Tuning -- Front-End to IMS:FC34.

(V.1.0.1.1 Edited for text clarity by Rene Tanaja, July 17, 2010)

(V.1.0.1 Edited by Jon Aoki after consulting Mike Trinczek)

(V.1.0. As advised by Friedhelm Ames, for the front-end, and Mike Trinczek, for the magnets to IMS:FC34, and as understood by Rene, and edited and approved by Friedhelm and Mike.)

 Rick has provided a description of the tuning function of the IMS quads and the IMS benders here: Descriptions of tuning functions of IMS quads and benders

    0.  Load theory optics values (using Rick's Beam Dynamics page, remember to include IMS:B23), magnet theory values from the mass scaling program (always insert source cups to move magnets) and set steerers to zero steering effect (the source CCB0 should be set lower than mid-range:  maybe ~700V).  Ensure IMS:MB1 polarity is set correctly (East/West).

1. Start from the front-end: Optimizing on IMS:FC3, with the IMS:DB0 slits at running size (nominally, 2.0mm), & centered iteratively play the EE (this is less effective in a febiad source & may run low) against the EL, while adjusting XCB0, YCB0, and the pre-separator magnet, MB1, to compensate for steering effects from the procedure. Changing the EE will probably cause a steering change, but changing the EL should not (but may). Try to center beam on the source harps rather than overlapping the saved traces (if comparing w/ previous harps, ensure the source is the same surface/febiad etc).  Also try using ITW/ITE:Q1 and Q2, especially after a good shape has been attained:

• Optimizing with Q1 and Q2: Increase Q1 until the current drops slightly on FC3, then try to restore with Q2. Repeat. If there are no gains, try decreasing Q1 instead.

2. While doing the above step, occasionally remove harps & do an FC0 scan (on EPICS optics: IMS Optics (1) -> Correlations -> IMS:FC0 vs. position) to check the beam size and shape (which should be Gaussian). The optimal size is ~2.2mm near the bottom of the scan. If the size is too small it means that you are over-focusing.  Use source y-steerers (upstream is better) & IMS:MB1 to center the FC0 scan while maintaining intensity on FC3 (reducing YSLIT0  to ~0.75mm may assist but remember to restore to 2mm!).  (It has been observed that good agreement between IMS:FC0 and FC3 seems to transport well.)  Harp5B should be a mirror of this scan as an intermittent indication.  In the event that no beam is present on this scan, it should be run again from -30-->30; if beam still not present a mass scan should be run.

3. Once you have a good beam shape, even if it's not perfect, and size, send the beam to IMS:FC10A. Make sure the width of slit IMS:YSLIT10A is set to 0.8mm and that it is centred (the centre position is around 31.2 +/- 1mm), but it can vary depending on recent calibration and backlash; Friedhelm Ames / Jon Aoki will check this at the start of each target’s tune). Optimize the FC10A signal with the pre-separator magnet, IMS:Q1, Q2, (Q4 and Q5), XCB3, and XCB6. Steering between the separator magnets is not desirable (it reduces the effectiveness of the Mass Scaling program among other things). Try to minimize steerer changes from zero-steering. If steering is needed, revisit the front-end tune for steering.  The transmission from IMS:FC3 to IMS:FC10A should be close to 100%.

4. Send the beam to IMS:FC14, and optimize with the separator magnet, IMS:XCB3, and IMS:XCB6, (Q7, Q8, Q9, Q10). IMS:YSLIT11 should be set to 0.8 mm.  It may take ~1-2h to get to this point.

• Set the IMS:MB2 beta coil according to the following formula: [(magnet field value in Tesla)x(23)-0.5] -- example: if the magnet is at 1159 Gauss: (0.1159)x(23)-0.5 = 2.16A
• Set the IMS:MB2 alpha coil according to the following formula: [(magnetic field value in Tesla)x(0.62)+0.28] -- example: if the magnet is at 1159 Gauss: (0.1159)x(0.62)+0.28=0.35A
• Spreadsheet with Alpha and Beta calculator: file:///home/opsi2cr3/Documents/ISACDocuments/Tuning Notes/Alpha&Beta calculator.ods
  
• *can't find beam on IMS:FC14?  Help is here:  https://isacops.triumf.ca/beam-delivery-tuning-1/tuning-tips/Place-Holder-Finding-Beam-after-the-Mass-Separator.  (IMS:XWS10A may be inserted to centre position to check for presence of beam w/ IMS x-steerers, though centre position may not be trusted.)
  

