wakefields by x-rays to accelerate particles along crystal channels. Such nano-scale plasma wakefield accelerators promise to offer an acceleration gradient that is two-to-three orders of magnitude even higher than that in LWFA and PWFA based on gaseous plasmas.

Trained as a theoretical particle physicist, Professor Chen has made prolific contributions across many fields of physics, including particle astrophysics, cosmology, and gravity, in particular classical and quantum black hole physics. While pursuing these fields, he always connected them with the plasma wakefield principle. In astrophysics, he, together with Prof. Toshiki Tajima, proposed in 2002 plasma wakefield acceleration as the origin of the observed ultra-high energy cosmic rays. In 2017, he proposed, with Prof. Gerard Mourou, the 2018 Nobel Laureate, to accelerate laser-induced relativistic flying plasma mirror by tailoring the target density, which acts as an analog black hole to investigate the celebrated black hole Hawking radiation and its associated information loss paradox in the laboratory setting (see Fig.2). Currently, he leads the international AnaBHEL (Analog Black Hole Evaporation via Lasers) Collaboration that includes research teams from Taiwan, France, and Japan, to carry out such an experiment. These efforts have helped to extend the role of plasma physics in tackling critical issues in other frontier fields of physics.