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Sodium Handling at the LEBT Polarized Beam Line at ISAC

Unpolarized ⁸Li⁺ or other alkali-metal ion beams are neutralized in a sodium vapour cell, with typically 50% efficiency (although over 90% efficiency is possible at high Na vapour densities). The jet-type cell construction, which minimizes the cell length while efficiently trapping the sodium, is shown in Figure 1. Sodium vapour flows from the reservoir, which is typically at 390 C, through a pipe and nozzle, at about 410 C, forming a jet target that is condensed by a collector, which is at less than 175 C. The liquid Na drains under gravity back to the reservoir, through a trap also maintained below 175 C. The relatively cool trap prevents the backflow of vapour from the reservoir into the condenser, since the vapour pressure of Na is negligible below 175 C. The heater leads to the reservoir and nozzle are water-cooled. The collector and trap are air-cooled. The air flow rate is controlled by manually operated metering valves. A previous active feedback system on the air cooling has been removed.

Loading and installing the sodium cell

Sodium reacts vigorously with water, via the reaction 2Na + 2H₂O -> 2NaOH + H₂, so some precautions need to be taken in handling it. If the above reaction occurs in air, the evolved hydrogen will often ignite explosively with oxygen. When disposing of large amounts of Na metal it is also possible for enough heat to be generated to ignite direct oxygen burning of the sodium. In spite of this, our previous experience with I4 shows that water in small amounts is the preferred reagent for disposing of waste sodium. The hazard is greatly mitigated by the fact that most of the Na is recycled and relatively small amounts are disposed of.

The reservoir is initially loaded in a nitrogen-filled glove box with about 100 gm of solid sodium, taken from a sealed stainless steel storage container. A stainless steel knife is used to cut the sodium, which has the consistency of hard cheese. The reservoir is plugged and taken over to the ISAC beam line. It is there unplugged, and attached to the previously cleaned and assembled remainder of the Na cell, then inserted into the beam line, which is then quickly pumped down. Slight oxidation of the Na in the reservoir during this procedure is not important.The beam line is to be purged with dry nitrogen if it is necessary to preserve the sodium while working on an open beam line.

Servicing a used cell

A used cell needing servicing is cooled to room temperature and vented with dry nitrogen. The amount of Na residue to be cleaned is minimized by cooling the reservoir first to ensure that most condensed Na returns to the reservoir. The cell is then removed from the beam line, and the reservoir is then quickly removed from the rest of the cell and plugged. This minimizes oxidation of the useful Na metal left in the reservoir. The rest of the cell is taken over to the ISIS area and disassembled and cleaned in the fume hood..The Na metal is disposed of by spraying a fine mist of water on it. The fume hood must be clear of all inflammable materials, and the worker must wear a lab coat, eye protection and rubber gloves.

Servicing a contaminated beam line

Small amounts of Na will deposit on beam line components close to the sodium cell after a day or more of operation. This sodium will rapidly convert to NaOH on exposure to the atmosphere and can be wiped away with wet Kimwipes, after removal of the Na cell. Heavier deposits will be seen after several weeks of operation. These can be removed, in situ, by cautiously spraying a fine mist of water on them and wiping up the resulting NaOH solution with Kimwipes. The worker must wear a lab coat, eye protection and rubber gloves.

Sodium cell interlocks

The sodium cell heaters cannot operate if vacuum is lost, water cooling to the cell is off, or the collector or trap temperatures rise above 175C. The reservoir heater goes off if the reservoir temperature rises above the nozzle temperature, otherwise the nozzle could clog up with condensed Na. The trap and collector temperatures are measured by two thermocouples. The interlocks latch and must be reset by the operator before the heaters can be switched on again.

The sodium cell bias (up to 10 kV) cannot be applied unless the high voltage enclosure is properly installed and the beam line is under vacuum. Once the enclosure is locked in place using two keys, the keys are removed from the locks and inserted into a transfer unit. The master key can then be removed from the transfer unit and inserted into the key switch. Turning the master key in the key switch closes the interlock circuits to the HV and isolation transformer power supplies. It is not physically possible to turn the enclosure lock keys without the enclosure in place.