In 1980, Malmberg was appointed to the first Plasma Sciences Committee of the
National Research Council.[citation needed] In that capacity, he was a strong voice for the importance of basic plasma experiments in maintaining the health of plasma science. In an era when small-scale and basic plasma physics research was nearing an ebb, Malmberg emphasized the importance of being able to follow the internal logic of the science, which he believed to be of paramount importance in doing basic research.
Scientific contributions
Landau damping of plasma waves
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, )[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 () 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.
Honors and awards
In 1985, Malmberg received the
James Clerk Maxwell Prize for Plasma Physics from the
American Physical Society for "his outstanding experimental studies which expanded our understanding of wave-particle interactions in neutral plasmas and increased our confidence in plasma theory; and for his pioneering studies of the confinement and transport of pure electron plasmas".[5]
In 1993, the UCSD physics department established the John Holmes Malmberg Prize in his honor. It is awarded annually to an outstanding undergraduate physics major with interests in experimental physics.[32]
^Landau, L. D. "On the vibrations of the electronic plasma". Zh. Eksp. Teor. Fiz. 16: 574–86 (reprinted 1965 Collected Papers of Landau ed D ter Haar (Oxford: Pergamon) pp 445–60).
^Prasad, S. A.; o'Neil, T. M. (1979). "Finite length thermal equilibria of a pure electron plasma column". Physics of Fluids. 22 (2): 278.
Bibcode:
1979PhFl...22..278P.
doi:
10.1063/1.862578.
^Dubin, Daniel H. E.; o'Neil, T. M. (1999). "Trapped nonneutral plasmas, liquids, and crystals (The thermal equilibrium states)". Reviews of Modern Physics. 71 (1): 87–172.
Bibcode:
1999RvMP...71...87D.
doi:
10.1103/RevModPhys.71.87.
^Beck, B. R.; Fajans, J.; Malmberg, J. H. (1992). "Measurement of collisional anisotropic temperature relaxation in a strongly magnetized pure electron plasma". Physical Review Letters. 68 (3): 317–320.
Bibcode:
1992PhRvL..68..317B.
doi:
10.1103/PhysRevLett.68.317.
PMID10045861.
^Bollinger, J. J.; Mitchell, T. B.; Huang, X.-P.; Itano, W. M.; Tan, J. N.; Jelenković, B. M.; Wineland, D. J. (2000). "Crystalline order in laser-cooled, non-neutral ion plasmas". Physics of Plasmas. 7 (1): 7–13.
Bibcode:
2000PhPl....7....7B.
doi:
10.1063/1.873818.
^Amoretti, M.; Amsler, C.; Bonomi, G.; Bouchta, A.; Bowe, P.; Carraro, C.; Cesar, C. L.; Charlton, M.; Collier, M. J. T.; Doser, M.; Filippini, V.; Fine, K. S.; Fontana, A.; Fujiwara, M. C.; Funakoshi, R.; Genova, P.; Hangst, J. S.; Hayano, R. S.; Holzscheiter, M. H.; Jørgensen, L. V.; Lagomarsino, V.; Landua, R.; Lindelöf, D.; Rizzini, E. Lodi; MacRì, M.; Madsen, N.; Manuzio, G.; Marchesotti, M.; Montagna, P.; et al. (2002).
"Production and detection of cold antihydrogen atoms". Nature. 419 (6906): 456–459.
Bibcode:
2002Natur.419..456A.
doi:10.1038/nature01096.
PMID12368849.
S2CID4315273.
^Surko, C. M.; Gribakin, G. F.; Buckman, S. J. (2005). "Low-energy positron interactions with atoms and molecules". Journal of Physics B: Atomic, Molecular and Optical Physics. 38 (6): R57–R126.
doi:
10.1088/0953-4075/38/6/R01.
S2CID15031194.
^Ahmadi, M.; Alves, B. X. R.; Baker, C. J.; Bertsche, W.; Capra, A.; Carruth, C.; Cesar, C. L.; Charlton, M.; Cohen, S.; Collister, R.; Eriksson, S.; Evans, A.; Evetts, N.; Fajans, J.; Friesen, T.; Fujiwara, M. C.; Gill, D. R.; Hangst, J. S.; Hardy, W. N.; Hayden, M. E.; Isaac, C. A.; Johnson, M. A.; Jones, J. M.; Jones, S. A.; Jonsell, S.; Khramov, A.; Knapp, P.; Kurchaninov, L.; Madsen, N.; et al. (2018).
"Characterization of the 1S–2S transition in antihydrogen". Nature. 557 (7703): 71–75.
Bibcode:
2018Natur.557...71A.
doi:
10.1038/s41586-018-0017-2.
PMC6784861.
PMID29618820.