1 cm diameter tungsten target

1. Goran Skoro University of Sheffield 1 cm diameter tungsten target

2. Potential problems with the high temperature, 1cm diameter, tungsten target(s) Shock: it seems to be too high for 4MW beam power (50 Hz); unexplored territory. Cooling: peak temperature rise ~ 600 K per pulse; cooling by radiation – equilibrium temperature too high for 4MW beam power (50 Hz). Next pages show a few late-night thoughts (with a little help from MARS and ANSYS) about the possibility to use the target(s) at the room temperature. This would (almost) eliminate the fear of shock effects (material is ‘stronger’) so the cooling scheme becomes the main concern. Forced cooling has to be included (this could be difficult) but the problem of high stress persists (could be even worse). Goran Skoro

3. Neutrino Factory target Tungsten; 1cm diameter, 25 cm length Beam energy = 10 GeV Parabolic beam, r_beam = r_target Optimal beam pulse length 3 x 2 ns bunches energy deposition from MARS B = 20 T - Energy deposition density – (for 3 different target-beam scenarios) Goran Skoro “Study II” “MERIT” “Best yield”

4. Neutrino Factory target Tungsten; 1cm diameter, 25 cm length Beam energy = 10 GeV Parabolic beam, r_beam = r_target Optimal beam pulse length 3 x 2 ns bunches energy deposition from MARS B = 20 T - Temperature rise per pulse – (for 4 MW beam power, 50 Hz frequency) ~ 600 K at the centreline ~ 200 K at the surface Goran Skoro “Study II” “MERIT” “Best yield”

5. For cooling schemes such as natural convection (air, water), forced convection (air, water, perfluorocarbons), spray cooling (perfluorocarbons) the heat transfer coefficients are well below 1 W/cm2K. Candidates: FC77 (poor thermal properties), glycol-water mixture or ‘pure’ water. The question is: Can we reach the heat transfer coefficient of 8 W/cm2K needed for the scenario of 200 x 1cm diameter target by using the impinging water jet(s)? Minimal value of the heat transfer coefficient needed to remove heat from the target between the pulses as a function of the cooling time (number of targets) Number of targets ANSYS result for the “Best yield” scenario Only solution: impinging liquid jet(s). Goran Skoro Cooling time [s] 25 50 100 200 300 400 Heat transfer coefficient [W/cm2K]

6. In order to reach 8 W/cm2K we need: Mass flow rate > 20 grams/s; Jet velocity, v > 10 m/s (more important); Nozzle diameter (d) and nozzle-to-target separation (S) should be carefully chosen in order to have the core of the jet as bigger as possible (4 < S/d < 10); Higher number of jets instead of single jet Other (non-negligible) reasons for concern in the case of high velocity water jets: - Instability of jet(s); - Geometry of the target (cylinder) – positions of the nozzles (number of jets); - Corrosion (should be small for tungsten); - Additional ‘stress’; - Erosion (for high velocity jets, v > 5 m/s). Problem: How to construct such a complicated cooling system (inside an already non-trivial target concept)? How to enhance heat transfer: Jet pulsations, Surface enhancements. Even if the the erosion rate for tungsten is 100 smaller than for aluminium, the projected erosion after (let’s say) 1 Million beam pulses is at the level of 0.5 mm (10% of the target radius). And the long term erosion effect is much worse. Data: scarce, but there are few short duration tests with aluminium and copper (haven’t found data for tungsten) Erosion, for example Goran Skoro