Long citation

Professors Pisin Chen, James Benjamin Rosenzweig and Chandrashekhar Janardan Joshi have explored the interaction of bunched particle beams – extreme forms of non-neutral plasmas – with plasma across wide parameter regimes. In particular, they have invented and pioneered beam-driven plasma wakefield acceleration (PWFA). In PWFA, intense particle beams (electrons, positrons or protons) excite strong plasma wave oscillations, which can provide focusing and accelerating electric fields three to four orders of magnitude larger than in conventional accelerators. Their applications range from compact, intense, relativistic electron beam sources to novel intense photon pulse sources to ultra-relativistic beams for high energy physics research. 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 CERN.

Pisin Chen is regarded as the inventor of PWFA, and has contributed numerous seminal theoretical publications on the plasma wakefield accelerator principle, energy transfer and beam loading, and plasma lenses at SLAC in the 1980s. The extreme gradients and energy gains obtainable from PWFA was a pathbreaking innovation at the energy frontier, and the extreme focusing with plasma lenses was a pathbreaking innovation at the luminosity frontier. Pisin led the E-150 Plasma Lens experiment at SLAC, that successfully demonstrated the plasma lens principle as predicted. He then went on to explore cosmology and gravity, and today, as Director of the Leung Center for Cosmology and Particle Astrophysics and after numerous honours such as the 2018 Blaise Pascal Chair, Ile de France, he works on bringing together plasma wakefield accelerators with black hole physics.

James Benjamin Rosenzweig has experimentally demonstrated PWFA for the first time in the 1980s, and is regarded as father of the non-linear “blowout” interaction regime, where the beam driver is so intense that it expels all plasma electrons and forms a spherical blowout with arising linear accelerating and focusing electric field structures. He contributed many theoretical and experimental foundations of the field and e.g. together with Chen and other pioneers, developed plasma lenses. He is a pioneer also in various other high-gradient accelerator technologies and light source development such as free-electron lasers, for which he received the 2007 International FEL prize. Jamie has not only held various fellowships himself, but has influenced the field at UCLA and facilities across the US such as SLAC, but also in Europe, and a remarkable large number of former students are now field leaders themselves.

Chandrashekhar Janardan Joshi has since the early 1980s contributed pioneering works to both, laser-driven plasma interaction as well as particle beam driven plasma wakefield interaction. Likewise at UCLA, he was able to translate many techniques and concepts from LWFA, where the plasma wave is driven by intense laser pulses, to PWFA, and pushed forward a pioneering program of PWFA R&D first with SLAC’s Final Focus Test Beam and then at the FACET facility. This included milestones such as energy doubling of 42 GeV electrons in the blowout regime in a meter-scale plasma, positron-driven PWFA, acceleration of positrons with PWFA, or plasma wigglers. This has set off the modern era of PWFA, which is now more vibrant and successful than ever. Chan has received various honours for his plasma research, such as the 1996 John Dawson Award for Excellence in Plasma Physics Research, or the 2006 James Clerk Maxwell Prize for Plasma Physics. Together, Chen, Rosenzweig and Joshi have pioneered and steered the field of particle beam-plasma interaction over many decades.

Long citation

Professors Tünde Fülöp and Per Helander have significantly advanced plasma theory through a number of fundamental contributions. Their research spans many topics including runaway electrons, kinetic instabilities and transport processes in magnetized plasmas. By pinpointing key physical mechanisms, they have catalyzed innovative developments in experimental devices for nuclear fusion applications.

Professor Tünde Fülöp has been a leader in the field of disruptions and relativistic “runaway” electrons associated with these events. She has systematically explored the physics of runaway electrons in tokamaks and beyond, and conducted unprecedented modelling efforts applicable to both existing experiments and future devices. As part of this endeavour, Professor Fülöp has investigated various facets of the problem, including runaway-electron-driven electromagnetic instabilities, the impact of different collision types, radiation reaction effects, and the role of partially ionized impurities on runaway electron dynamics. Additionally, she has evaluated and optimized disruption mitigation strategies involving external magnetic perturbations and massive material injection. Furthermore, Professor Fülöp has played a crucial role in overseeing the development of several open-source, state-of-the-art runaway modelling tools and synthetic diagnostics. These tools have gained widespread use in the scientific community, reflecting her commitment to advancing collective knowledge in the field.

Professor Per Helander has obtained seminal results in the theory of stellarator plasmas by systematically exploring the question of how the properties of magnetically confined plasmas depend on the geometry of the magnetic field. In most such plasmas, turbulent transport caused by micro-instabilities arising from plasma density and temperature gradients poses a significant challenge. Professor Helander foresaw a crucial development, the absence of the most important density-gradient-driven instability in certain types of magnetic fields. This prediction is believed to underpin the remarkable record plasma performance achieved in the Wendelstein 7-X stellarator. In addressing long-standing concerns about neoclassical impurity accumulation in stellarators, professor Helander demonstrated a possible route to avoiding it in collisionality regimes relevant to reactors. Furthermore, he identified important differences between stellarators and tokamaks concerning plasma rotation. On large scales, it is relatively slow in stellarators and governed by neoclassical processes even in the presence of turbulent transport, and on small scales zonal flows behave differently. These and other revelations have shaped the general understanding of stellarator plasmas and the burgeoning field of stellarator optimization.