Electron Beam Ion Trap

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An electron beam ion trap ( EBIT ) or electron beam ion trap is a special type of ion trap . This type of trap is particularly suitable for the generation and storage of highly charged ions . Low-charge ions are captured in it and further ionized by collisions with electrons that are "shot" through the trap as a beam. The low-charge ions arise from atoms flying through the electron beam .

Ions of high charge states can be generated in an EBIT without having to bring them up to high speeds, for example in accelerators ; rather, the ions remain practically at rest. This allows the use of different spectrometry methods to investigate the different states. This also means that the effort required to generate the highly charged ions is relatively low compared to other methods. In some cases, instead of neutral atoms, the trap is injected with low-charge ions that were created separately.

An Electron Beam Ion Source ( EBIS , German  electron beam ion source ) works very similar, but omitted the full capture of the ions. The particles are ionized here as they fly through and carried out as an ion beam. In this case, there is usually no direct line of sight to the point of generation of the ions, which is why electron beam ion sources are not suitable for spectroscopy.

Working principle

In an EBIT, ions are generated by electron impact ionization . For this purpose, a bundled, intense electron beam with well-defined kinetic energy is shot in a vacuum at neutral atoms. When the electrons collide with the shell electrons , the latter are accelerated and, if the transferred energy exceeds the binding energy , they are “knocked out” of the shell of the atom. A positively charged ion is created .

The charge of the electrons creates a negative space charge in the area of ​​the beam , which traps the positively charged ions in the electron beam. In order to prevent the electron beam from moving out of the trap in an EBIT, the center of the trap is set to a lower electrical potential than the areas further out. This traps the ions completely.

Since the electron beam is intensely and strongly focused and the ions are held in the beam, further electrons are removed from the atomic shell . This process is only limited by the energy of the electron beam and the binding energy of the remaining shell electrons. This creates highly charged ions.

In order for the highly charged ions not to collide with neutral atoms and thereby partially catch electrons again ( recombination ), an extremely good vacuum (UHV - Ultra High Vacuum) of typically fractions of a trillionth of the atmospheric pressure is required in an EBIT .

The number of ions stored in an EBIT varies in the range from a few thousands to several billion. This is a very small amount of matter, less than a trillionth of a gram.

construction

Basic structure of an EBIT. Red: electron source, black: electrodes, green: magnet

As explained above, only a strongly focused electron beam and a longitudinal electric field are ultimately required for an EBIT.

In order to generate the electron beam, a thermal electron source is usually used which is at a lower potential than the following trap. An aperture close to the trap potential accelerates the electrons. Due to the high space charge of the electrons, the beam would expand quickly, so a (often superconducting ) magnet is used, whose field constricts the electron beam along the axis of the trap. At the end of the path of the electrons, the magnetic field is weaker, which causes the beam to expand. A larger electrode that is at a lower potential than the trap (but still higher than the source) partially decelerates the electrons and collects them (“collector”). This collector has to be cooled in a vacuum because the power of the electron beam is deposited almost completely in it. For this reason, the magnetic field that constricts the beam is also partially compensated in this area by a further magnetic coil, as a result of which the beam diverges and distributes its power over a larger area. This relieves the collector.

Typical values ​​for the electron current are around 150 mA. In the trap area, the beam often reaches a minimum diameter of less than one millimeter. The energy can be selected in order to be able to set the degree of ionization. Typical values ​​are a few thousand volts. In order to achieve high levels of charge in atoms with a high atomic number , for example , much higher voltages are required, for example 300 kV for the super-EBIT of the LLNL .

In order to prevent the ions from leaving the trap parallel to the electron beam , cylindrical electrodes known as drift tubes are used, which generate an electrostatic potential. For permanent trapping of the ions, this potential in the trap center must be lower than in the immediate vicinity. With a suitable choice of the voltages on the drift tubes, the ions can be extracted from the trap in order to make them available for other experiments. If spectroscopy is to be carried out, there are windows in the middle drift tube to allow visual access to the stored ions. At least three drift tubes are required to trap the ions. Most EBITs, however, have more in order to be able to realize more complex potential landscapes.

advantages

EBITs are compact devices, some of which can be set up on a table, and are a cost-effective alternative for most experiments compared to the particle accelerators otherwise required to generate highly charged ions . This advantage over accelerators is based on the fact that the ionization in an EBIT can take place gradually over a longer period of time and does not have to take place in a single short process.

An arrangement with permanent magnets also eliminates the need for complex cooling processes (liquid nitrogen, liquid helium), which makes the system easy to handle. However, such EBITs do not yet achieve the performance of devices that work with superconducting magnets, because their up to ten times stronger magnetic field (several Tesla ) causes a much stronger focus of the electron beam. In addition, cryogenically operating EBITs create a better vacuum, which slows down the recombination of ions through charge exchange . This leads to the possibility of producing the highest levels of charge, such as B. naked, i.e. 92-fold positively charged uranium ions.

Such a highly charged ion can otherwise only be produced at a few particle accelerators worldwide (e.g. at the GSI Helmholtz Center for Heavy Ion Research ) by first bringing ions to almost the speed of light and then shooting them through a thin metal foil. The electrons are practically stripped off by electron-electron collisions; this is known as a stripper reaction. In contrast to EBIT, the ions are accelerated here instead of the electrons.

application

The charged ions in an EBIT make it possible to realize conditions in the laboratory for the smallest amounts of matter. B. in stellar atmospheres, active galactic nuclei ( AGN ) or supernovae at temperatures of z. T. occur many millions of degrees. Processes that play an important role in the plasma of fusion reactors, such as tokamaks or stellarators , can be examined in detail in EBITs because the ionization conditions can be well controlled. As a result, EBITs play an important role in the spectroscopy of plasmas at high temperatures.

