John H. Malmberg

Malmberg and Charles Wharton made the first experimental measurements of Landau damping of plasma waves in 1964,[2] two decades after its prediction by Lev Landau.[8] Since this damping is collisionless, the free energy and phase-space memory associated with the damped wave are not lost, but are subtly stored in the plasma. Malmberg and collaborators demonstrated explicitly the reversible nature of this process by observation of the plasma wave echo[9][10] in which a wave “spontaneously” appears in the plasma as an ‘echo’ of two previously launched waves that had been Landau damped.

Neutral plasmas are notoriously difficult to confine. In contrast, Malmberg and collaborators predicted and demonstrated experimentally[3][4][11] that plasmas with a single sign of charge, such as pure electron or pure ion plasmas, can be confined for long periods (e.g., hours). This was accomplished using an arrangement of electric and magnetic fields similar to that of a Penning trap, but optimized to confine single-component plasmas. In recognition of Malmberg’s contributions to the development of these devices, they are now referred to as Penning–Malmberg traps.

Malmberg and collaborators, realized that non-neutral plasmas offer research opportunities not available with neutral plasmas. In contrast to neutral plasmas, plasmas with a single sign of charge can reach states of global thermal equilibria.[12][13] The possibility of using thermal equilibrium statistical mechanics to describe the plasma provides a large advantage to theory. [14] Furthermore, states near such thermal equilibria can be more easily controlled experimentally and departures from equilibrium studied with precision.

When a neutral plasma is cooled, it simply recombines; but a plasma with a single sign of charge can be cooled without recombination. Malmberg constructed a trap for a pure electron plasma with walls at 4.2 K. Cyclotron radiation from the electrons then cooled the plasma to a few Kelvin. Theory argued that electron-electron collisions in such a strongly magnetized and low temperature plasma would be qualitatively different than those in warmer plasmas. Malmberg measured the equipartition rate between electron velocity components parallel to and perpendicular to the magnetic field and confirmed the striking prediction that it decreases exponentially with decreasing temperature.[15]

Malmberg and Thomas Michael O'Neil predicted that a very cold, single-species plasma would undergo a phase transition to a body-centered cubic crystalline state.[16] Later, John Bollinger and collaborators created such a state by laser cooling a plasma of singly ionized beryllium ions to temperatures of a few millikelvin.[17] In other experiments, trapped pure electron plasmas are used to model the two-dimensional (2D) vortex dynamics expected for an ideal fluid.[18][19]

In the late 1980s, pure positron (i.e., antielectron) plasmas were created using the Penning–Malmberg trap technology.[20] This, and advances in confining low-energy antiprotons,[21] led to the creation of low-energy antihydrogen a decade later.[22][23] These and subsequent developments[24][25] have spawned a wealth of research with low-energy antimatter.[26] This includes ever more precise studies of antihydrogen and comparison with the properties of hydrogen[27] and formation of the di-positronium molecule (Ps${\displaystyle _{2}}$, ${\displaystyle e^{+}e^{-}e^{+}e^{-}}$)[28] predicted by J. A. Wheeler in 1946.[29] The Penning–Malmberg trap technology is now being used to create a new generation of high-quality positroniumatom (${\displaystyle e^{+}e^{-}}$) beams for atomic physics studies.[30][31]

In the broader view, Malmberg’s seminal studies with trapped single-component and non-neutral plasmas have stimulated vibrant sub-fields of plasma physics with surprisingly broad impacts in the wider world of physics.