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Download BNMRfit istallation script Download BNMRfit Download BNMRminfit	Tuning Alkali-Metal Ion Beams Through the Polarizer 3.4The polarizer neutralizes a fraction of the ion beam at the Na cell neutralizer, polarizes the resulting neutral beam with laser optical pumping, and then re-ionizes a fraction of the neutral beam at the He cell re-ionizer. The ion beam not neutralized at the Na cell cannot be polarized, and is therefore removed by the deflector plate ILE2:DEF15C onto Faraday cup ILE2:FC15. The fraction of neutral beam not re-ionized at the He cell goes straight through the bender ILE2:B21 into the neutral beam monitor. The Helmholtz coils controlled by ILE2:SOL15A are essential to producing polarization. Preserving polarization to the neutral beam monitor also requires that ILE2:SOL15B is on. Tuning procedure: 1. Make sure that the polarizer is completely off i.e. beam line components ILE2: BIAS15, RESEVR, NOZZ, DEF15C, SOL15A, and SOL15B are all off, the Na cell reservoir and nozzle temperatures are both below 200 deg C, and ILE2:FG16 (helium flow) is set to zero. ii. Tune stable pilot ion beam through the polarizer to the experimental target. The smallest restriction in the polarizer is an 8 mm diameter aperture at the entrance to the He cell chamber. Because of that restriction, the best transmission through the polarizer of a 30 keV ion beam is about $82\%$ . iii. Turn on the helium flow (typically 1.0 ccm). This will increase the emittance of the ion beam. Slight retuning of elements downstream of the He cell may be required. vi. Turn on ILE2:SOL15A (typically 5 amps). This should have no effect on the beam tune, since the magnetic field is small ($\sim$ 10 gauss) and along the beam axis. If required by the experimenters, turn on SOL15B as well (typically 10 amps). This will move the beam slightly in the vertical direction at the bender ILE2:B21 and will probably need correcting downstream of that. You have then finished tuning the unpolarized ion beam. v. Now turn on the deflector ILE2:DEF15C. Typical value for 30.6 keV beam is 1926 V. The value should scale linearly with the ion beam energy and be independent of the isotope mass. The ion beam current can be measured at ILE2:FC15. CAUTION: Oversteering the beam can lead to high radiation fields when running radioactive beam. vi. Turn on the Na cell.Typical reservoir and nozzle temperatures are 470 and 480 deg C, respectively. The nozzle must be turned on first, so as to keep it hotter than the reservoir, otherwise an interlock will shut off the reservoir heater. The trap and collector, ILE2:TRAP and: COLL, are passively air-cooled and only read back temperature (a previous closed loop control has been removed). As the Na cell heats up, one sees a decrease in the ion beam current at ILE2:FC15 and an increase in ion beam current after the He cell. At typical operating temperatures and He flows, the overall transmission efficiency of the polarizer is $\sim$ 30- 40 $\%$ . vii. Turn on the Na cell bias ILE2:BIAS15. The experimenters should know the approximate setting. The final ion beam energy is decreased by that amount.This requires decreasing the voltage on all electrostatic elements downstream of the He cell by a percentage equal to the percentage decrease in beam energy. NOTE: the bias only reduces the energy of beam that is neutralized within the Na cell. Any ion beam passing through is re-accelerated at the Na cell exit to the initial energy, albeit with worsened emittance, since there are nice field-shaping electrodes only at the entrance to the Na cell, none at the exit. viii. Switch over to radioactive beam.
	Page last modified: 07/23/09 02:45 by Andrew MacFarlane.