Skeleton contributions to targets section Chris Densham, Otto Caretta, Tristan Davenne, Mike Fitton, Peter Loveridge, Matt Rooney

Conceptual design of target 1: Separate vs combined target and horn Targets separately supported inside horns proposed since: Target cooling – key challenge. Horn cooling at high rep rate also significant issue. A combined target and horn would require a HTC x 10 higher than existing state-of-the-art Separate target and horn permits separate cooling solutions Reduced thermal stress in target Thermal stress wave in target is in addition to high steady-state stress Reduced magnetic stress in horn, with increase in horn inner conductor radius Possible to tune target and horn geometry separately, both radially and longitudinally More favourable target design and cooling options possible Increased tolerance to accidental off-centre beam Failure modes are not combined, possibly longer lifetimes for both Targets can be replaced separately within horn

Conceptual design of target 2: Target cooling Helium cooling of target proposed since: Possible for coolant to be within beam footprint Negligible interaction with beam No generation of stress waves in coolant Low activation of coolant No corrosion problems Several different target/cooling geometry options possible Challenges/disadvantages of helium cooling (vs water) Lower heat transfer coefficient than water Needs high pressure (>10 bar) for sufficient mass flow and low pressure drops

Conceptual design of target 3: Helium cooled candidate Target concepts Solid rod appears too challenging (ref Peter slides) High heat load at front end Pebble bed target appears feasible But lower material fraction Either beryllium or titanium spheres appear possible Titanium would permit recovery of material mass and consequent pion production within focal length of horn Distributed mass target ‘Tapered pencil – like ’ target front end

Packed Bed Target A couple of relevant papers: A helium gas cooled stationary granular target (Pugnat & Sievers) 2002 The “Sphere Dump” – A new low-cost high-power beam dump concept (Walz & Lucas) 1969 Why consider a packed bed as a high power target? Surface to volume ratio through out target enables significant heat removal with reasonable target temperature Small target segments result in low thermal stress and also low inertial stress (stress waves and excited natural frequencies) Structural integrity not dependant on target material Points to note Lends itself to gas cooling High power designs will require pressurised gas (need to consider in design) Bulk density lower than material density (approx factor of 2) may result in a reduction in yield compared to a solid target. Suitable alternative materials may be able to increase bulk density

Temperature and stress in a uniformly heated sphere

Ideas for a transversal helium cooled target Packed bed. Helium cooled spheres could be arranged with either longitudinal or transversal cooling (e.g. with triple channels) reducing drastically the temp difference (and stresses) within each sphere. Stack of face cooled disks with triple helium cooling channels to prevent longitudinal deformation (banana effect) and transversal stresses A higher Z material (e.g. Titanium) could be used to make up the loss of material in the beam line

Preliminary packed bed modelling Transverse flow packed bed 45 kW deposited power in a titanium packed bed 3 input channels Single output channel Staggered holes to encourage cross flow 10bar helium as coolant Packed bed modelled as a porous media with Ergun equation Hole size and pressure drop under optimisation Initial results confirm significant heat removal is possible with a reasonable operating pressure and pressure rise

Significant heat load & excessive thermal stresses High heat flux requires optimisation of the cooling pattern to achieve high heat transfer Thermal stresses are considerable both longitudinally (e.g. off-centre beam produces banana effects) and on the cross section (i.e. temp difference between the core and the skin is substantial and may result in surface cracking) Water cooling near the beam is to be avoided! Water near the beam would experience significant secondary heating with consequent pressure pulses and possible cavitation Target segmentation prevents longitudinal deformations and stress propagation and allows localised cooling as well as reducing the temperature difference within each segment Transversal (parallel) cooling reduces temp difference between the core and the edges Helium cooling can be routed to deliver cooling through on the warmest spots on the beam line whilst avoiding activation and pressure effects