Rotating wall technique

Last updated

The rotating wall technique (RW technique) is a method used to compress a single-component plasma (a cold dense gas of charged particles) confined in an electromagnetic trap. It is one of many scientific and technological applications that rely on storing charged particles in vacuum. This technique has found extensive use in improving the quality of these traps and in tailoring of both positron and antiproton (i.e. antiparticle) plasmas for a variety of end uses.

Contents

Overview

Single-component plasmas (SCP), which are a type of nonneutral plasma, have many uses, including studying a variety of plasma physics phenomena [1] and for the accumulation, storage and delivery of antiparticles. Applications include the creation and study of antihydrogen, [2] [3] [4] beams to study the interaction of positrons with ordinary matter and to create dense gases of positronium (Ps) atoms, [5] [6] [7] and the creation of Ps-atom beams. [8] [9] The “rotating wall (RW) technique” uses rotating electric fields to compress SCP in PM traps radially to increase the plasma density and/or to counteract the tendency of plasma to diffuse radially out of the trap. It has proven crucial in improving the quality and hence utility of trapped plasmas and trap-based beams.

Principles of operation

For this application, a plasma is stored in a Penning–Malmberg (PM) trap [1] in a uniform magnetic field, B. The charge cloud is typically cylindrical in shape with dimension along B large compared to the radius. This charge produces a radial electric field which would tend to push the plasma outward. To counteract this, the plasma spins about the axis of symmetry producing a Lorentz force to balance that due to the electric field, and the plasma takes the form of a spinning charged rod. Such cold, single-component plasmas in PM traps can come to thermal equilibrium and rotate as a rigid body at frequency

,

where n is the plasma density. [10] As illustrated in Fig. 1, the RW technique uses an azimuthally segmented cylindrical electrode covering a portion of a plasma. Phased, sinusoidal voltages at frequency fRW are applied to the segments. The result is a rotating electric field perpendicular to the axis of symmetry of the plasma. This field induces an electric dipole moment in the plasma and hence a torque. Rotation of the field in the direction of, and faster than the natural rotation of the plasma acts to spin the plasma faster, thereby increasing the Lorentz force and producing plasma compression (cf. Figs. 2 and 3). [11]

Fig. 1. Apparatus used to radially compress electron plasmas in a Penning-Malmberg trap using the RW technique by applying phased sinusoidal electrical signals to a segmented (RW) electrode. Electron Compression Apparatus.png
Fig. 1. Apparatus used to radially compress electron plasmas in a Penning–Malmberg trap using the RW technique by applying phased sinusoidal electrical signals to a segmented (RW) electrode.
Fig. 2. Radial compression of an electron plasma vs time with the RW fields turned on at t = 0. Note the log scale for density and the flat density profiles, before and after compression, that are characteristic of rigid plasma rotation. Surko - Electron Compression Plot.png
Fig. 2. Radial compression of an electron plasma vs time with the RW fields turned on at t = 0. Note the log scale for density and the flat density profiles, before and after compression, that are characteristic of rigid plasma rotation.

An important requirement for plasma compression using the RW technique is good coupling between the plasma and the rotating field. This is necessary to overcome asymmetry-induced transport which acts as a drag on the plasma and tends to oppose the RW torque. For high quality PM traps with little asymmetry induced transport, one can access a so-called “strong drive regime." [11] [12] In this case, application of a rotating electric field at frequency results in the plasma spinning up to the applied frequency, namely fE = fRW (cf. Fig. 3). This has proven enormously useful as a way to fix plasma density simply by adjusting fRW.

Fig. 3. Density of a positron plasma as a function of applied RW frequency. The solid line corresponds to fE = fRW, characteristic of the strong drive regime. For this experiment, B = 0.04 T, and the maximum density achieved is 17% of the Brillouin density limit, which is the maximum possible density for a SCP confined in a field of strength B. Surko - Density Plot.png
Fig. 3. Density of a positron plasma as a function of applied RW frequency. The solid line corresponds to fE = fRW, characteristic of the strong drive regime. For this experiment, B = 0.04 T, and the maximum density achieved is 17% of the Brillouin density limit, which is the maximum possible density for a SCP confined in a field of strength B.

