A Positron-Electron eXperiment Collaboration

Matter-antimatter plasma research at IPP:

The APEX collaboration is focused on the creation of confined, low-temperature, long-lived, electron-positron plasma in the laboratory. Doing so will allow us to conduct experiments involving this unique state of matter, in order to test fundamental plasma physics predictions and gain insight into the physics of the early universe.

In contrast to conventional ion-electron plasmas, electron-positron "pair" plasmas consist of charged particles with identical mass. This symmetry results in pair plasmas having certain novel properties, a topic that has been studied theoretically and numerically for decades; experimental studies, however, are just getting started.
The generation of matter-antimatter plasmas in the laboratory is a significant challenge.  To achieve this, we are developing and bringing together techniques from a variety of fields of physics.  Once the different projects we are working on are all brought together, positrons from a world-class source (NEPOMUC) will be accumulated into ever denser bunches in a series of non-neutral plasma traps, then injected along with electrons into either of two toroidal magnetic confinement configurations:

Selected publications (experiment strategies):

Stoneking, M.; Pedersen, T. S.; Helander, P.; Chen, H.; Hergenhahn, U.; Stenson, E. V.; Fiksel, G.; Linden, J. v. d.; Saitoh, H.; Surko, C. M. et al.; Danielson, J. R.; Hugenschmidt, C.; Horn-Stanja, J.; Mishchenko, A.; Kennedy, D.; Deller, A.; Card, A.; Nißl, S.; Singer, M.; König, S.; Willingale, L.; Peebles, J.; Edwards, M. R.; Chin, K.: A new frontier in laboratory physics: magnetized electron–positron plasmas. Journal of Plasma Physics 86 (6), 155860601 (2020)
Stenson, E. V.; Horn-Stanja, J.; Stoneking, M. R.; Pedersen, T. S.: Debye length and plasma skin depth: two length scales of interest in the creation and diagnosis of laboratory pair plasmas. Journal of Plasma Physics 83 (1), 595830106 (2017)
Pedersen, T. S.; Danielson, J. R.; Hugenschmidz, C.; Marx, G.; Sarasola, X.; Schauer, F.; Schweikhard, L.; Surko, C. M.; Winkler, E.: Plans for the creation and studies of electron-positron plasmas in a stellarator. New Journal of Physics 14, 035010 (13pp) (2012)

Selected publications (theory predictions):

Horn-Stanja, J.; Biancalani, A.; Bottino, A.; Mishchenko, A.: Linear gyrokinetic studies with ORB5 en route to pair plasmas. Journal of Plasma Physics 85 (3), 905850302 (2019)
Kennedy, D.; Mishchenko, A.; Xanthopoulos, P.; Helander, P.; Banon Navarro, A.; Görler, T.: Linear gyrokinetics of electron–positron plasmas in closed field-line systems. Journal of Plasma Physics 86 (2), 905860208 (2020)
Helander, P.: Microstability of Magnetically Confined Electron-Positron Plasmas. Physical Review Letters 113, 135003 (2014)

Research topics

Positron beams & diagnostics:

The NEutron-induced POsitron source MUniCh (NEPOMUC), installed at the FRM II at the Heinz-Maier-Leibnitz Zentrum (MLZ), is one of the most intense antimatter sources in the world, capable of producing more than a billion low-energy (< 1 keV) anti-electrons (i.e., positrons) every second.   These can be used for a wide variety of surface science, materials science, solid state physics, and atomic physics applications.  More details can be found on the NEPOMUC website.

Within the scope of the APEX collaboration, we have conducted a number of studies to order to develop new settings for the NEPOMUC beam, so as to optimize for high rates of positrons with phase space distributions suitable to be injected into the traps we are building.  We also develop diagnostic techniques for positrons, non-neutral plasmas, and pair plasmas --- all of which require significantly different methods than are typically used for quasi-neutral, electron-ion plasmas.

Selected publications:

Horn-Stanja, J.; Stenson, E.; Stoneking, M. R.; Singer, M.; Hergenhahn, U.; Nißl, S.; Saitoh, H.; Pedersen, T. S.; Dickmann, M.; Hugenschmidt, C. et al.; Danielson, J. R.: Injection of intense low-energy reactor-based positron beams into a supported magnetic dipole trap. Plasma Research Express (PREX) 2, 015006 (2020)
Stenson, E. V.; Hergenhahn, U.; Stoneking, M. R.; Pedersen, T. S.: Positron-Induced Luminescence. Physical Review Letters 120, 147401 (2018)
Stanja, J.; Hergenhahn, U.; Niemann, H.; Paschkowski, N.; Pedersen, T. S.; Saitoh, H.; Stenson, E. V.; Stoneking, M. R.; Hugenschmidt, C.; Piochacz, C.: Characterization of the NEPOMUC primary and remoderated positron beams at different energies. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 827, pp. 52 - 62 (2016)


Non-neutral positron plasma accumulation & storage:

Non-neutral plasma traps are traditionally linear devices that use electric and magnetic fields to confine a single species of charged particles.  A common implementation is with hollow cylindrical electrodes in a uniform magnetic field.  

Buffer-gas trap (BGT) system: In order to trap and accumulate positrons from the NEPOMUC source, low-energy positrons will be magnetically guided into a buffer-gas trap.  This is the standard method to convert a low-density DC positron beam into cold, dense, tailorable pulses. The electrode structure and vacuum pumps create a pressure gradient that, along with a stepped potential profile, optimizes between competing mechanisms: annihilation on the gas and inelastic collisions with nitrogen gas that trap positrons in the electrostatic potential well. With the voltages and nitrogen flow carefully tuned, it should be possible to accumulate hundreds of millions of positrons from NEPOMUC into a non-neutral plasma, which can then be delivered to downstream experiments in a very short and intense pulse (< 10 ns).

