Description

Introduction

DESY provides three test beam lines (21, 22 and 24). These electron or positron beams are converted bremsstrahlung beams from carbon fibre targets in the lepton synchrotron DESY II with up to 1000 particles per cm² and second, energies from 1 to 6 GeV, an energy spread of ~5% and a divergence of ~2mrad.

A detailed description of the beam generation can be found below. More information about the operation, infrastructure and equipment and safety aspects can be found here.

The time schedule is made after the requests by test beam coordinator (Email: testbeam-coor@DESY.de). At the moment the co-ordination is done by Ingrid Gregor, F1 and Norbert Meyners, MEA1. Beam time can be requested via a request form or by email according to the points of the form.

(Please report the attendees via this list (Excel) before you come. Be aware that the German and DESY regulation forces all participants to attend a briefing about general, special and radiation safety before you can start to work at the test beam.)

The EU-funded AIDA project has been approved to improve the Detector R&D for detectors at accelerators. As a part of it, support for travel expense can be given for European, non-German DESY test beam user.

Brief Description

The counting rooms for beam 24 and 21/22 are close to the areas. The entire controls are located devices 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 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 they need to be in the DESY network they have to fullfill the DESY regulations (PDF) for computers - most important an up-to-date virus scanner - to get one or more IP numbers.)

The beam areas are surrounded by shielding blocks and are equipped such that flammable gases could be used in the tests. (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.)

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

Fig. 1: Schematic Layout of a Test Beam at DESY (pdf, ps)

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.

DESY II

Fig. 2: Ideal DESY II cycle; No Extraction

Fig. 3: Ideal DESY II cycle; Extraction

The electron/positron synchrotron DESY II is mainly an injector for DORIS and PETRA (picture of the beam lines in the DESY tunnel as they were in 2006). The average radius of the DESY II ring is 46.601 m (so the circumference is 292.8 m).

It accelerates and decelerates in sinusoidal mode with a frequency of 12.5 Hz (= 50 Hz/4 = 1/80 ms). Therefore one DESY II magnet cycle takes 80 ms (see Fig. 2). The revolution frequency is 1 MHz, the RF frequency 500 MHz, and the bunch length around 30 ps.

In theory, particles could be injected into the synchrotron every cycle. But typically, when running in DORIS mode, DESY II injects every second cycle (160 ms) single bunches with about 3*10⁹ positrons at 4.5 GeV/c into DORIS. For PETRA it injects every fourth cycle (320 ms) single bunches with up to 3*10¹⁰ (1*10¹⁰ positrons) at 7 GeV/c. The ejection to DORIS and PETRA happens at maximum energy 40 ms after injection. Therefore, a burst of 40 ms can be seen every 160 or 320 ms (see Fig. 3).

On the beam current scope this looks like Fig. 4 and 5 if the particles are not extracted or like Fig. 6 if they are extracted.

Fig. 4: Beam current scope picture: Electrons, No Extraction

Fig. 5: Beam current scope picture: Positrons, No Extraction

Fig. 6: Beam current scope picture: Positrons, Extraction

  Fig. 4: Beam current scope picture:
  Electrons, No Extraction
  Upper signal: Beam Current;
  Lower signal: Spill Counter Signal

  Fig. 5: Beam current scope picture:
  Positrons; No Extraction
  Upper signal: Beam Current;
  Lower signal: Spill Counter Signal

  Fig. 6: Beam current scope picture:
  Positrons; Extraction
  Upper signal: Beam Current;
  Lower signal: Spill Counter Signal

Table 1: Estimated Rates
Rates	Target
Energy	3mm Cu	1mm Cu
1 GeV	~3 kHz	~1 kHz
2 GeV	~5 kHz	~1.5 kHz
3 GeV	~4.5 kHz	~1.2 kHz
5 GeV	~15Hz (6GeV in DESY II) - 600 Hz (7GeV in DESY II)	~3Hz (6GeV in DESY II) - 200 Hz (7GeV in DESY II)
6 GeV	~3 Hz (7GeV in DESY II)	~1 Hz (7GeV in DESY II)

Rates

In table 1 you can find a rough estimate of the rates.

Figures 7 and 8 show the rates vs momentum for Testbeam 21 and 24, with 6 GeV Electrons or Positrons in DESYII. Figure 9 shows a comparison of the rates for 6 GeV and 7 GeV in DESY II (measurements done 10/2008).

The rates are influenced by many parameters. In practice, the maximum rate is around 5 kHz (3 GeV, 3mm Cu convert, Collimator ca. 5mm x 5mm, DESY II maximum energy at 7 GeV, no beam extraction, no DESY III ramp, single carbon wire).

Fig. 7: Rate vs p for Testbeam 21 (e+ @ 6 GeV, >1 wire)

Fig. 8: Rate vs p for Testbeam 24 (e- @ 6 GeV, 1 wire)

Fig. 9: Rate vs p for Testbeam 21 (e+, >1 wire)

  Fig. 7: Rate vs p for Testbeam 21:
  about 7*10e9 positrons @ 6 GeV,
  more than one carbon wire,
  different targets.

  Fig. 8: Rate vs p for Testbeam 24:
  about 14*10e9 electrons @ 6 GeV,
  one carbon wire,
  different targets.

  Fig. 9: Rate vs p for Testbeam 21:
  about 7*10e9 positrons @ 6 and 7 GeV,
  more than one carbon wire,
  5mm Cu target.

Beam Attributes

A detailed description of the test beams at DESY II is given in the EUDET-Memo-2007-11: Test Beams at DESY

Some details about the T24 beam line can be found in the document
" Characterization of the T24 electron beam line available at DESY " (PS / PDF)
courtesy D.Autiero, P.Migliozzi et.al.; OPERA Collaboration; March 12, 2004

Details

The carbon fibre has a thickness of 25 µm. Six fibres are prepared inside the fibre holder. 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.).

Geometry

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 (pdf; ps); whole hall (pdf; ps)).

Authors: Isabell Melzer-Pellmann, Norbert Meyners; Last change: 04.04.2012, Imprint