History

The RW technique was first developed by Huang et al., to compress a magnetized Mg+ plasma. [13] The technique was soon thereafter applied to electron plasmas, where a segmented electrode, such as that described above, was used to couple to waves (Trivelpiece-Gould modes) in the plasma. [14] The technique was also used to phase-lock the rotation frequency of laser cooled single-component ion crystals. [15] The first use of the RW technique for antimatter was done using small positron plasmas without coupling to modes. [16] The strong drive regime, which was discovered somewhat later using electron plasmas, [17] has proven to be more useful in that tuning to (and tracking) plasma modes is unnecessary. A related technique has been developed to compress single-component charged gases in PM traps (i.e., charge clouds not in the plasma regime). [18] [19]

Uses

The RW technique has found extensive use in manipulating antiparticles in Penning–Malmberg traps. One important application is the creation of specially tailored antiparticle beams for atomic physics experiments. [5] Frequently one would like a beam with a large current density. In this case, one compresses the plasma with the RW technique before delivery. This has been crucial in experiments to study dense gases of positronium (Ps) atoms and formation of the Ps2 molecule (e+e−e+e−) [5-7]. It has also been important in the creation of high-quality Ps-atom beams. [8] [9]

The RW technique is used in three ways in the creation of low-energy antihydrogen atoms. Antiprotons are compressed radially by sympathetic compression with electrons co-loaded in the trap. The technique has also been used to fix the positron density before the positrons and antiprotons are combined. [2] [3] Recently it was discovered that one could set all of the important parameters of the electron and positron plasmas for antihydrogen production using the RW to fix the plasma density and evaporative cooling to cool the plasma and fix the on-axis space charge potential. The result was greatly increased reproducibility for antihydrogen production. [4] In particular, this technique, dubbed SDREVC (strong drive regime evaporative cooling), [20] was successful to the extent that it increased the number of trappable antihydrogen by an order of magnitude. This is particularly important in that, while copious amounts of antihydrogen can be produced, the vast majority are at high temperature and cannot be trapped in the small well depth of the minimum-magnetic field atom traps. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Antimatter</span> Material composed of antiparticles of the corresponding particles of ordinary matter

In modern physics, antimatter is defined as matter composed of the antiparticles of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charge, parity, and time, known as CPT reversal. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay, but only a tiny fraction of these have successfully been bound together in experiments to form antiatoms. Minuscule numbers of antiparticles can be generated at particle accelerators; however, total artificial production has been only a few nanograms. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Nonetheless, antimatter is an essential component of widely-available applications related to beta decay, such as positron emission tomography, radiation therapy, and industrial imaging.

<span class="mw-page-title-main">Positronium</span> Bound state of an electron and positron

Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states. The energy levels of the two particles are similar to that of the hydrogen atom. However, because of the reduced mass, the frequencies of the spectral lines are less than half of those for the corresponding hydrogen lines.

ATHENA, also known as the AD-1 experiment, was an antimatter research project at the Antiproton Decelerator at CERN, Geneva. In August 2002, it was the first experiment to produce 50,000 low-energy antihydrogen atoms, as reported in Nature. In 2005, ATHENA was disbanded and many of the former members of the research team worked on the subsequent ALPHA experiment and AEgIS experiment.

Resolved sideband cooling is a laser cooling technique allowing cooling of tightly bound atoms and ions beyond the Doppler cooling limit, potentially to their motional ground state. Aside from the curiosity of having a particle at zero point energy, such preparation of a particle in a definite state with high probability (initialization) is an essential part of state manipulation experiments in quantum optics and quantum computing.

The Antihydrogen Trap (ATRAP) collaboration at the Antiproton Decelerator facility at CERN, Geneva, is responsible for the AD-2 experiment. It is a continuation of the TRAP collaboration, which started taking data for the TRAP experiment in 1985. The TRAP experiment pioneered cold antiprotons, cold positrons, and first made the ingredients of cold antihydrogen to interact. Later ATRAP members pioneered accurate hydrogen spectroscopy and observed the first hot antihydrogen atoms.

Plasma acceleration is a technique for accelerating charged particles, such as electrons, positrons, and ions, using the electric field associated with electron plasma wave or other high-gradient plasma structures. The plasma acceleration structures are created either using ultra-short laser pulses or energetic particle beams that are matched to the plasma parameters. These techniques offer a way to build high performance particle accelerators of much smaller size than conventional devices. The basic concepts of plasma acceleration and its possibilities were originally conceived by Toshiki Tajima and John M. Dawson of UCLA in 1979. The initial experimental designs for a "wakefield" accelerator were conceived at UCLA by Chandrashekhar J. Joshi et al. Current experimental devices show accelerating gradients several orders of magnitude better than current particle accelerators over very short distances, and about one order of magnitude better at the one meter scale.