5. Optimize the beam on FC14 with the alpha and beta coils, readjusting the separator magnet after each coil setpoint change. Remember that the alpha/beta coils & the magnet affect each other.

• Start with the beta coil, adjusting in larger steps first, then fine tuning with smaller and smaller steps (eg. start with ~0.5 steps, then 0.1 steps, then, finally, with 0.01 steps). Note that when changing the beta coil value, there is always a “recoil” effect – the larger the step, the larger the recoil – that takes a few seconds to settle to the real effect of the field change, so be patient.

• Adjust the alpha coil, starting with larger steps (eg. 0.1), moving to smaller steps for finer adjustment (eg. 0.01). There is a small lag while the coil field settles, but no recoil, with the alpha coil.

• Iterate the above steps until the current on IMS:FC14 is maximized (although slits should cut ~2-5% to ensure good beam size for transport, can adjust slits to verify).  For fine-tuning of alpha/beta coils, IMS:YSLIT11 may temporarily be reduced to 0.5mm.

6. Make adjustments on IMS:FC14 with ITE:XCB0 and YCB0, IMS:XCB3 and XCB6, and IMS:Q1, Q2, (Q4-Q8), Q9, and Q10 (be careful not to over-adjust).  With any gains made in x, the front end should be revisited; with any gains in y, the magnets should be revisited.

7. Once you are satisfied with the result, begin taking emittance measurements (*requires >1nA on IMS:FC14, use reverse bias for currents <3nA) using IMS:SCAN11 page. Send the platform to the 'in' position.  Set an FC gain of 1nA (but increase if it saturates during the scan).; sample rate of 0.1second.  Set Scan Values to 'horizontal' & adjust as per the note in 'Help for emittance.' Ignore the note on the emittance rig page regarding slit sizes, and leave the IMS slit sizes at running values. Friedhelm and Mike have decided that it makes more sense to measure the real characteristics of the beam you are sending to the experimenter, rather than changing those characteristics for the sake of making the scans easier to do.  Click 'Start.'  When the position stops incrementing on IMS:SCAN11 page & 'File written' is displayed below,  click 'Keep Data.' Click on the emittance icon in the taskbar, or open an xterm & run /isac/ISACDocuments/emittance/runemit.  Choose 'mass sep' from the menu that pops up, the most recent previous file is displayed.   From the 'New File' menu, click 'Open.'

8. Notes on emittance measurements with MatLab displayer:

• On the IMS:EMIT11 page, adjust the offset and range of the “Field Scan Parameters” (vertical parameters on the plot) according to the energy of the beam. Note that the standard settings are for a 30keV beam. If the energy is higher or lower, these voltage settings and steps may need to be adjusted to compensate.

• Make sure the Kepco high-voltage power supply is working (readback should vary between +/- "Range" value + "Offset".)  If in doubt that it is working, striptool its output.  If you need a reminder of how to reset the Kepco supply, read the “Help for emittance” information.

• On the lower left of the resulting scan window is a square button with an “S” on it. Pressing it will pop up a surface map of the scan which you can rotate with the mouse when you press the button with an arrow in a circle at the top (just right of centre) of the surface map page. This map will let you know if your scan is saturated, as well as giving you a better idea of the scan shape. If the scan is saturated, the height of the signal goes up to 10 and it will have a flat top. In such a case, increase the gain setting and rescan. If you are at the lowest gain setting and the signal height is below 1 switch off the bias and rescan. If the signal is still below 1 switch to reverse bias.

• The signal should be centred within the emittance picture. If part of the beam is cut off vertically, adjust the offset  &/or range values in “Field Scan Parameters” (an increase in offset moves the field of view upward, an increase in range increases the vertical field of view).  If part of the beam is cut off horizontally, adjust the start/end values in 'Position Scan Parameters.'

• The expected horizontal scan is historically a vertical ellipse, slightly tilted to the left (due to the scanner position, otherwise it would be perfectly vertical) though this may have been fixed during 2013 emittance scan upgrades.  The desired ellipse tilt (or to be made vertical) is achieved via the alpha coil.  Correction for a C-shape is achieved via the beta coil (0.1 step).  (An S-shape correction would theoretically be achieved via the octopole IMS:M8, but this is not used.)
  

