HD Still at James Madison University

To produce the SPHICE target requires a supply of high purity (<99.99%) HD gas. Since this gas is not commercially available, we must produce the gas ourselves. This is done by purchasing HD gas from Cambridge Isotopes and distilling it.

Since it is easy to remove all contaminants heavier than He by lowering the temperature and since He is not involved in the production process, the raw HD gas has essentially only H₂, HD and D₂ in it. A typical assay of the gas will be 1.5% H₂, 98% HD and 0.5% D₂. Since concentrations of H₂ and, in particular D₂, at these levels do not permit target production, the gas is distilled to reduce these two contaminants.

This system uses a two stage Joule-Thompson cooling loop using high purity He as the operating gas. Attached to the second stage is the still. The still is itself in two parts. The upper portion is made of stainless steel. This section has a tube down the center that contains H₂. This gas is liquified during operation and is used to stabilize the temperature of the upper portion. As HD gas condenses on this H₂ filled tube, the H₂ is boiled rather than raising the temperature.below this stainless steel section is the glass tail-piece. This section contains the Steadman packing that is the heart of the still. The Steadman packing consists of “pillows” of stainless steel wire gauze. These circular pieces are made by joining two concave pieces of wire mesh so that they bulge outwards. A stack of about 25 or 30 of these enhance the separation of the gases. At the bottom of the tail-piece is a heater used to boil the liquid HD.

The distillation process works because the vapor pressures of H₂, HD and D₂ each differ by about 10-15% at low temperature. When the liquid hydrogen is boiled, the most volatile (H₂) comes off most abundantly. At each layer in the column, the gas can condense and re-evaporate giving again another 10-15% enhancement of the lighter isotopes. The column used in our still is capable of poducing as much as 20-30 times the bulk concentration of H₂ in the initial stages of the distillation.

Distillation is done at about 20K. The gas is distilled in batches. The processing of one batch begins with the cooling of the still to its base temperature of about 12K. Valves to a gas manifold, to which tanks containing 10-12 moles of HD are attached, are opened and the HD condenses into the still. The valves are then closed and the still is warmed to the operating temperature.

The boil-up heater is then turned on to boil the hydrogen. Over the course of a couple days, a steady state is established in the column. The hydrogen boils up from the bottom and is condensed at the top and drips back down. The Steadman packing produces a separation with the H₂ at the top and D₂ at the bottom. The gas is then extracted at a constant mass flow back into the tanks on the manifold.

The relative concentrations are monitored with a residual gas analyzer to determine the efficiency of the process. A batch of 12 moles will produce about 6 moles of “good HD”. Generally, two passes through the system are required to achieve the desired purity. This requires three runs, two to get enough single distilled gas to make the double distilled run. Thus, 24 moles of raw HD will produce about 6 moles of double distilled HD of a quality high enough to use for making targets.