David Edward Pritchard is a professor at the Massachusetts Institute of Technology (MIT), working on atomic physics and educational research.

<span class="mw-page-title-main">Electron beam ion trap</span>

Electron beam ion trap (EBIT) is an electromagnetic bottle that produces and confines highly charged ions. An EBIT uses an electron beam focused with a powerful magnetic field to ionize atoms to high charge states by successive electron impact.

Di-positronium, or dipositronium, is an exotic molecule consisting of two atoms of positronium. It was predicted to exist in 1946 by John Archibald Wheeler, and subsequently studied theoretically, but was not observed until 2007 in an experiment performed by David Cassidy and Allen Mills at the University of California, Riverside. The researchers made the positronium molecules by firing intense bursts of positrons into a thin film of porous silicon dioxide. Upon slowing down in the silica, the positrons captured ordinary electrons to form positronium atoms. Within the silica, these were long lived enough to interact, forming molecular di-positronium. Advances in trapping and manipulating positrons, and spectroscopy techniques have enabled studies of Ps–Ps interactions. In 2012, Cassidy et al. were able to produce the excited molecular positronium angular momentum state.

High-precision experiments could reveal small previously unseen differences between the behavior of matter and antimatter. This prospect is appealing to physicists because it may show that nature is not Lorentz symmetric.

A non-neutral plasma is a plasma whose net charge creates an electric field large enough to play an important or even dominant role in the plasma dynamics. The simplest non-neutral plasmas are plasmas consisting of a single charge species. Examples of single species non-neutral plasmas that have been created in laboratory experiments are plasmas consisting entirely of electrons, pure ion plasmas, positron plasmas, and antiproton plasmas.

<span class="mw-page-title-main">Scissors Modes</span> Collective excitations

Scissors Modes are collective excitations in which two particle systems move with respect to each other conserving their shape. For the first time they were predicted to occur in deformed atomic nuclei by N. LoIudice and F. Palumbo, who used a semiclassical Two Rotor Model, whose solution required a realization of the O(4) algebra that was not known in mathematics. In this model protons and neutrons were assumed to form two interacting rotors to be identified with the blades of scissors. Their relative motion (Fig.1) generates a magnetic dipole moment whose coupling with the electromagnetic field provides the signature of the mode.

Jürgen Mlynek is a German physicist and was president of the Helmholtz Association of German Research Centres from 2005 to 2015.

<span class="mw-page-title-main">ALPHA experiment</span> Antimatter gravitation experiment

The Antihydrogen Laser Physics Apparatus (ALPHA), also known as AD-5, is an experiment at CERN's Antiproton Decelerator, designed to trap antihydrogen in a magnetic trap in order to study its atomic spectra. The ultimate goal of the experiment is to test CPT symmetry through comparing the respective spectra of hydrogen and antihydrogen. Scientists taking part in ALPHA include former members of the ATHENA experiment (AD-1), the first to produce cold antihydrogen in 2002.

AEgIS, AD-6, is an experiment at the Antiproton Decelerator facility at CERN. Its primary goal is to measure directly the effect of Earth's gravitational field on antihydrogen atoms with significant precision. Indirect bounds that assume the validity of, for example, the universality of free fall, the Weak Equivalence Principle or CPT symmetry also in the case of antimatter constrain an anomalous gravitational behavior to a level where only precision measurements can provide answers. Vice versa, antimatter experiments with sufficient precision are essential to validate these fundamental assumptions. AEgIS was originally proposed in 2007. Construction of the main apparatus was completed in 2012. Since 2014, two laser systems with tunable wavelengths and synchronized to the nanosecond for specific atomic excitation have been successfully commissioned.

<span class="mw-page-title-main">Buffer-gas trap</span> Device used to accumulate positrons

The buffer-gas trap (BGT) is a device used to accumulate positrons efficiently while minimizing positron loss due to annihilation, which occurs when an electron and positron collide and the energy is converted to gamma rays. The BGT is used for a variety of research applications, particularly those that benefit from specially tailored positron gases, plasmas and/or pulsed beams. Examples include use of the BGT to create antihydrogen and the positronium molecule.

