Changing Post-Stripper Charge States

On rare occasions, the Fragment Separator physicists will request that the FS1 charge selecting slits be moved to allow/block certain charge states after the charge stripper. The controls for the FS1 charge selecting slits can be found by going to Diag. Launcher → Charge Selector. The slits can be configured by adjusting the slit center and slit gap motors to send only one charge state or many charge states. After configuring the slits, it will be necessary to measure and update the stripping efficiency for the loss monitors and the records of the Fragment Separator physicists (they sometimes ask for this value). Once this is done, beam can be sent to them again.

Be sure to note down the positions of the center and gap along with the stripping efficiency before making any changes.

On the right side of the Charge Selector page is a list primary beam species that can be selected. After selecting the correct species, a list of selectable charge states as well their corresponding center positions will be shown further to the right. To set the slits to a specific charge state:

1. Stop beam with the chopper.
2. Set the slit gap to 15 mm and wait for the slits to stop moving.
3. Select the desired charge state and hit the 'Go' button.
4. In the TCT, update the charge state being sent by clicking the 'AP Config.' tab and type the charge state into the Charge State post-Stripper setpoint.

The slits should be in the correct position for the charge state after they have finished moving.

• Most of the charge state buttons for the primary beam species are to only send that specific charge state. The only exception (as of August 08, 2025) is the '19.4+' charge state for 48Ca. This setting centers the slits to send the 19+ and 20+ charge states assuming the slits are wide open (see the Reference section screenshot).
• While there are no formal instructions to send multiple (but not all) charge states, we could potentially calculate the slit gap and center based on a single charge state gap of 15 mm and the use of the charge state centers.
  • To find the position of the left slit, subtract 15/2 mm from the slit center of the left most charge state of interest (the more negative the center, the more left the charge state is in the beamline). To find the position of the right slit, add 15/2 mm to the slit center of the right most charge state of interest (the more positive the center, the more right the charge state is in the beamline). We can then calculate the gap and center from: Gap = Right Slit - Left Slit and Center = (Left Slit + Right Slit)/2.
  • Example: Want to send only the 74+ and 75+ charge states of 238U. The respective centers are 10.5 mm and -1.1 mm. If we assume that each charge state needs a 15 mm gap, then the left slit should be at -1.1 - (15/2) = -8.6 mm and the right slit should be right slit should be at 10.5 + (15/2) = 18 mm. Using these two values, we should set the gap to 18 - (-8.6) = 26.6 mm and the center to ((18 + -8.6)/2) = 4.7 mm.
• For sending multiple charge states, we could probably just take the average of the states and then use that as the post-Strip charge state. AP might use a weighted average, but this should be close enough.
• When in doubt, call AP about what to set the slits to.

After changing the slits to the desired charge state(s), the stripping efficiency will have changed and needs to be measured again so that the TCT can properly adjust the loss monitor thresholds. To measure the stripping efficiency:

• Send beam to FC D1102.
• Remove any attenuation and adjust the chopper duty factor to send a pulse of ~2 W to target.
  • You could also use CW beam, but chopped beam might be easier.
  • To be safe, use a stripping efficiency of 100% in your calculation. This will most likely underestimate the beam current needed to produce 2 W, but this can be adjusted at will using the chopper duty factor.
• Set the TCT destination power to 2 W.
• Confirm with the Fragment Separator physicists that it is ok to send beam to the target for the measurement. If it is, then send.
• In the 'AP Configuration Parameters' box of the TCT, there is a live readback for the stripping efficiency. Type the measured stripping efficiency into the stripping efficiency setpoint in the 'AP Config.' tab of the TCT.

Beam can be handed over to the Fragment Separator physicists once the measurement is done and the beam has been set back to what they want.

Some of the information on setting the FS1 slits was taken from an email from July 11, 2025. A screenshot of this email is posted below.