The Qweak high performance LH2 target

A high-power liquid hydrogen target was built for the Jefferson Lab Qweak experiment, which measured the tiny parity-violating asymmetry in ep scattering at an incident energy of 1.16 GeV, and a Q2 = 0.025 GeV2. To achieve the luminosity of 1.7 x 1039 cm-2 s-1, a 34.5 cm-long target was used with a beam current of 180 mu A. The ionization energy-loss deposited by the beam in the target was 2.1 kW. The target temperature was controlled to within +/- 0.02 K and the target noise (density fluctuations) near the experiment's beam helicity-reversal rate of 960 Hz was only 53 ppm. The 58 liquid liter target achieved a differential pressure (head) across the pump of 7.6 kPa (11.4 m) and a mass flow of 1.2 +/- 0.3 kg/s (corresponding to a volume flow of 17.4 +/- 3.8 l/s) at the nominal 29 Hz rotation frequency of the recirculating centrifugal pump. We describe aspects of the design, operation, and performance of this target, the highest power LH2target ever used in an electron scattering experiment to date.