Log entry time 16:46:44 on June11,2004

Entry number 125371

This entry is a followup to: 125365

Followups:

• 125450 06/12/04 13:42 Armstrong Propose target load test

 Here's a better fit than the earlier log entry, along with the 
fit values. 

 Looks to me like the target can get to about 50 µA at the 
operating point we have been using. If I take the target temperature
as 6.6 K (the setpoint of the PID loop, and the value read by
the two thermometers on the intake to the cell), and the pressure
as 230 psi, then our target density should be about 0.138 gm/cc,
which is just under our "design" value of 0.14 gm/cc.
  (design operating conditions were 5.8 K and 180 psi).

 If we increase the operating temperature we will lose density,
however we will gain in cooling power (more delta T with the 4.5
K coolant provide by CHL). One Kelvin will (at constant pressure)
drop the density (and thus the luminosity) by 7%, but it may
increase the cooling power significantly. A naive calculation
would say that it increases delta T by 50%, thus potentially
increasing the power by a similar large factor (forgetting things
like static heat loads due to the target fan, etc.).

  If the target is indeed limiting our luminosity, Jian-Ping 
thinks we should try raising the temperature, and I agree. 
Thanks to J-P for the detailed advice!

  You might ask about the pressure. The density is a less steep
function of pressure for cold He gas than it is of temperature.
Dimitri's pressure tests of the 20 cm racetrack cells with around 
0.010 inch wall thicknesses (similar to the present cell) typically
went up to 575 psi beore some of them would burst. So, 230 psi 
represents a factor of two safety margin.

  The third factor to throw in the mix is the septum heating issue. 
We don't know, for a fixed luminosity seen by the HAPPEX detectors,
if the septum heating would be less for higher beam current on a 
less dense target or for lower beam on a denser target. Perhaps
we can explore this briefly as well.