COMET Target Design COherent Muon to Electron Transition

COMET Target Design (COherent Muon to Electron Transition) Satoshi MIHARA

COMET Experiment • Mu-e conversion search • COMET Target group – Charged lepton flavor violation – GUT, ν mass origin Proton beam π μ Production target Muon stopping target – RAL • Chris Densham • Peter Loveridge • Tristan Davenne – KEK • Makoto Yoshida • Satoshi Mihara proton pulse prompt background muon decay Electron spectrometer

COMET Staging Approach • • Phase I – Beam background study and achieving an intermediate sensitivity of <10 -14 • 8 Ge. V, ~3. 2 k. W, ~3 weeks of DAQ • 2016 -2017 Phase II – 8 Ge. V, ~56 k. W, 1 year DAQ to achieve the COMET final goal of < 10 -16 sensitivity Phase II • Starts around 2019 -2020 μ- μ+ Phase I 104 Me. V/c 0. 03 BG expected in 1. 5 x 106 sec running time 10/Sep/2013 Satoshi MIHARA, PSI 2013 3

Beam Power • Phase I – 8 Ge. V, 3. 2 k. W – # of protons per MR bunch equivalent to that of 3. 2 x(30/8)x 2 = 24 k. W operation at 30 Ge. V • Phase II – 8 Ge. V, 56 k. W – Faster repetition cycle is necessary (1. 47 sec)

Production Target • Phase I (Radiation cooling) – Graphite • Refractory material and so is tolerant to high temperature operation • Experience in T 2 K – Tungsten • Larger muon yield • Radiation cooling may be OK but need careful assessment • Phase II (Active cooling) – Tungsten • Bad chemistry between tungsten and water • Helium cooling instead of water cooling

Radiation cooled tungsten (Phase I) Values used in simulations (not necessarily COMET baseline) Beam power 3. 2 k. W Target heat load 194 W Target radius 4 mm Beam radius rms 1 mm Tungsten emissivity 0. 3 Temperature Max = 1298°C Von Mises stress Max = 3. 56 MPa

Phase II: How about helium cooling? Values used in simulations (preliminary) Beam power 56 k. W Target heat load 3. 4 k. W Target radius 4 mm Beam size rms 1 mm Helium annulus thickness 1 mm Helium inlet pressure 8 bar Helium mass flow 5 g/s NB effect of beam cycle not included: 1/3 duty factor -> x 3 higher stress! Temperature Max = 921°C Von Mises stress Max = 63 MPa

Tungsten yield strength CW operation

Outline layout for annular cooling of target Coolant streamlines

Effect of off-centre beam Temperature profile for beam displacement of 2σ Deformation from beam displacement of 2σ Maximum displacement = 0. 07 mm