<span class="mw-page-title-main">John H. Malmberg</span> American physicist

John Holmes Malmberg was an American plasma physicist and a professor at the University of California, San Diego. He was known for making the first experimental measurements of Landau damping of plasma waves in 1964, as well as for his research on non-neutral plasmas and the development of the Penning–Malmberg trap.

Clifford Michael Surko is an American physicist, whose works involve plasma physics, atomic physics, nonlinear dynamics and solid state physics. Together with his colleagues, he developed techniques for laser scattering at small angles to study waves and turbulence in tokamak plasmas and invented a positron trap that was used in experiments worldwide to study antimatter. Surko also developed other techniques for studying positron plasmas and examined atomic and plasma physics with positrons.

<span class="mw-page-title-main">Penning–Malmberg trap</span> Electromagnetic device used to confine particles of a single sign of charge

The Penning–Malmberg trap, named after Frans Penning and John Malmberg, is an electromagnetic device used to confine large numbers of charged particles of a single sign of charge. Much interest in Penning–Malmberg (PM) traps arises from the fact that if the density of particles is large and the temperature is low, the gas will become a single-component plasma. While confinement of electrically neutral plasmas is generally difficult, single-species plasmas can be confined for long times in PM traps. They are the method of choice to study a variety of plasma phenomena. They are also widely used to confine antiparticles such as positrons and antiprotons for use in studies of the properties of antimatter and interactions of antiparticles with matter.

Toshiki Tajima is a Japanese theoretical plasma physicist known for pioneering the laser wakefield acceleration technique with John M. Dawson in 1979. The technique is used to accelerate particles in a plasma and was experimentally realized in 1994, for which Tajima received several awards such as the Nishina Memorial Prize (2006), the Enrico Fermi Prize (2015), the Robert R. Wilson Prize (2019), the Hannes Alfvén Prize (2019) and the Charles Hard Townes Award (2020).

