The Q_weak high performance LH₂ target

A high-power liquid hydrogen target was built for the Jefferson Lab Q_weak experiment, which measured the tiny parity-violating asymmetry in e→p scattering at an incident energy of 1.16 GeV, and a Q²=0.025 GeV². To achieve the luminosity of 1.7×10³⁹ cm⁻² s⁻¹, a 34.5 cm-long target was used with a beam current of 180 μA. 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.