Proton Background

• CJC-2 current vs beam current:

  

  We'd like to operate CJC2 below 150 uA.

  Protons-only (green stars) are fine. The point from 12.5. was taken with a 21h old proton beam, and after 14h of collisions.

  Positrons-only (blue squares) are also OK.
  Only points with best synchrotron radiation is properly steering and collimation are shown.
  e⁺ at 27.5 GeV is not worse than at 12 GeV (blue open squares), demonstrating that synchrotron radiation can be reduced to negligible levels.
  The contribution from positron-gas scattering remains, with an apparently quadratic dependendce on I_e.

  With ep collisons (red circles) the chamber current is higher than the sum of the currents from single beams.
  The points are plotted vs the e⁺ current. The proton current and date are printed next to the symbol.
  When scaling to a proton current of 30 mA (open red circles), all points lie around the red dotted line with a slope of 6 uA per mA of I_e.
  Scaling to 50 mA e⁺ and 120 mA p leads to a chamber current above 1 mA, clearly intolerable.
   

• A typical background event:

  
   

• Since April, our z-vertex distribution has not changed:

  

  The peak at 0 is due to genuine ep collisions.

  The peak at -z = 80 cm is from interactions at C5a.

  The peak at -z = 150 cm is from interactions at C5b.
   

• C5a and C5b are tungsten collimators inside the H1 beam pipe, shielding against backscattered synchrotron radiation:

  
   

• Hypothesis: the chamber current in ep collisions is due to hadrons interacting at the collimators and beam pipe walls.

  The hadrons originate from proton-gas interactions upstream of H1.

  Their rate is high due to the bad vacuum in the presence of e⁺ beam.
   

• Long lumi run of Sat night, 6.7.2002:

  The CJC was at full HV for 12h.
  The positron current dropped from 7.8 to 3.0 mA.
  The proton current dropped from 25.2 to 22.3 mA:

  

  The CJC-2 current dropped from 87 to 51 uA.
  The pressure at pump NR8.3 (North Right 8.3m) dropped from 5.9 to 4.1 10^-9mbar:

  

  Pressure and CJC current (after an initial decay) are proportional to I_e:

  

  And finally, CJC current vs pressure:

  

  The lower plot corrects for the small change in I_p during the fill by scaling to 30 mA.

  The pressure must be reduced to about 2 10^-9mbar at full e⁺ intensity, to allow for a safe operation of the chamber.
   

• The vacuum must be a factor 3 better than at HERA-I, where we had 6 10^-9mbar at I_e = 50 mA: