test setup with beam (draft)

test setup (draft)

interior view (draft)

test setup sectional drawing (draft)

Design and development of a Cryo-Collimator-prototype for superconducting SIS100 at FAIR

 The main SIS100 ring accelerator will provide high intense uranium beams for different kinds of experiments. For this purpose intermediate charged heavy ions are used to reduce beam losses due to the stripping process. Therefore electrons are stripped from ions through special foils. By operating with low energies, not only the wanted charge state will be generated, but a range of different charge states with low beam intensity behind the foil, which mostly won’t be used. Without the stripping process the high intensity of the beam can be hold and the maximum amount of generated ions of the beam will be shift due to space charge. Intermediate charged heavy ions also possibly can collide in the accelerator vacuum with residual gas molecules and exchange charge furthermore. Due to this accelerated U²⁸⁺ ions can exchange to U²⁹⁺ ions and because of the difference to the signifying ion, they will be separated therefore from the circulating beam and get useless. Where the collision takes place, ion induced desorption will set from the chamber walls adsorbed gas molecules free, and local gas clouds can appear which again raise the probability for charge exchange. At the worst case this could amplify itself up to the loss of the entire beam.

Collimator and Ion Catcher

 The desorption rate, amount of free molecules / invasive ion for grazing incidence, is about approx. 25.000 free molecules / invasive ion. A special surface coating of the cryo-chamber, a low desorption material OFHC-copper (Oxygen-Free High Thermal Conductivity - copper) with gold plating and nickel diffusion barrier, reduces grazing particles for perpendicular incidence down to ~50 / invasive ion than common stainless steel surfaces. By installing a high throughput close to the collimator, the few free gas molecules could be removed quickly from the accelerator-vacuum and the residual gas pressure stabilised. Less beam ions will be charge exchanged and lost, the high beam intensity will be stabilised. At the same time the activation of surrounding accelerator components will be reduced as the exposure to beam losses is lower.

Collimation system of SIS100

 The ion optical lattice (assembly of the magnets) of SIS100 is optimised for the operation of collimators in comparison to the lattice of SIS18. Charge exchanged ions normally getting lost at defined positions between the quadrupole magnets, where now will be installed the collimators. Since SIS100 will be superconducting, the collimators have to be fit in a cold environment. The advantage is that cold chamber walls can additionally act as cryo-pumps and desorbed gases could be bond easily. To keep the walls of the collimator as cold as possible, the stainless steel chamber is coated with a copper surface from outside which is connected with a LHe-supply line (Liquid Helium) via flexible copper bends. In order to assure a moderate cold temperature of the collimator-chamber, it is built from stainless steel coated with explosive welded copper. This technique, also known as explosive bonding, welds two plates of dissimilar metals by using the force of controlled detonation to accelerate one metal plate into another creating an atomic bond. Secondary chamber plates inside the chamber prevent the beam axes from desorbed gases and provide additionally cold surfaces. The head of the collimator is mounted insulated and hits of ionisation losses can be measured directly. In case of rapidly increasing losses due to bad vacuum conditions, an interlock will be activated and the beam will be kicked out of the machine. The collimation system avoids incidences of ionisation losses with the cold, superconducting magnets which would get heat by them.

Prototype

 The cryo-collimator prototype will be connected with a beam line at GSI and hit directly with a test beam. By means of pressure rise in the chamber, important parameters will be determined. Therefore the chamber will be built with a surrounding cryostat with thermal shielding and super insulation, connected via cold-warm-transition with a warmed beam line. As SIS18 will work as injector for SIS100 tests under realistic conditions will be possible.