Targets Neutrino Factory J. R. J. Bennett roger.bennett@rl.ac.uk Rutherford Appleton Laboratory Chilton, Didcot, Oxon. OX11 0QX, UK

The Target Specification Current Status What we would like Reality FP6

Specification Proton Beam - Pulsed at 10-50 Hz Energy 2-30 GeV Average Power ~4 MW

pulse train, several ms long Proton Bunch Train There are a number of individual bunches which make up the bunch train, which may be several microsecond long. RAL design has 4 bunches, CERN design over 100. individual bunches, ~1 ns long pulse train, several ms long

Target (high z material such as tantalum) Not a stopping target The Target Proton beam Energy 2-30 GeV Current 2-0.1 mA Power 4 MW Pulse ~2 ms, (2 ns substructure) 50 Hz Target (high z material such as tantalum) Not a stopping target Dimensions 20 cm long (2 interaction lengths), 1-2 cm diameter Power Dissipation ~1 MW (average) Power Density ~16 kW/cm3 (average) Energy Density ~320 J/cm3/pulse 20 cm 1-2 cm diameter target cylinder

Typical Schematic Arrangement of a Muon Collider Target protons

High Power Pulsed Targets energy density 0.03 kJ/cm3 energy density 0.3 kJ/cm3 mean power dissipation 1-10 MW energy dissipated per pulse 20-200 kJ p-bar target at Fermi Lab 6 kJ/cm3 mean power dissipation 1 MW pulse length 1-2 ns pulse repetition rate 50 Hz energy dissipated per pulse 20 kJ pulse length 1 ms Power Dissipation Spallation Neutron Source Target Neutrino Target

The Importance of Target Studies The proton accelerator looks good The collection and cooling channel is inefficient but will work The target is the only “Show Stopper” Know how to design a target for lower power dissipation - ~100 kW

Design Considerations Need to know Beam current density profile and target geometry Power density distribution within the target Apply thermal calculations Cooling, Stresses including Pulsed Effects, Temperatures Radiation Effects Shielding, Activity, Remote Handling, Beam Dump, Radiation Damage, Maintenance, Target Changes, Disposal Magnet Magnetic field, sc magnet (heat and radiation), Forces, Induced Currents Design Considerations

Mainly a problem of power density Cooling Mainly a problem of power density 1. Fluid Cooling Water (limited to ~5-10 kW/cm3) Liquid metals Gas Increase the Effective Volume Larger target- less dense, larger cross-section, less radiation damage Moving target - rotating wheel, moving band Flowing Target - liquid metal in tube, liquid metal jet, solid powders in fluid - radiation damage no problem 2. Conduction 3. Radiation Limited to ~400 W/cm2

Water Cooled Target 2 cm diameter x 20 cm long Insufficient heat transfer across the water boundary Just possible (probably) one central water channel 2 mm diameter water channels, taking up 50% of the cross sectional area

Current Status CERN have had considerable success with studies of mercury jets (with BNL), including within solenoidal fields. They are putting forward mercury jet and granular targets. CERN ISOLDE have experience of the problems of radioactivity and of shock waves. They have a laboratory suitable for handling active materials and molten metals - mercury. PSI are building a liquid metal target. They are involved with the US in liquid metal targets for high power spallation sources. RAL has done preliminary tests on shock waves in hot tantalum using electron beams. CERN, PSI (not pulsed) and RAL have facilities providing high power proton beams. Also in the US at Los Alamos, Brookhaven and FNAL.

Programmes under way The USA are building SNS. The Japanese are active as well. LAL/Orsay are developing Horns.

Continue with present lines of study Target Studies Continue with present lines of study 1. Molten Metal Jets 2. Flowing Contained Molten Metal 3. Helium Cooled Solid Spheres 4. Moving Solid Targets or Rotating Band 5. New Ideas?

The ISIS Target

Flowing Metal Target Problems with Cavitation. Problem for SNS and ESS Solid Target Water cooled metal (tantalum) for up to 1-2 MW Tantalum is a good material. a. Little radiation damage. b. OK with water.