• Once you have a good scan, press “re-zero”, then use the mouse to form a rectangle around the area of interest.  Press 'De-Ripple' & select 3 points with the crosshairs:  2 on the same ripple, & 1 on another ripple.  Press “cut” to eliminate most of the background noise. Repeatedly press “cut” until the numbers on the lower right of the scan graphic stop changing.

• Press “erase”, then use the mouse to form a rectangle around noise not eliminated by the previous step to erase them. Repeat as necessary. Be careful, so that you don't erase significant parts of the beam, or erase too much of the real signal.  To remove unnecessary information for the elog, toggle 'noGUI' then take a grabscreen for the elog.

8. Repeat Steps 5 and 6 as necessary to improve the emittance.

9. Change emittance scan values to vertical & adjust as per note in 'Help for emittance,' wait for the rotation to reach setpoint, then take a vertical emittance scan. Evaluate the emittance as in step 7.  Note that the vertical emittance scan shape is taken as-is. We do not have the ability to adjust the vertical emittance scan shape.

10. Notes on using the beam envelopes program:

• After making the emittance measurements, start the beam envelope program via right clicking on the background of the computer. Choose (double-click) Main_sep-->“IMS_YSLIT11” from the menu that pops up.

• Select scale to energy, input the correct beam energy & press “return”/”enter” (not on the numeric keypad!) and the quad values should change.

• Press the “emit” button at the bottom of the generated page.

• Enter the numbers generated by the vertical scan into the left entry boxes: X_2rms into X(mm), r₁₂ into r₁₂ (including the sign of the value), and e_2rms into e(πµm).  Emittance can be compared w/ previous values from a similar target in the elog to verify reasonableness.

• Enter the numbers generated by the horizontal scan into the right entry boxes: X_2rms into X(mm), r₁₂ into r₃₄ (including the sign of the value), and e_2rms into e(πµm).

• Press “return”/”enter” (but not on the numeric keypad!) after each value is entered and the ellipse should redraw.

• Press “exit”.

• Double-click on these quads in the “elements – values” list, then press optimize” (button on the upper right), one by one: Q11, Q12, Q15, Q16, and Q18, – Do not do the same with Q17 or the other quads. These are the suggested starting values. They can still be adjusted for further improvement, as can the other quads.

11. Enter the resulting quad values into the corresponding quads in EPICS.

12. Restore IMS:DB11 for transmission by clicking 'set' for all 3 parameters.  Calibrate from SCAN11 page if led's won't turn green.
    

13. Adjust the tune with steering to center rpm's (ignore traces). From Box 14 to Box 18, ideally, only vertical steering should be needed (ensure B18 is in closed loop!, benders may run 2-3% off theory). Do the same with Box 18 to Box 34/36 (ie. IMS:FC34).  Once beam is visible on FC34, tune for transmission there rather than FC19.  Separator magnets & alpha/beta coils may be further optimized.

14. Attempt to balance movement in XCB14 with movement in XCB18, though other vertical steering may still be needed.

15. IMS:RPM31 is perhaps the most useful and indicative of the beam shape and position in the periodic transport section. The vertical position MUST absolutely be centred, and the horizontal should be as centred as possible. The green trace on its “RPM Compare” page is a good one to compare to.

16. Use the IMS slits at 5.5mm for steering. DO NOT use them at 5.5mm to adjust quads. Adjust the quads with the slits set to 8mm.

17. Adjust the beam shape as needed. Optimize the quads iteratively in pairs: Q11 balanced with Q12 and repeat; Q15 balanced with Q18 (or so) and repeat; etc. IMS slits have to be set to 8 mm.

18. Adjust the quads up to Q21, but no further.  (Try to achieve a centered, tight double focus on IMS:RPM31 for good transport into ILT.)  (Ensure IIS:IV1 is open & optimize IMS:B23 kicker onto IIS:FC2.)

19. Expected transmissions for 28keV (lower energies will have slightly worse transmissions, and higher energies, better):

• FC3 -> FC34 ~85% (FC3 -> FC10A ~100%; FC10A -> FC14 ~95-100%; FC14 -> FC34 ~90%)

• FC34 -> ILY:FC4 ~100%

19. Continue tuning to the experimental destination.

20. Save and Document the tune. This includes posting front-end/IMS harp scans, saving RPM data, and taking a full set of Faraday Cup readings.