The stored ions are constantly electronically excited by collisions with the electron beam. The energy temporarily “stored” by the bound electrons is released through electronic transitions into the energetically lowest, non-excited electronic ground state in the form of photons . Similarly, photons are also generated in recombination processes when an ion captures a free electron. These photons can have quite different energies. The high electron beam energy emits X-ray photons , but also z. B. those in the ultraviolet or visible range.

A highly charged ion with a heavy nucleus and only a few electrons is a system bound by electromagnetic interaction with extremely high binding energies . But because it contains only a few, in some cases just a single electron, such a structure is easier to describe theoretically than a neutral atom, in which the mutual interactions of the many bound electrons are difficult to deal with. In these structures, which have been simplified by removing most of the electrons, extremely high binding forces on a single electron can be scientifically investigated. This makes experiments possible in which the quantum electrodynamics of bound relativistic electrons is investigated in an area in which this theory (which otherwise has very high accuracy in its theoretical predictions) still suffers from mathematical difficulties.

The interactions of highly charged ions with atoms in the gas phase and on surfaces are a very active area of ​​research. When a highly charged ion approaches a surface, a so-called hollow atom is created for a short time , as the ion tries to compensate for its lack of electrons very quickly by sucking in electrons that are abundant on the surface. Laws of quantum mechanics prevent the immediate creation of a neutral atom and allow these highly excited systems to exist for a very short time. Investigations of these hollow atoms have contributed to the clarification of some questions about the dynamics of electrons on surfaces.

The ions generated in an EBIT can also e.g. B. can be used in mass spectrometry in the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) method, i.e. for material analysis.

Thanks to the economical generation of high-carbon ions, the EBIT also appears to be suitable for future medical applications (irradiation of cancerous tumors in ion therapy ). Up to now, accelerators and storage rings have been used here.

HCIs can also be used in nanotechnology to generate surface structures in the nanometer range. Various applications in this area are currently being tested. For this purpose, ions are generated with an EBIT / EBIS and, outside of this, shot onto solid samples with a well-defined charge state and well-defined energy. The resulting surface changes are examined using imaging methods (such as atomic force microscopy ).

literature

  • Roscoe E. Marrs, Peter Beiersdorfer, Dieter Schneider: The Electron Beam Ion Trap , Physics Today, October 1994, p. 27
  • MA Levine, RE Marrs, JR Henderson, DA Knapp, Marilyn B Schneider: The Electron Beam Ion Trap: A New Instrument for Atomic Physics Measurements , Physica Scripta, T 22, 1988, p. 157
  • RE Marrs, MA Levine, DA Knapp, JR Henderson: Measurement of electron-impact-excitation cross sections for very highly charged ions , Physical Review Letters, Vol. 60, 1988, p. 1715.
  • RE Marrs, SR Elliott, DA Knapp: Production and Trapping of Hydrogenlike and Bare Uranium Ions in an Electron Beam Ion Trap , Physical Review Letters, Vol. 72, 1994, p. 4082.
  • P. Beiersdorfer, AL Osterheld, J. Scofield, JR Crespo López-Urrutia, K. Widmann: Measurement of QED and Hyperfine Splitting in the 2s1 / 2- 2p3 / 2 X-Ray Transition in Li-like 209Bi , Physical Review Letters, Vol. 80, 1998, p. 3022.
  • JR Crespo López-Urrutia, P. Beiersdorfer, DW Savin, K. Widmann: Direct Observation of the Spontaneous Emission of the Hyperfine Transition F = 4 to F = 3 in Ground State Hydrogenlike 165Ho66 + in an Electron Beam Ion Trap , Physical Review Letters, Vol. 77, 1996, p. 826.
  • P. Beiersdorfer, AL Osterheld, J. Scofield, B. Wargelin, RE Marrs: Observation of magnetic octupole decay in atomic spectra , Physical Review Letters, Vol. 67, 1991, p. 2272.
  • P. Beiersdorfer, CM Lisse, RE Olson, GV Brown, H. Chen: X-Ray Velocimetry of Solar Wind Ion Impact on Comets , Astrophysical Journal Letters , Vol. 549, 2001, L147
  • SR Elliott, P. Beiersdorfer, MH Chen: Trapped-Ion Based Technique for Measuring the Nuclear Charge Radii of Highly Charged Radioactive Isotopes , Physical Review Letters, Vol. 76, 1996, p. 1031
  • CA Morgan, FG Serpa, E. Takacs, et al. a .: Observation of Visible and UV Magnetic Dipole Transitions in Highly-Charged Xenon and Barium , Physical Review Letters, Vol. 74, 1995, p. 1716.
  • HP Cheng, JD Gillaspy: Nanoscale modification of silicon surfaces via Coulomb explosion , Physical Review B, Vol. 55, 1997, p. 2628.
  • JM Laming, I. Kink, E. Takacs, et al. a .: Emission-line intensity ratios in Fe XVII observed with a microcalorimeter on an electron beam ion trap , Astrophysical Journal Letters, Vol. 545, 2000, L161-L164
  • JD Gillaspy JD, DC Parks, LP Ratliff: Masked ion beam lithography with highly charged ions , Journal of Vacuum Science and Technology B, Vol. 16, 1998, p. 3294.
  • FJ Currell, J. Asada, K. Ishii, et al. a .: A new versatile electron-beam ion trap , Journal of the Physical Society of Japan, Vol. 65, 1996, p. 3186.
  • H. Kimura, N. Nakamura, H. Watanabe, et al. a .: A scaling law of cross-sections for multiple electron-transfer in slow collisions between highly-charged ions and atoms , Journal of Physics B (Atomic, Molecular and Optical Physics), Vol. 28, 1995, L 643

Web links

Individual evidence

  1. ↑ nist.gov: NIST EBIT Introduction