Electron cooler

Figure 1 shows three configurations for colliding-beam experiments that have been used for studies of dissociative recombination. These are the inclined-beam apparatus at the University of Newcastle-upon-Tyne,30 the merged-electron—ion-beam experiment (MEIBE) at the University of Western Ontario,31 and an electron-cooler apparatus at the Manne Siegbahn laboratory in Stockholm.32 In the inclined-beam method (Figure la), the ion beam is accelerated to an energy of 30... 

The role of electron-electron interaction is one of the main topics of atomic, molecular physics and quantum chemistry. The normal helium atom is then naturally one of the most fundamental systems. Doubly excited states are as almost bound states of special interest since the role of the electron-electron interaction is important in describing energies and also autoionization rates. Dielectronic recombination processes where one of the two excited electrons falls down to a lower level while the other is ejected appears to be a fundamental process where electron-electron interaction plays a dominant role[6]. The recently built electron-cooler storage rings [7] have made it possible to study dielectronic recombination and thereby doubly excited states with high experimental accuracy. 

Figure 1. Schematic view of CRYRING. Molecular ions are created in the ion source MINIS, accelerated and mass selected. In some cases they are further accelerated by the Radio Frequency Quadrupole (RFQ), and injected into the ring. The accelerating system is used to further increase the ion energy. Reaction products from the electron cooler section exit the ring and hit detectors located on the 0° arm. The scintillation detector, which detects neutral particles arising from collisions of the stored beam with rest gas molecules, is used as a beam monitor.

The electron cooler, which is positioned in a dispersive-free straight section, serves two different purposes. First, it reduces the phase space volume occupied by the ion beam by beam cooling. Coulomb collisions between electrons and ions give rise to a friction force which slows the velocity of those ions travelling with non-zero velocity in the reference frame of the electron beam. Optimal cooling is obtained when the ion and electron beams have the same velocity. Second, the electron beam acts as a target in the studies of collisions between ions and electrons. 

The merged-beams geometry in the electron cooler section gives the possibility to study electron-molecular ion interactions at meV collision energies. The spread of relative energies of electrons and ions can be made very small and depends essentially only on the quality of the electron beam. 

Figure 3, The dipole magnet in CRYRING located immediately after the electron cooler. The ion beam is HD and the dipole magnet separates negatively charged products from resonant ion-pair formation in HD from the main beam. (Reproduced with permission from Ref. [55].)...

The electron cooler was supplied with a supraconducting magnet, which made it possible to expand the electron beam a factor of 100. This led to a transverse electron temperature only slightly above 1 meV. 

Figure 16 shows a pulse height spectrum recorded in an ion-implanted-silicon surface barrier detector mounted in the zero degree direction of the electron cooler in CRYRING. A stored beam of 4.4 MeV D30" ions interacts with a beam of velocity matched electrons, thus the collision energy... 

Figure 16, Pulse height spectrum recorded at the CRYRING with a 4.4 MeV stored beam of 030 ions and the electron cooler set to the velocity-matching condition. The peak at 4.4 MeV, corresponding to particles with a combined mass of 22 amu, originates from dissociative recombination, whereas the six other peaks are due to collisions of 030 with rest gas molecules. (Reproduced with permission from Ref. [116].)...

FIGURE 5.3 Scheme of the experimental setup at the electron-cooler device of the ESR storage ring. X-rays are observed at almost 0 deg vrith respect to the beam axis and recorded in coincidence with the down-charged ions (U + ... 

FIGURE 5.4 Left side X-ray spectra for capture into bare and H-like uranium (forming H- and He-like uranium, respectively) as measured at ESR electron cooler. Right side Schematic presentation of the RR process of free electrons into the initially bare and H-like ions. The energy difference AE = — ha>ne gives exactly the two-electron contribution to the... 

There are, however, other interesting experiments that were successful. For instance, in the electron cooler ions and atoms move with nearly equal velocities and form an unusual kind of a plasma. Here laser spectroscopy may help to better understand the physics of highly ionized plasmas [1245]. 

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