HERA meeting 21.5.2002

HERA

• Wed 15.5. a special e⁺ run was taken with 1 mA in 10 filled bunches spaced by 19 empty bunches, for timing studies in ZEUS and H1 drift chamber.
• Wed 15.5. afternoon: short access to check NR GI magnet coils: OK.
• Wed 15.5. evening: H1 GO magnet raised by 1.7 mm with external hydraulic moving system. An e⁺ beam of 0.6 mA at 12 GeV was used for orbit measurements. The vertical correction coil in GO could be reduced from -34 A to 0.5 A.
  A new lumi file was prepared Thursday.
• Wed 15.5. night: 14 mA e⁺ beam for Hermes.
• Thu 16.5.: no positrons for several hours due to a short in a Leistungsschalter in DESY II. Repaired by HEW.
  Proton studies at 40 GeV during this time.
• Thu 16.5. evening: beam based alignment studies at 27.5 GeV in the North.
• Thu 16.5. night: 28 mA stable proton beam for HERA-B
• Fri 17.5. day: repeat the special timing run with 11 widely spaced bunches, starting with 2.4 mA at 27.6 GeV. A screen shot of the bunch pattern is here. The idea is to have a bunch spacing larger than any drift time to avoid pile-up (1.1 us in H1 CJC, about 0.6 us in ZEUS CTD).
• Fri 17.5. night: 22 mA e⁺ beam for Hermes.
• Sat 18.5. morning: background studies in H1 with 6 mA e⁺. Using a few 3-coil bumps like CH 44 11 02 and CV 56 27 04 the CJC2 current was reduced from 58 to 30 uA at full HV. Opening the 6 m vertical collimator NL06o and NL06u from 8 mm to 12 mm reduced the chamber current to 21 uA. The cosmics level is 14 uA. A screen shot of the CJC2 current during this time is here. The CJC1 current was then 17 uA, with a cosmics level of 7 uA. An updated plot of chamber currents vs beam current is here. The effect of the collimator is attributed to increased beam-gas interactions from outgassing when the collimator is hit by direct synchrotron radiation. This was confirmed by observing a small increase in the local pressure, see picture. All collimators need bake-out before they are fully beneficial.
• Sat afternoon 5 attempts to ramp the e⁺ beam beyond 21 GeV failed due to high rates in H1 and ZEUS radiation monitors.
• Sat 18.5. night: 24 mA stable p beam for HERA-B.
• Sun 19.5. morning: ramped 15 mA e⁺ to 21.4 GeV, where the background could be reduced to acceptable levels after beam steering. Ramping further had to be aborted due to extremely high rates (up to saturation at 10 MHz) in H1 Spacal and Radiation Monitor. Also the ZEUS diode rates were very high. Bad vacuum in the IP may have been the cause. A plot of the pressure at NR 6 m is here. This presure peak was in fact seen also at NR 3.6 and NL 5.8 and, to a smaller extent, at NL 8.1, see plot.
• It was then decided to regenerate the NEG pumps around the experiments, which uses a current of 20 A at up to 300 V to heat the Ti strips and drive adsorbed molecules deeper into the material. During this procedure a large vacuum leak occured in the South section. The e and p beam pipes between the shutters at +-26 m were vented.
• The leak was localized in the proton beam pipe in the first GM magnet at SR 13 m. This is the half-quad proton final focus magnet, which can be opened at the mirror plate by step motors. This was done by Sun evening.
• A 4 m proton beam pipe section was replaced by a spare part on Monday morning and pumping started a noon time.
• The GM magnet was surveyed and e⁺ beam operation at 12 GeV commenced on Tue evening when the vacuum was just below 10^-6mbar for bake out.

H1

• A first analysis of Wednesday's timing run is here. The upper plot shows the early part of the drift time spectrum for track, defining the rising edge as t₀. The lower plot shows the drift time spectrum for random hits, presumably from synchrotron radiation. The rising edge is much flatter, indicating a distributed source and multiple bounces for the backscattered radiation. The midpoint of the rising edge is delayed by 28 ns, corresponding to a backscattering from around 4.2 m North Right, which could be the photon chamber. Also indicated are the expected arrival times for backscattering from 11 m and 24 m, which must be present at some level, but is swamped in this run.
• More timing studies are underway on Friday's data, when different collimator settings were tried.

ZEUS

• The silicon detector received a dose of about 6 krad in one incident during the weekend, raising the accumulated dose to about 24 krad.
• p-only beam: the C5A collimator is a strong source of beam-wall interaction, as seen by the central track detector. Interaction vertices cluster ring inwards and slightly upwards. They are caused either by off-momentum protons bent towards the inside or by beam halo touching the inner edge of the collimator. In the first case better vacuum will help, in the second the proton collimators and even the HERA-B wire might help. The calorimeter energy spectrum for these events has a peak at around 250 GeV and extends out to 600 GeV, mainly in the FCAL. However, it is not possible to distinguish off-momentum particles from beam-halo due to acceptance losses in the beam pipe.
• e⁺ timing runs: Drift time spectra for tracks and for random triggers were shown. The later show a slowly rising edge. The midpoint corresponds to a delay of 72 ns or backscattering from 10.8 m. The z-distribution has a maximum in the backward direction (SR, electron downstream). The timing spectrum changes with z, which is not understood. The φ distribution of hits is flat in the forward half, while it has strong maxima in the up and down directions in the backward half, which might indicate shadowing effects from a collimator. The method is now established. Runs with different beam steering and collimator settings have now to be taken.

Schedule

• Beam pipe bake-out in the South with 12 GeV e⁺ beams for several days to improve the vacuum. In addition, bake out of synchrotron radiation collimators.
• Beam based alignment and other optics studies with 12 GeV e⁺ beam.
• Investigate background problem during ramp to 27.5 GeV.
• Reproduce good e⁺ background conditions in H1 at 12-15 mA.
• Proton beam as soon as the vacuum allows.
• Machine studies on Mon and Tue; program to be defined by the Task Forces.