CJC currents

CJC2 current history Oct 2003 until May 2004:

CJC gain changes due to atmospheric pressure variations are compensated since day 54 of 2004. Before that I ~ p^-5.

CJC2 current for the 49.5 mA e⁺ fill:

The increased chamber current at the beginning is due to the pressure peak at injection.
The proton current varied from 90 to 70 mA during this fill. 85 mA is some average.

For the H1 shift crew:

CJC2 current vs positron beam current, scaled to 105 mA protons from 8.4.2004:

CJC2 current vs positron beam current, scaled to 105 mA protons from 17.3.2004:

Shown at DPG 2004.

The progress was first seen on Mon 9.2.2004:

Lumi run with 57 mA protons and 23 mA e⁺:

After some beam steering we observed the lowest chamber currents ever, at these beam currents.

A 17% increase in CJC gain due to increased HV is taken out in this plot.

Coefficients at nominal gain are:

I_CJC2 = I₀ + α_SR I_e + α_egas I_e² + p₀ I_p + α_pgas I_e I_p

I_0,CJC2 = 14 μA from dark current

p₀ = 0.15 μA/mA from base pressure

α_pgas = 0.023μA/mA² from dynamic pressure

The reduction of the p-gas background is directly seen in the DCRphi trigger rate:

We are approaching a DCRphi t₀ rate of 1 Hz/mA².

The coincidence of DCRphi with Backward Tagging Scintillors at -6m and -8m shows the origin of the p-gas background.

Extrapolating to I_e*I_p = 50*100 mA² we now expect 200 μA in CJC2, which was tolerable in 2000.

The recent progress was simply due to steady operation at the highest possible positron current,
without vacuum leaks,
possibly accelerated by GO/GG warmup and/or NEG pump regeneration
in GI1,2, and GJ magnets around H1 during the maintenance day on 5.2.2004.

The synchrotron radiation an e-gas background has also improved:

Coefficients from Dec 2003 and Feb 2004:

I_CJC2 = I₀ + α_SR I_e + α_egas I_e²

α_SR = 0.6 μA/mA from backscattered synchrotron radiation

α_egas = 0.007 μA/mA² from e-gas

Subtracting the SynRad and e-gas contribution leaves us with the p-gas part:

The slope (dynamic pressure increase) has decreased by a factor of two from 1.1 to 14.2.2004.

Full CJC2 current history since 2002:

• Subtract 15 μA pedestal for CJC2 (constant since 2002).
• Subtract SynRad = 1.2 μA per mA of e⁺.
  When the beam is well steered (flat in x and y up to 100 m), only the irreducible backscattering from ABS4 remains.
• Normalize to e-gas and p-gas contributions as determined in Sep 2002, as function of beam currents.

CJC2 current on 2.1.2004.

Lumi run with 50 mA protons and 26 mA e⁺:

The CJC2 limit of 150 μA will be reached at I_e· I_p = 40·50 mA²,
or at 30·60 mA², which is possible with 120 bunches.

Another factor of 2 in proton-gas background reduction is needed.