TARGETS With thanks to many people whose data I have used from various reports. Roger Bennett email@example.com Rutherford Appleton Laboratory Chilton, Didcot, Oxon. OX11 0QX, UK
Reading - See the web sites Targets http://puhep1.princeton.edu/~mcdonald/mumu/target/ General http://www.cap.bnl.gov/mumu/ http://www.fnal.gov/projects/muon_collider/ http://bc1.lbl.gov/CBP_pages/ACP/muon.html http://www.hep.princeton.edu/mumu/ http://muonstoragerings.web.cern.ch/muonstoragerings/ http://hepunx.rl.ac.uk/~edgecock/muons.html
High Power Targets 1. Some existing targets 2. Design considerations Simple thermal principles 3. Some possible solutions
High Power Targets Beam power and target power are not the same Target Beam Material Target Size Power Cooling Power cm Density 3 kW W/cm ISIS protons, 800 MeV, tantalum 130 10x10x30 average -- 43 heavy neutron m slabs stopping peak --- 340 water 200 A, 50 HZ, ESS* protons, 1.3GeV mercury 2240 6x20x60 average -- 310 mercury neutron 4 mA, 50 Hz stopping peak --- 2500 PSI SINQ protons,600 MeV zircalloy 600 30x15 dia. average -- 110 heavy neutron 1.5 mA, cw lead stopping peak --- 300 water APT* protons tungsten ~200000 ~20x200x100 average --- ~1 heavy tritium ~2 GeV,~100 mA stopping water GANIL heavy ions, cw graphite 2.1 ~3 dia. cone 5250 radiation RNB e.g.Argon,3456 MeV cone stopping m 1 p A RIST protons, 800 MeV, tantalum 20 - 40 20x4 dia. average 80 -160 radiation RNB m foils transmission peak 170 -340 100 A, ~10Hz PSI protons, 600 Mev, graphite 6 0.6x5 average -- 60 radiation muon 1.5 mA, cw rotating wheel transmission peak --- 280 protons, 600 Mev, heavy PSI MEGAPIE (constrctn) test lead/ bismuth liquid 1000 beam power average - 1700 dia. 30x5 water 2.0 mA, cw stopping * Proposed
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
Pion Production • Hadron production models, e.g. MARS, used to predict pion production as few measurements exist • Some relevant data from E910 used to check MARS: E910 Layout • Detailed measurements will come from HARP
MARS Predictions Pion production higher with higher atomic number target
MARS Predictions Yield as a function of capture field strength Yield as a function of target radius magnetic capture required
MARS Predictions Yield/proton Pion production on a carbon target. Peaks at ~6 GeV Yield/proton/energy is low at low energy. Possibly a problem for CERN proton driver design
Pion Capture • Two capture methods proposed: US: 20T solenoid CERN: magnetic horn (as used for “conventional” neutrino beams) • Each feeds pions into ~1.5T solenoid decay channel • 20T solenoid formed from a resistive inner core and superconducting outer • Inner core shields outer “Old” 20T capture solenoid layout
Horn Capture Current of 300 kA p To decay channel Protons B = 0 Hg target B1/R
target protons Typical Schematic Arrangement of a Muon Collider Target
The Target 1-2 cm diameter target cylinder 20 cm Proton beam Energy 2-30 GeV Current 2-0.03 mA Power 1-4 MW Pulse 2 ns 10-50 Hz Target (high z material such as tantalum) Dimensions 20 cm long (2 interaction lengths), 1-2 cm diameter Power Dissipation ~200 kW Power Density ~13-3 kW/cm3 (average) ~10-100 GW/cm3/pulse Energy Density ~1000-60 J/cm3/pulse
Proton Pulse Length Imagine an infinitely short proton pulse hitting the target at time t = 0. The pions all originate at t = 0. The pions have a very large momentum spread axially and transversely. They decay to muons over a period of time (average tp = 26 ns) and the muons also have a wide range of momenta. The time width of the muon production is given by: for Ep = 300 MeV (max) dtdecay = 1.4 ns We need to keep the proton bunch reasonably short and not to lengthen the muon bunch too much. So the proton bunch should be ~ few nanosec.
Proton Repetition Rate Limited to ~50 Hz max due to time taken for the acceleration process in the muon acceleration and storage/decay ring.
individual bunches, ~1 ns long 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.
High Power Targets Target Beam Material Target Size Power Cooling Power cm Density 3 kW W/cm ISIS protons, 800 MeV, tantalum 130 10x10x30 average -- 43 heavy neutron m slabs stopping peak --- 340 water 200 A, 50 HZ, ESS* protons, 1.3GeV mercury 2240 6x20x60 average -- 310 mercury neutron 4 mA, 50 Hz stopping peak --- 2500 PSI SINQ protons,600 MeV zircalloy 600 average -- 110 heavy neutron 1.5 mA, cw lead peak --- 300 water 30x15 dia. APT* protons tungsten ~200000 ~20x200x100 average --- ~1 heavy stopping tritium ~2 GeV,~100 mA stopping water GANIL heavy ions, cw graphite 2.1 ~3 dia. cone 5250 radiation RNB e.g.Argon,3456 MeV cone stopping m 1 p A RIST protons, 800 MeV, tantalum 20 - 40 20x4 dia. average 80 -160 radiation RNB m foils transmission peak 170 -340 100 A, ~10Hz PSI protons, 600 Mev, graphite 6 0.6x5 average -- 60 radiation muon 1.5 mA, cw rotating wheel transmission peak --- 280 heavy PSI MEGAPIE (constrctn) test lead/ bismuth liquid 1000 beam power average - 1700 water Muon Collider protons, 2 GeV, 200 20x1-2 dia. 3000-13000 protons, 600 Mev, muon 1 mA transmission dia. 30x5 2.0 mA, cw stopping * Proposed
Design Considerations Need to knowBeam current density profile and target geometry Power density distribution within the target Pulsed or continuous Apply thermal calculations Temperatures,Cooling, Stresses including Pulsed Effects 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
Peak Power density vs . beam size and profile 2.5 1 GeV Protons on a Lead Target 2 Gaussian 1.5 Parabolic MW/litre Peak Power Density per MW of Beam Power 1 Uniform 0.5 0 4 6 8 10 12 14 16 Proton Beam Diameter cm
THE TARGET SHOULD NOT MELT • The temperature rise, DT per bunch (or pulse) for a given energy dissipated, DQ is given by; • where S is the specific heat and M the mass. This ignores cooling. For short high power pulses the cooling is generally negligible during the pulse. • The final temperature can be calculated by taking account of the cooling and successive pulses. Generally this leads to a saw tooth superimposed on an exponential rise to an equilibrium temperature. T time
Cooling Mainly a problem of power density 1. Fluid Cooling Water (limited to ~5-10 kW/cm3) Liquid metals Gas 2. Conduction 3. Radiation Limited to ~400 W/cm2 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
Water Cooled Target 2 cm diameter x 20 cm long water one central water channel 2 mm diameter water channels, taking up 50% of the cross sectional area Insufficient heat transfer across the water boundary Just possible (probably)
Power/cm2 radiated from surface at temperature T1 with emissivity 0.8 flux from 1 cm diameter target 3000 2000 radiated flux density, W/sqcm flux from 2 cm diameter target 1000 flux 0 500 1000 1500 2000 2500 3000 T1 temperature, Kelvin Flux = e s (T14-T24)
Radiation Cooled Target temperature at the centre surface temperature T1 T0 -T1= 796 K target cross-section, diameter D, length 20 cm, power 200 kW
Radiation cooled target 10 cm bore diameter, 0.5 cm wall thickness, 20 cm long T = 2890 K e = 0.8 ignores additional cooling from the bore T = 2890 + 37 K cross-section through the target tube If diameter increased to 20 cm, T = 2430 K
Pulsed Effects • 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
Tilt Angle Particles in a solenoidal field originating at the axis will rotate back to the axis. The target can be tilted at a small angle so that the particles will miss the target. The optimum angle can be calculated. target Axis of the solenoid ~6° pion
“EXCITING TIMES” • Challenge • At the edge and beyond present knowledge • New Ideas Required 1. The Liquid Metal (Mercury) Jet 2. Spheres in Flowing Coolant 3. Rotating Band 4. Bars
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. 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.
The velocities measured from the pool were twice that from the jet • At full power beam power there may be problems unless the pulse length is made 100 ms, i.e. problems with high power density. • Studies continuing at CERN/Grenoble.
Distribution of beam/target interactions as a function of z Mercury Pool Layout of the target area. From US Study
From US Study Yield as a function of capture field strength