High-field, multi-cell trap: To reach the 1010–1011 cold (∼1 eV) positrons needed to ensure plasma densities in the toroidal traps, multiple pulses from the BGT system's "accumulator" will need be transferred, combined, and stored for hundreds of seconds in another, higher-capacity trapping stage.  This consists of a nested array of several non-neutral traps in ultra-high vacuum in a 5-T magnet.  A "master cell" is used to transport plasmas into and out of the storage cells by means of controlled excitation and damping of a the m=1 diocotron mode.

Selected publications:

Singer, M.; König, S.; Stoneking, M. R.; Steinbrunner, P.; Danielson, J. R.; Schweikhard, L.; Pedersen, T. S.: Non-neutral plasma manipulation techniques in development of a high-capacity positron trap. Review of Scientific Instruments 92, 123504 (2021)


Injection and trapping in a prototype dipole trap

To address key questions that needed to be answered early in the experiment planning process, we have conducted a number of proof-of-principle tests in a trap based on a supported permanent magnet.  These have demonstrated that positrons can be losslessly transported from "open" magnetic field lines (connecting back to the NEPOMUC beam line) onto "closed" dipole field lines using ExB drifts.  These drifts are created by strategically tailoring the 3D electrostatic potential landscape by biasing electrodes just outside the edge of the confinement region.  By switching off those potentials, positrons that were transiting through the confinement region on the closed field lines in the trap become trapped there.  We have also found that the drift injection technique works for a range of relevant energy distributions, and it is not impeded by the trap being pre-filled with an electron space charge.

Selected publications:

Singer, M.; Stoneking, M. R.; Stenson, E. V.; Nißl, S.; Deller, A.; Card, A.; Horn-Stanja, J.; Pedersen, T. S.; Saitoh, H.; Hugenschmidt, C.: Injection of positrons into a dense electron cloud in a magnetic dipole trap. Physics of Plasmas 28, 062506 (2021)
Nißl, S.; Stenson, E. V.; Hergenhahn, U.; Horn-Stanja, J.; Pedersen, T. S.; Saitoh, H.; Hugenschmidt, C.; Singer, M.; Stoneking, M. R.; Danielson, J. R.: Positron orbit effects during injection and confinement in a magnetic dipole trap. Physics of Plasmas 27, 052107 (2020)
Horn-Stanja, J.; Nißl, S.; Hergenhahn, U.; Pedersen, T. S.; Saitoh, H.; Stenson, E. V.; Dickmann, M.; Hugenschmidt, C.; Singer, M.; Stoneking, M. . et al.; Danielson, J. .: Confinement of Positrons Exceeding 1 s in a Supported Magnetic Dipole Trap. Physical Review Letters 121, 235003 (2018)
Stenson, E.; Nißl, S.; Hergenhahn, U.; Horn-Stanja, J.; Singer, M.; Saitoh, H.; Pedersen, T. S.; Danielson, J. R.; Stoneking, M. R.; Dickmann, M. et al.; Hugenschmidt, C.: Lossless Positron Injection into a Magnetic Dipole Trap. Physical Review Letters 121, 235005 (2018)


Pair plasma traps

Because the properties of a plasma system are often inextricably linked to its magnetic topology, being able to compare and contrast the behavior of pair plasmas in highly distinct but complementary magnetic field configurations is expected to significantly extend our understanding beyond what we will learn from each experiment individually.  Both dipoles and stellarators have been shown to effectively confine both non-neutral and quasi-neutral plasmas without requiring internal plasma currents.  We are designing and building two different tabletop-sized, superconducting traps for our electron-positron pair plasmas.

Levitated dipole: Dipole magnetic fields occur copiously in nature, from permanently magnetized minerals, to planets like the Earth, to stellar objects.  In the laboratory, dipole magnetic field lines that touch no material surfaces can be achieved by levitating a current-carrying, superconducting coil.  The "floating coil" is suspended in the vacuum chamber by a feedback-controlled "lifting coil" above it, providing an attractive force to balance gravity.   The confining magnet field lines are purely poloidal, with large mirror ratios and short connection lengths; the magnetic field gradient and curvature cause particles to drift toroidally around the trap on surfaces of constant magnetic flux.

Optimized stellarator: In a stellarator, magnetic field lines are predominantly toroidal but also have a poloidal component, so that they twist around the torus; this "rotational transform" is generated by external coils.  The design of those coils, the vacuum magnetic field they produce, the properties of the plasma that will be confined by the magnetic field, and the required engineering tolerances are determined through the sophisticated process of stellarator optimization, the forefront of which our collaborators in Stellarator Theory at IPP and the Simons Hidden Symmetries in Fusion Energy Collaboration are actively developing.  While optimizing a stellarator for a laboratory electron-positron plasma is not the same as optimizing a stellarator for a fusion plasma, it uses many of the same computational tools and techniques; in return, a pair plasma will offer a highly sensitive test of the effectiveness of the optimization.

Selected publications:

Saitoh, H.; Stoneking, M.; Pedersen, T. S.: A levitated magnetic dipole configuration as a compact charged particle trap. Review of Scientific Instruments 91, 043507 (2020)
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