Forgot your password?
-- Obsolete -- Polarizer Start up
Date: Oct 24, 2008
Startup of the Polarizer
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 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:
-
Make sure that the polarizer is completely off i.e. beam line components ILE2:
BIAS15
RESEVR
DEF15C
SOL15A
SOL15B
are all off, the Na cell reservoir temperature is below 250 deg C, and ILE2:FG16 (helium flow) setpoint is zero.
-
Tune stable pilot ion beam through the Polarizer to the experimental target. The smallest restriction in the Polarizer is a nominal 8 mm diameter aperture (variable) at the entrance to the He cell chamber. Because of that restriction, the best transmission through the Polarizer of a 30 keV 8Li ion beam is about 82%.
-
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.
-
Turn on ILE2:SOL15A (nominal 4 amps). 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 8 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.
-
Now turn on the deflector ILE2:DEF15C. Nominal value for 28.0 keV beam is 1762 V. 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.
-
Record in the E-log the currents at ILE2:FC11 and ILE2:FC15.
-
Turn on the Na cell after checking that the propylene glycol circulator is on (as evidenced by ILE2:TRAP and :COLL temperatures of ~94 deg C) and raise the reservoir temperature in 50 deg steps, then 10 deg steps as the final temperature is approached (to avoid too much overshoot) to about 480 deg C. As the Na cell heats up, one sees a decrease in the ion beam current at ILE2:FC15. The percentage drop at FC15 equals the neutralization efficiency of the Na cell (assuming source current is constant, monitored at ILE2:FC11). At typical operating temperatures and He flows, the overall transmission efficiency of the polarizer is ~ 25 %. That is ~50% neutralization at the Na cell and ~50% reionization at the He cell. 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 manually set to 102 deg C) and only read back temperature (a previous closed loop control has been removed).
-
Once ILE2:RESEVR and ILE2:NOZZ have stabilized, record in the E-log the currents at ILE2:FC11 and ILE2:FC15.
-
Turn on the Na cell bias ILE2:BIAS15. The nominal value is 100 V. The experimenters will later tune it to a precise value. The ion beam energy to the experiment 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.
-
Switch over to radioactive beam.
Phil Levy
Revision 6. October 24, 2008
