Electron stretcher system
The electron stretcher system ( ELSA for short ) is a particle accelerator operated by the University of Bonn . The three-stage system can store an internal beam current of a maximum of 200 mA with an energy of up to 3.5 GeV and extract it to various experiment stations.
construction
ELSA is the largest particle accelerator operated by a German university. The entire facility, which has been in operation in its current form since 1987, consists of three accelerators connected in series.
Linear accelerator
First, one of the two linear accelerators LINAC 1 (under construction) or LINAC 2 accelerates the electrons to approx. 20 MeV. Polarized or thermally generated electrons can be used as required. A single bunch injector will also be put into operation at LINAC 1, with which it will be possible to store only a single electron packet in the stretcher ring.
Booster synchrotron
The pre-accelerated electrons are brought to typically 1.2 to 1.6 GeV in the so-called booster synchrotron. The booster synchrotron works with the mains frequency of 50 Hz, which means that every 20 milliseconds a beam is injected by the LINAC, accelerated and extracted into the stretcher ring. The accelerated beam is then only available in very short pulses of less than a millisecond. For a long time, the pulsed beam was used directly.
Stretcher ring
The downstream, 164 m long stretcher ring can now either store the pulses of the synchrotron for 20 milliseconds and extract them evenly for the external experiment during this time or collect several pulses, possibly accelerating them to energies of up to 3.5 GeV and finally over long periods (seconds to minutes) extract evenly. After its construction in the eighties, the experiments (in the past: PHOENICS, ELAN, SAPHIR, GDH and currently Crystal-Barrel and BGO-OD ) could be supplied with almost constant electricity.
Experiments
Experiments with synchrotron light
Synchrotron radiation is a by-product of the acceleration of lightly charged particles in circular accelerators . It arises in the deflection dipoles of the machine, is extremely intense and also contains high-energy components over a wide spectrum (in addition to visible light, especially UV and X-rays). In the past, in addition to high-resolution lithography, X-ray fluorescence spectroscopy was particularly important for real-time investigation of chemical processes, but structural investigations on special materials, e.g. B. depth profile analyzes of surface treated material, e.g. B. after implantation or doping with foreign atoms. No experiments with synchrotron radiation are currently being carried out at ELSA.
Hadron Spectroscopy
For current experiments, polarized photons are required, which can be generated by means of polarized electrons from ELSA by bremsstrahlung on a photon marking system (tagging system). In combination with an unpolarized or polarized proton target, e.g. B. baryon resonances are measured. Liquid hydrogen or deuterium or alternatively a polarized frozen-spin butanol target is used as the target. The electrons in the atomic shell can almost be neglected in the investigations if the projectile particles are not charged particles like electrons, but uncharged ones like photons. Then these only interact with the nuclei, which in the simplest cases only consist of protons.
Detector tests
In the former synchrotron light laboratories, there is an external beam guide that delivers a primary electron beam that is used to test detector components.
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Coordinates: 50 ° 43 ′ 38.3 " N , 7 ° 5 ′ 18.2" E