References

  1. 1 2 Dubin, Daniel H. E.; O’Neil, T. M. (1999-01-01). "Trapped nonneutral plasmas, liquids, and crystals (the thermal equilibrium states)". Reviews of Modern Physics. American Physical Society (APS). 71 (1): 87–172. doi:10.1103/revmodphys.71.87. ISSN   0034-6861.
  2. 1 2 Amoretti, M.; Amsler, C.; Bonomi, G.; Bouchta, A.; Bowe, P.; et al. (2002-09-18). "Production and detection of cold antihydrogen atoms". Nature. Springer Nature. 419 (6906): 456–459. doi: 10.1038/nature01096 . ISSN   0028-0836. PMID   12368849. S2CID   4315273.
  3. 1 2 Gabrielse, G.; Bowden, N. S.; Oxley, P.; Speck, A.; Storry, C. H.; et al. (2002-10-31). "Background-Free Observation of Cold Antihydrogen with Field-Ionization Analysis of Its States" (PDF). Physical Review Letters. American Physical Society (APS). 89 (21): 213401–213404. doi:10.1103/physrevlett.89.213401. ISSN   0031-9007. PMID   12443407.
  4. 1 2 Ahmadi, M.; Alves, B. X. R.; Baker, C. J.; Bertsche, W.; Capra, A.; et al. (2018-04-04). "Characterization of the 1S–2S transition in antihydrogen". Nature. Springer Science and Business Media LLC. 557 (7703): 71–75. doi: 10.1038/s41586-018-0017-2 . ISSN   0028-0836. PMC   6784861 . PMID   29618820.
  5. 1 2 3 Danielson, J. R.; Dubin, D. H. E.; Greaves, R. G.; Surko, C. M. (2015-03-17). "Plasma and trap-based techniques for science with positrons". Reviews of Modern Physics. American Physical Society (APS). 87 (1): 247–306. doi: 10.1103/revmodphys.87.247 . ISSN   0034-6861.
  6. ↑ Cassidy, D. B.; Mills, A. P. (2007). "The production of molecular positronium". Nature. Springer Science and Business Media LLC. 449 (7159): 195–197. doi:10.1038/nature06094. ISSN   0028-0836. PMID   17851519. S2CID   11269624.
  7. ↑ Cassidy, D. B.; Hisakado, T. H.; Tom, H. W. K.; Mills, A. P. (2012-03-30). "Optical Spectroscopy of Molecular Positronium". Physical Review Letters. American Physical Society (APS). 108 (13): 133402–133405. doi: 10.1103/physrevlett.108.133402 . ISSN   0031-9007. PMID   22540698.
  8. 1 2 Jones, A. C. L.; Moxom, J.; Rutbeck-Goldman, H. J.; Osorno, K. A.; Cecchini, G. G.; et al. (2017-08-02). "Focusing of a Rydberg Positronium Beam with an Ellipsoidal Electrostatic Mirror". Physical Review Letters. American Physical Society (APS). 119 (5): 053201. doi: 10.1103/physrevlett.119.053201 . ISSN   0031-9007. PMID   28949762.
  9. 1 2 Michishio, K.; Chiari, L.; Tanaka, F.; Oshima, N.; Nagashima, Y. (2019). "A high-quality and energy-tunable positronium beam system employing a trap-based positron beam". Review of Scientific Instruments. AIP Publishing. 90 (2): 023305. doi:10.1063/1.5060619. ISSN   0034-6748. PMID   30831693.
  10. ↑ O’Neil, T. M.; Driscoll, C. F. (1979). "Transport to thermal equilibrium of a pure electron plasma". Physics of Fluids. AIP Publishing. 22 (2): 266–277. doi:10.1063/1.862577. ISSN   0031-9171.
  11. 1 2 Danielson, J. R.; Surko, C. M. (2005-01-24). "Torque-Balanced High-Density Steady States of Single-Component Plasmas". Physical Review Letters. American Physical Society (APS). 94 (3): 035001–035004. doi:10.1103/physrevlett.94.035001. ISSN   0031-9007. PMID   15698274.
  12. ↑ Danielson, J. R.; Surko, C. M.; O’Neil, T. M. (2007-09-28). "High-Density Fixed Point for Radially Compressed Single-Component Plasmas". Physical Review Letters. American Physical Society (APS). 99 (13): 135005. doi:10.1103/physrevlett.99.135005. ISSN   0031-9007. PMID   17930602.
  13. ↑ Huang, X.-P.; Anderegg, F.; Hollmann, E. M.; Driscoll, C. F.; O'Neil, T. M. (1997). "Steady-State Confinement of Non-neutral Plasmas by Rotating Electric Fields". Physical Review Letters. 78 (5): 875–878. Bibcode:1997PhRvL..78..875H. doi:10.1103/PhysRevLett.78.875.
  14. ↑ F. Anderegg, E. M. Hollmann, and C. F. Driscoll, Rotating Field Confinement of Pure Electron Plasmas Using Trivelpiece-Gould Modes, Phys. Rev. Lett. 81, 4875-4878 (1998).
  15. ↑ X. P. Huang, J. J. Bollinger, T. B. Mitchell, and W. M. Itano, Phase-Locked Rotation of Crystallized Non-Neutral Plasmas by Rotating Electric Fields, Phys. Rev. Lett. 80, 73-76 (1998).
  16. ↑ R. G. Greaves and C. M. Surko, Radial Compression and Inward Transport of Positron Plasmas Using a Rotating Electric Field, Phys. Plasmas 8, 1879-1885 (2001).
  17. ↑ J. R. Danielson and C. M. Surko, Radial Compression and Torque-Balanced Steady States of Single-Component Plasmas in Penning–Malmberg Traps, Phys. Plasmas 13, 055706-055710 (2006).
  18. ↑ R. G. Greaves and J. M. Moxom, Compression of Trapped Positrons in a Single Particle Regime by a Rotating Electric Field, Phys. Plasmas 15, 072304 (2008).
  19. ↑ C. A. Isaac, C. J. Baker, T. Mortensen, D. P. v. d. Werf, and M. Charlton, Compression of Positron Clouds in the Independent Particle Regime Phys. Rev. Lett. 107, 033201-033204 (2011).
  20. ↑ M. Ahmadi, B. X. R. Alves, C. J. Baker, W. Bertsche, A. Capra, C. Carruth, et al., Enhanced Control and Reproducibility of Non-Neutral Plasmas, Phys. Rev. Lett. 120, 025001 (2018).
  21. ↑ C. Amole, M. D. Ashkezari, M. Baquero-Ruiz, W. Bertsche, P. D. Bowe, E. Butler, et al., Resonant Quantum Transitions in Trapped Antihydrogen Atoms, Nature 483, 439-444 (2012).
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.