DRAFT Metal hydride storage system for MICE experiment at Rutherford Appleton Laboratory. Technical specification. MICE Collaboration Contact persons: Dr Thomas Bradshaw Head of Cryogenics Section Tel: +44 (0) 1235 446149 E-mail: T.W.Bradshaw@rl.ac.uk Dr Iouri Ivaniouchenkov roject engineer Tel: +44 (0) 1235 445757 E-mail: i.ivaniouchenkov@rl.ac.uk Engineering Department Rutherford Appleton Laboratory Chilton, Didcot Oxfordshire OX11 0QX United Kingdom
1. INTRODUCTION International Muon Ionization Cooling Experiment (MICE) Collaboration has proposed to carry out at Rutherford Appleton Laboratory (RAL), UK an experiment called MICE, in order to demonstrate the reduction of transverse emittance ( cooling ) of a beam of muons using the technique of ionisation cooling. The experiment will employ three hydrogen absorbers as well as a set of RF cavities and particle detectors, all placed inside strong solenoidal magnetic field provided by a system of superconducting coils. A schematic structure of MICE experiment is shown in Fig.1, a more detailed description can be found in [1]. Figure 1. MICE experiment at RAL. 2. HYDROGEN SYSTEM DESCRITION 2.1 Hydrogen absorber In MICE the hydrogen absorber and focus superconducting coil elements will be mounted within a single module (3 such modules in total ) as it is shown in Fig.2. The absorber assembly comprising the hydrogen absorber vessel and its dedicated vacuum jacket, is a complete unit which is installed in the bore of the solenoid. The absorber assembly includes two sets of thin aluminium windows. The space between the inner and outer windows is vacuum pumped. The absorber is cooled with a helium coolant passing through a built-in heat exchanger. The module cryostat provides structural support for the above components, means for connections to external services, and vacuum insulation.
Figure 2. Cross section (left) and side view(right) of the MICE hydrogen absorber/focus coil assembly. Normal operating parameters for the LH2 absorber are summarized in the following table: Liquid-hydrogen volume (at 20K), litres 21 Hydrogen volume (at ST), litres 16548 LH 2 operating temperature, K 20.8 LH 2 operating pressure, bar abs 1.2 LH 2 max pressure, bar abs 1.7 LH 2 min pressure, bar abs 1.05 Max. heat removal, W 100 Refrigerant mass flow, g/s <2 Refrigerant inlet (outlet) temperature, K 14 (18) Refrigerant inlet (outlet) pressure, bar 18(14) Absorber vacuum volume (within the module), litres 91 2.2. Hydrogen system 2.1.1 System description The hydrogen system for the MICE absorbers is shown schematically in Fig.3. The system is duplicated for the each of 3 absorbers.
Hydrogen flow and safety system (option with a metal hydride storage unit) Version: 06/08/2003 V He urge system Metal hydride hydrogen storage unit Vent outside flame arrester 14 K He from Cold box 18 K He to Compressor via Radiation shield Fill valve Chiller/ heater unit X 2 Hydrogen module enclosure X 2 1.7 bar 2.1 bar H 2 Gas bottle Liquid level gauge Vacuum Internal Window LH 2 Absorber 70 K Safety window He Heat exchanger Vacuum vessel Vent valve H 2 Detector Vent valve H 2 Detector Ventilation system H 2 Detector Evacuated vent buffer tank Vent outside flame arrester Vent outside flame arrester V V ressure ressure ressure Non-return Valve V Vacuum pump relief valve Bursting disk gauge regulator valve Figure 3. Schematic diagram of the hydrogen system using metal hydride storage unit. 2.1.2 System components The main subsystems required for the hydrogen system include: Hydrogen storage system Hydrogen fill system urge system Window failure safety vent system 2.1.2.1 Hydrogen storage system In our chosen design, a hydrogen storage unit and a hydrogen absorber form a closed system such that hydrogen is either as a liquid in the absorber at the normal operating conditions when the absorber is cold, or is stored as a gas in the storage unit when absorber was warmed up and empty. ressure in the absorber - storage unit system is always higher than the atmospheric one, this prevents air from leaking into the system. The hydrogen is stored in hydride beds which are located ideally in the experimental hall close to the absorber. These beds absorb hydrogen when cooled and release hydrogen when heated,
normal absorber filling and empting time is about 5 hours. The temperatures at which these processes take place depend on the hydrogen material but are assumed to be near room temperature. The size of the storage unit for a MICE absorber is expected to be about the size of a small desk. It is believed that if there were a leak in the system when the hydride bed is cold, very little hydrogen would be evolved as it is chemically bound to the hydride. This is a big advantage compared with conventional hydrogen gas storage systems. 2.1.2.2 Hydrogen fill system While there are various options for this system, the simplest is to connect up a hydrogen gas bottle and fill the gas storage system to the required pressure. Bottles will be stored in a compound outside the hall. 2.1.2.3 urge system The purpose of the purge system is to remove all traces of hydrogen from the hydrogen system. The purge gas must be helium (if any other gases contaminate the system there is a risk of blockage. It will not matter if there is a small amount of helium present in the hydrogen.) The helium can be used to leak test the absorbers after installation. 2.1.2.4 Window failure vent system This emergency vent system is connected to the absorber vacuum space between the absorber and the vacuum windows. In the event of a rupture of the absorber window, it allows a safe path for venting the hydrogen gas. The system consists of a pipe leading from the top of the absorber vacuum space. This pipe leads through a low-pressure relief valve into a buffer volume that is sized to keep the pressure during the venting low. After this volume, a further relief valve leads to an external vent, outfitted to a flame arrester, on the roof of the building. All external piping will be insulated to decrease the pressure drop along the pipe from the cold liquid and to prevent the exposure of cold surfaces inside the absorber module that could serve to trap oxygen. Hydrogen monitors will be installed in the relief volume and in the piping to the absorber vacuum space to give adequate warning of a rupture. 3. HYDROGEN STORAGE UNIT SECIFICATION 3.1 General specifications Type Hydrogen metal hydride urpose Configuration Location referable size To fill and empty a MICE hydrogen absorber with hydrogen See diagram Fig.3. Inside the experimental hall close to a hydrogen absorber module Less then 1 m 3 in volume
Environment temperature 15-25 C Operating pressure in the system, bar abs 1.2 Max. pressure in the system, bar abs 1.7 Min. pressure in the system, bar abs 1.05 Hydrogen storage capacity (at ST), litres 20000 Absorber filling/empting time, hours 5 3.2 Control system A storage unit should be equipped with a pressure gauges and temperature sensors (with a computer read out) which can be implemented into the MICE hydrogen safety system. The specification will be agreed in due course. 3.3 Safety consideration A storage unit must be intrinsically safe. lease note that all the equipment supplied to the Rutherford Lab must confirm to the European, the UK and the RAL standards and codes. 4. ROJECT STATUS AND TIME LINE MICE Collaboration is currently seeking for funds and at the same time is working on the preliminary design for the experiment. If financial support is received in 2003/04, experiment can start in 2005/06. This means that the hydrogen system should be ready sometime in 2004/05. 5. REFERNCES [1] An International Muon Iionization Cooling Experiment (MICE). roposal to the Rutherford Appleton Laboratory. http://hep04.phys.iit.edu/cooldemo/micenotes/public/pdf/mice0021/mice0021. pdf