Relativistic dynamics of electrons in strong fields
Final Report Abstract
Collisions of energetic electrons with highly charged ions (HCI) and heavy atoms provide unique opportunities to investigate dynamics of electrons in extremely strong electric fields. For example the uranium nucleus generates an average field of about 10^16 V/cm in the space region of its 1s orbital. Such fields are many orders of magnitude stronger comparing to fields achieved today by high-power lasers. A great detail of electron dynamics can be uncovered by measuring polarization of x rays emitted in collisions. Within this research project we have constructed high resolution x-ray polarimeters, developed several novel experimental techniques, and investigated dynamics of the most fundamental atomic processes — bremsstrahlung, photoelectric effect, dielectronic recombination. We have perfected the well known technique of Compton polarimetry, improving its polarization resolution down to record 0.3 deg, ten times better that any competitor. Using several recently developed technologies, we created efficient polarimeters in the energy range from 10 keV up to 2.1 MeV. At low energies we achieved very high energy resolution by employing segmented silicon drift detectors; at high energies we used segmented germanium detectors with the techniques of the pulse shape analysis and the Compton imaging to suppress strong radiation backgrounds. This is the broadest energy range covered by a single research group. With these polarimeters we executed a very broad experimental physics program. Based on new phenomena, which we discovered in our experiments, we developed a completely new x-ray detection concept — imaging circular polarimetry. This technique opens unprecedented possibilities for astrophysics: searches for bulk Antimatter and the Dark Matter in the Universe. While studying bremsstrahlung we observed rotation of x-ray polarization as a function of the incoming electron spin orientation. We have interpreted it as a result of the precession of the electron spin and its angular momentum induced by the electron motion in the extremely strong Coulomb field. This precession was postulated in the early days of quantum physics as a manifestation of the Spin-Orbit interaction. Until our experiment this precession was not observed. Our observation shows, surprisingly, that during the electron scattering, the electron is not moving within a fixed plane. Instead, its motion plane is precessing. We have also shown that at energies of a few MeV the electron deflection due to the Spin-Orbit interaction becomes comparable to or even stronger than the deflection caused by the Coulomb force. Furthermore, we have even developed the first practical application of this new phenomenon — we used it for precise polarimetry of electron beams. This polarimetry can detect all three electron beam polarization components. This makes it superior to the technique of Mott scattering, previously the only technique available at these energies. Next, we focused our attention on the photoelectric effect at relativistic energies. We investigated it via its time-reverse process of radiative recombination using heavy ion beams. By applying, for the first time, the technique x-ray coincidences, we observed that the electron orbital momentum rotates during the collision, and this rotation is much stronger than that predicted by the non-relativistic theory. In the same experiment we, for the first time, produced and observed coherently populated magnetic sublevels of a highly charged ion. Thus, our experimental technique opens a way to study effects of coherence at energies orders of magnitude higher than previously possible. At low x-ray energies we addressed polarization of x rays produced by dielectronic recombination with ions populated in an electron beam ion trap. Through polarimetry we have studied the details of the electron-electron interaction and observed a very strong effect of the magnetic interactions and timedilation, termed the Breit interaction. After that, we focused our efforts on the development of the plasma polarization diagnostics. The recent progress in fusion energy projects ITER and Wendelstein 7- X, and a strong will of astrophysics community to build new polarimetry missions, such as XIPE ESA M4 mission, motivate urgent development of the plasma polarization diagnostics. We took a leading role and performed systematic measurements and plasma modeling, and discovered that, surprisingly, trielectronic and quadruelectronic recombination, previously considered to be weak and insignificant, dominate polarization of plasma x rays. This calls for an urgent update of atomic databases used in astrophysics and plasma research. To demonstrate the practical application of our techniques for fusion energy research, we studied a magnetically confined plasma of similar temperature at the electron cyclotron ion source. Through polarimetry we observed formation of hot spots within this plasma.
Publications
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“Circular polarimetry with gamma-ray tracking detectors”, Nuclear Instruments and Methods in Physics Research A 640, 164 (2011)
S. Tashenov
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“Measurement of the Correlation between Electron Spin and Photon Linear Polarization in Atomic-Field Bremsstrahlung”, Physical Review Letters 107, 173201 (2011)
S. Tashenov et al.
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“Comparative study of the electron-atom and positron-atom bremsstrahlung”, Physical Review A 86, 032708 (2012)
V. Yerokhin et al.
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“Production and diagnostics of spin-polarized heavy ions in sequential twoelectron radiative recombination”, Physical Review A 86, 032701 (2012)
A. Maiorova et al.
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“Bremsstrahlung polarization correlations and their application for polarimetry of electron beams”, Physical Review A 87, 022707 (2013)
S. Tashenov et al.
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“Observation of Coherence in the Time-Reversed Relativistic Photoelectric Effect”, Physical Review Letters 113, 113001 (2014)
S. Tashenov et al.
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“Linear polarization of x-ray transitions due to dielectronic recombination in highly charged ions”, Physical Review A 91, 042705 (2015)
H. Jörg et al.
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“Polarization measurement of dielectronic recombination transitions in highly charged krypton ions”, Physical Review A 92, 042702 (2015)
C. Shah et al.
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“Spin-orbit interaction in bremsstrahlung and its effect on the electron motion in a strong Coulomb field”, Physical Review A 92, 062707 (2015)
O. Kovtun et al.
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“Strong higher-order resonant contributions to x-ray line polarization in hot plasmas”, Physical Review E 93, 061201(R) (2016)
C. Shah et al.