The basic concept (left) and conceptual design (right) of the MEGAPIE target

Proposed for the USA and by CERN The Mercury Jet Proposed for the USA and by CERN The jet breaks up when the beam hits it, but the beam has interacted with the jet before it has time to break up. So no problem. Tests show that the “next jet” is not prevented from appearing in time. Jet velocity ~30 m/s. No power limit? The jet hits the walls and they must take the heat and the effect of the mercury hitting the walls. Not thought to be a problem. The mercury is condensed and recycled. It can also be distilled so removing some of the radioactive isotopes formed in the jet. Interaction of the jet with the magnetic field of the solenoid is not a problem. A mercury pool in the target chamber can serve as the beam dump.

Handling the mercury is hazardous Handling the mercury is hazardous. If the mercury escapes - severe hazard. Messy! Need windows between the other parts of machine.

Tests at BNL and CERN Calculations of injecting into a magnetic field showed small perturbations. The field provides damping of the motion in the jet. Tests at Grenoble (CERN) show this. Tests with a proton beam at BNL showed that the jet broke up.

Recent schematic of the CERN Target

Solid Metal Spheres in Flowing Coolant P. Sievers, CERN Small spheres (2 mm dia.) of heavy metal are cooled by the flowing water, liquid metal or helium gas coolant. The small spheres can be shown not to suffer from shock stress (pulses longer than ~3 ms)and therefore be mechanically stable.

A Cu-Ni Rotating Band Target (BNL and FNAL) Radiation Cooled Rotating Toroid, Magnetically Levitated (RAL)

TOROID OPERATES AT 2000-2500 K RADIATION COOLED ROTATES IN A VACUUM VACUUM CHAMBER WALLS WATER COOLED NO WINDOWS SHOCK? Pbar target OK. Tests using electron beam simulation indicate no problem.

collection and cooling reservoir Levitated target bars are projected through the solenoid and guided to and from the holding reservoir where they are allowed to cool. solenoid collection and cooling reservoir proton beam

Pulsed Effects pbar target The thermal shock can exceed the mechanical strength of the material causing it to break. The “pbar target” at FNAL is OK at 10 times the pulse power density. The pbar target has: 600-700 J/gm in nickel discs, DT per pulse (1.6 ms) ~1000 K (compare to 60 J/gm in the neutrino tantalum target) Stack of slowly rotating discs Gas cooling between discs proton beam pbar target

OTHER PROBLEMS Magnet Target operates in a magnetic field, Forces, Induced Currents SC magnet (heat and radiation), Radiation Beam Dump - 1-4 MW! Spread the beam to make cooling easier. Radiation Shielding. Maintenance. Remote handling essential. Target changes. Disposal Safety requirement/legislation -formidable.

Conclusions We have a number of possible solutions. None have been tested at full beam or lifetime (problem!). Other problems only partly addressed. Lots to do!!

Current Status

In-Beam Target Studies For a full-scale test with beam - need: 1. High Power Beam 2. Target Station with Facilities: Radioactive handling Chemical and contamination hazards Disposal EXPENSIVE!! - £100 M +

Note: Even with a full power beam test it is difficult to simulate life time studies in much shorter times.

Reality 1. CERN - no funds 2. ESS (neutrons) - no funds 3. RAL - some reasonably substantal funds over 3 years starting 2004 for accelerator and target R&D 4. Other Europe - no funds (?) 5. SNS (neutrons, USA) - big effort on the target 6. Japan (neutrons) - likely to have big effort on target

FP6 1. Network to include target studies 2. Design Study to be considered Will not provide enough money for a Target Station (or a full power proton beam)

Target Section FP6, Network Collaborators: Helge Ravn (CERN) Jacques Lettry (CERN) Peter Sievers (CERN) Bruno Autin (CERN) Francois Meot (CERN) Andre Verdier (CERN) Jean Marie Maugain (CERN) Günter Bauer (Juelich) Roger Bennett (RAL) Paul Drumm (RAL) Chris Densham (RAL) Dave Rodger (Bristol Univ.) Mohamed Farhat (Lausanne Univ.) Friedrich Groeschel (PSI) K. Thomsen (PSI) G. Heidenreich (PSI)

What can be Realistically Achieved? 1. Calculations. 2. Simulated tests using electron beams, lasers etc. 3. Pulsed proton beams of less than ideal intensity from CERN or ISIS or -? 4. Unlikely to be able to do full power in-beam tests prior to building a suitable accelerator.

“EXCITING TIMES” Challenge At the edge and beyond present knowledge New Ideas Required 1. The Liquid Metal (Mercury) Jet 2. Flowing, Contained Mercury 3. Spheres in Flowing Coolant 4. Rotating Band 5. Bars