1 cm diameter tungsten target

Goran Skoro

University of Sheffield

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).

Goran Skoro

Forced cooling has to be included (this could be difficult) but the problem of high stress persists (could be even worse).

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.

Neutrino Factory target Tungsten; 1cm diameter, 25 cm

lengthBeam energy = 10 GeV

Parabolic beam, r_beam = r_targetOptimal beam pulse length

3 x 2 ns bunchesenergy deposition from MARS

B = 20 T

Goran Skoro

- Energy deposition density – (for 3 different target-beam

scenarios)

“Study II”

“MERIT”

“Best yield”

Goran Skoro

- Temperature rise per pulse – (for 4 MW beam power, 50 Hz

frequency)

“Study II”

“MERIT”

“Best yield”

Neutrino Factory target Tungsten; 1cm diameter, 25 cm

lengthBeam energy = 10 GeV

Parabolic beam, r_beam = r_targetOptimal beam pulse length

3 x 2 ns bunchesenergy deposition from MARS

B = 20 T

~ 600 Kat the centreline

~ 200 Kat the surface

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)

Goran Skoro

ANSYS result for the “Best yield” scenario

Heat

tran

sfe

r coeffi

cie

nt

[W/c

m2K

]

Cooling time [s]Number of

targets

25 50 100 200 300 400

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.

Only solution: impinging liquid jet(s).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)?

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).

Goran Skoro

material Jet freq. [Hz]

V [m/s]

Dur. [h]

Erosion rate [nm/impact]

Al 33 5 5 ~ 0.1

Cu 4.2 125 4 ~ 5

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

How to enhance heat transfer:

Jet pulsations, Surface enhancements.

Erosion, for exampleData: scarce, but there are few short duration tests with aluminium and copper (haven’t found data for tungsten)

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.

Problem:

How to construct such a complicated cooling system

(inside an already non-trivial target concept)?