Brief Description

The counting rooms for test beam 24 and 21/22 are close to the areas. The entire control system for each area  are located in these huts. There is space for electronics and data acquisition, but it is also possible to place it in the area. The huts have 100MBit and wireless Ethernet connections and there are RJ45, BNC and HV cables connections to the areas. Cycle trigger and bunch trigger signals from the accelerator are also available in the huts. (The network now uses DHCP (Dynamic Host Configuration Protocol). So internal users can just bring their computers, and external users will be in the guest network not protected by the DESY firewall. If computers need to be in DESY network, please contact the administrator of the network segment (at the moment: Norbert Meyners.)

The beam areas are surrounded by shielding blocks and are equipped such that flammable gases could be used in the test beam areas. (All sorts of gas also premixed can be supplied by the DESY gas group MEA6. The use of flammable gas or other hazard materials needs special precautions. Please discuss this well before your test beam.)

The areas can be equipped with translation stages. The large one can carry up to 1000 kg and move the load 1 m horizontal and vertical. The smaller one can take 30 kg and is mainly used to carry the trigger counter.

Beam Generation

A bremsstrahlung beam is generated by a carbon fibre in the circulating beam of the electron/positron synchrotron DESY II. The photons are converted to electron/positron pairs with a metal plate (converter). Then the beam is spread out into a horizontal fan with a dipole magnet. Like a slice, the final beam is cut out of this fan with a collimator.

Therefore, the physics is simple. The bremsstrahlung spectrum has a 1/E dependence. The energy distribution of the electron/positron pair is nearly flat, the geometry is fixed by the beam pipe and setting the magnet current chooses the beam momentum. The real situation is more complicated because the energy of the synchrotron varies with time, i.e. the bremsstrahlung edge of the photon spectrum changes in time with cycles of the accelerator. The beam particles can reach the area obviously only when the accelerator energy/c is above the chosen momentum.

Schematic Layout of a Test Beam at DESY

Beam Attributes

The carbon fibre has a thickness of 25 µm. Six fibres are prepared inside the fibre holder (s.figure below). By rotation of the inner part, a broken fibre can be replaced without opening the machine vacuum.

The photon beam leaves the DESY II vacuum chamber through a 0.5 mm Aluminium window and then it has to pass through the DESY III vacuum chamber via two 0.5 mm Aluminium windows (The beam 24 is converted before DESY III).

The geometric arrangement is such that for beam 21 and 22 a bend of 32 mrad and for 24 of 80.6 mrad and a second of 32 mrad backward is needed to get the beam through the centre of the collimator (see detailed layout; whole hall.

Converter Target

There are different conversion targets available (Cu wire, Al and Cu plates of different thickness from 1 mm to 10 mm). It is controlled via a homemade NIM module (List of currently installed converters).

Conversion targets

Moveable Collimator

The four collimator jaws have their own control box. The beam shutter and the shielding wall follow the variable collimator behind the magnet.











Lead Collimator

Inside test beam area, it is possible to put a second lead collimator. It has exchangeable insertion. Several insertions with different apertures are available. This auxiliary collimator catches particles scattered off the yaws of the main collimator. Obivously its opening should be bigger than the one of the main collimator.

Second lead collimator

Beam signal oscilloscope

can be found in addition in the test beam hut.

The oscilloscope shows the beam current and the converter spill signal that is produced by a photo multiplier with scintillator close to each converter target. Therefore, it picks up a signal, if the photon beam hits the target. This allows deciding, if a "missing beam" problem is caused by the synchrotron and the fibre target, which is under control of the main control room or by the magnets, the converter, the collimators, the beam shutter or something else which should be under your control.