Setup of the Accelerator
The ELSA electron stretcher facility is operated by the University of Bonn in North Rhine-Westphalia. It consists of three stages (injector LINAC, booster synchrotron and stretcher ring) and delivers a beam of polarized or unpolarized electrons with variable energy of up to 3.2 GeV. For the main research area, hadron physics, currents of up to 1 nA can be extracted and transferred to the CB-ELSA and BGOOD experimental stations.
There is also a second electron extraction to a measuring station where detectors for high-energy experiments are tested in the direct electron beam. Currents in the femtoampere range are usually extracted there. Maximum currents that can be extracted are up to 100 pA
The Source of Spin Polarized Electrons
In February 2000, the source (50 keV) for polarized electron beams was put into operation at ELSA. By using a Be-InGaAs/Be-AlGaAs superlattice crystal in the source, pulse currents of 100 mA with excellent pulse stability at high polarization can be achieved. Due to the high transfer efficiency from the source to the linear accelerator (> 95%), currents of up to 1 nA are achieved in the hadron physics experiments.
The Linear Accelerator
The linear accelerator is equipped with two electron sources (energy 50 keV), one for polarized and one for unpolarized electrons. In the linear accelerator, the electrons are accelerated to an energy of 26 MeV and then transferred to the booster synchrotron through a transfer line.
The Booster Synchrotron
The electrons accelerated to about 26 MeV in the linac are transferred through a beam line to the booster synchrotron and injected there. In the booster synchrotron, the electrons are then accelerated to a maximum energy of 1.6 GeV and then transferred to the stretcher ring. The booster synchrotron runs at a fixed repetition rate of 50 Hz. Extraction usually takes place at an energy of 1.2 GeV.
The Stretcher Ring
The stretcher ring can be operated in three different modes:
- The stretcher mode can be used up to a maximum energy of 1.6 GeV. The extraction energy in this mode is identical to the transfer energy. Injection into ELSA takes place during each cycle of the synchrotron. The electrons are then extracted over 20 ms while the next acceleration cycle is already running in the booster synhrotron. In this way, a macroscopic duty cycle of almost 100% can be achieved.
- The post-accelerator (booster) mode can deliver electrons with an energy of up to 3.2 GeV. The transfer from the synchrotron to the stretcher ring normally takes place at 1.2 GeV. Since no beam can be delivered to the experiments during the ramp time, several injections from the synchrotron are accumulated in ELSA before the energy ramp to increase the duty cycle. The extraction times are typically 4 - 8 s.
- In storage mode, ELSA is used as a storage ring. Typically, storage times of several hours are achieved at energies of 1.6 to 3.0 GeV. This mode is mainly used for machine studies with synchrotron light.
In the post-accelerator and stretcher mode, the electrons are extracted by slow extraction at a horizontal betatron resonance. For this purpose, the horizontal operating point is shifted towards the 4 2/3 betatron resonance using 4 ironless quadrupoles.
External Beamlines / Extraction
The stretcher ring has 2 extraction points for electrons:
The electron extraction for the hadron physics experiments is located at the magnets M22/23:
- CB-ELSA and TAPS
This collaboration investigates the photoproduction of meson resonances (search for missing resonances, study of the decay modes of known resonances, study of less established resonances). The results should help in understanding QCD at low energies. The measurements are carried out with a polarized target and a (circularly or linearly) polarized photon beam.
- BGOOD
This collaboration investigates the photoproduction of mesons on nucleons. The main components of the detector system are the BGO ball detector (consisting of 480 individual modules, 11.3 sr acceptance), a dipole magnet with a large opening angle (horizontal 12°, vertical 8°) in the forward direction for momentum separation and eight drift chambers and four time-of-flight detectors for track reconstruction.
The electron extraction to the measuring station for detector tests and medical physics is located at the magnets M7/8:
- Detector tests
The electron beam with an energy of max. 3.2 GeV is extracted to this measuring station and can be used there to characterize new particle detectors. The intensity of the extracted electron current can be precisely varied in a wide range from attoamperes (corresponds to an electron rate of a few kHz) to 100 picoamperes.
- Medical physics
In addition, this measuring station is used to carry out investigations into the irradiation of cancer cells with doses of up to 50 Gy using ultra-high-energy electrons (UHEE) and pulse durations in the range of µs to ms (FLASH). In these investigations, the electrons are not stored after extraction from the booster synchrotron, but only pass through the ring to the extraction point. This allows pulses to be generated whose length is determined by the orbital time in the booster synchrotron.
