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-- Obsolete -- Tuning Alkali-Metal Ion Beams Through the Polarizer
From: Phil Levy
Date: July 3, 2003
Revision 1: October 6, 2005
Revision 2: April 12, 2006
Revision 3: May 2, 2006
Addendum: October 6, 2006
Revision 4: March 16, 2007
Revision 5: November 23, 2007
The 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 dumped by the deflector plate ILE2:DEF15C onto Faraday cup ILE2:FC15. The fraction of the 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, ie. beamline components ILE2:
- BIAS15
- RESEVR
- DEF15C
- SOL15A
- SOL15B
2. Tune stable pilot ion beam through the polarizer to the experimental target. The smallest restriction in the polarizer is a nominal 8mm diameter aperture (variable) at the entrance to the He cell chamber. Because of that restriction, the best transmission through the polarizer of a 30keV 8Li beam is about 82%. [Removed from the previous version: The aperture can be seen through the viewport on the He cell chamber. A manual knob to control the aperture size is on the other side of the chamber. The calibration has been lost, so the aperture size is best estimated visually while shining the laser beam through it.]
3. Turn on the helium flow (ILE2:FG16) (typically 1.0ccm). This will increase the emittance of the ion beam. Slight retuning of elements downstream of the He cell may be required [see Baartman's theoretical tunes in "Beam Dynamics"].
4. Turn on ILE2:SOL15A (nominal 4A). This should have no effect on the beam tune, since the magnetic field is small (~10 Gauss) and along the beam axis. If required by the experimenters, turn on SOL15B as well (nominal 8A). 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.
5. Now turn on the deflector ILE2:DEF15C. Nominal value for 28.0keV ion beam 1762V. The value should scale linearly with the ion beam energy and be independent of the isotope mass, although for unknown reasons, this has not always been true in practice. The ion beam current can be measured at ILE2:FC15. Tune the deflector voltage for maximum current on FC15. CAUTION: Oversteering the beam can lead to high radiation fields when running radioactive beam.
6. Turn on the Na cell by ensuring that the propylene glycol cooling is on (physically check at the beamline) and gradually raise the reservoir temperature (initially in 100°C steps, then less as the final temperature is approached) to about 480°C. Note: The nozzle heater has been removed, so that only temperature readback is left on that. The trap and collector, ILE2:TRAP and :COLL, are cooled by propylene glycol (circulator set to 102°C) 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. Please record the initial value at ILE2:FC15 and the final value. The difference is the amount of neutralized beam (assuming source current is constant, which can be monitored by ILE2:FC11).At typical operating temperatures and He flows, the overall transmission efficiency of the polarizer is ~25%. That is ~50% neutralization and ~50% reionization.
7. Turn on the Na cell bias ILE2:BIAS15. The nominal value is 100V. The experimenters will later tune it to a precise value. The ion beam energy to the experimenters is decreased by the amount of the bias. 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..
8. Switch over to radioactive beam.
Notes regarding the Na cell: The PID settings are optimized for keeping the reservoir temperature stable under operating conditions. They are not optimized for raising the temperature quickly to the setpoint. If the reservoir heater trips off, it saves a great deal of recovery time if the setpoint is frequently adjusted to keep the PID output near the operational value. Run a strip tool of the Na cell parameters, including reservoir PID output, to aid in diagnosing and recovering from Na cell trips or outages. If the Na cell tripped due to high temperature, the options are to lower the heating or (maybe) to increase the cooling.
