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

3. 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

4. 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

5. 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

6. 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

7. Temperature and stress in a uniformly heated sphere

8. 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

9. 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

10. 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