Tuning the Beamline Optics

This is a general guide to the process of tuning the beamline optics to optimize the total flux and beam shape.

The Elements

  • M1 Mirror
  • M1 Tilt
  • Monochromator
  • Theta2
  • M2 Mirror
  • M2 Tilt
  • Chi2
  • M2 Yaw
  • M2 Bend Up
  • M2 Bend Down
  • Optical Table
  • Stand Tilt
  • Collimator
  • Collimator X
  • Collimator Y

M1 Tilt

For purposes of tuning the beamline, the M1 mirror performs one very simple function: it directs the x-ray beam into the monochromator. The effects of changing M1 tilt can be seen directly by looking through the viewport just upstream of the monochromator.

The viewport through which we can see the effects of changing the M1 Tilt is circled in yellow. The storage ring is out of frame to the left, and the M2 mirror and endstation are off to the right.

Looking through the viewport, we can see the beam striking the aperture (the dark rectangle in the center), because the aperture is painted with phosphors. The beam is very wide at this point. On the left, the beam is a little low (1000 counts in M1 Tilt), so the whole lower edge of the aperture is glowing brightly. On the right, the beam is on target, so the only bright glow we see is to the left and right of the aperture.

Looking through the viewport makes it clear what is happening when we change M1 Tilt, but for practical tuning we measure the total light scattered from the aperture opening, known as "Imono in". (I for current, mono because it's measured at the monochromator, and in because this measurement is taken as the beam is going into the mono.)

(Show plot of what a scan of Imono in vs M1 Tilt looks like, and indicate where the optimum is.)

Monochromator Theta2

Theta2 is the angle of the second crystal in the monochromator.

(Picture of mono face, with x-ray path drawn in ...)

Monochromator, conceptual view

The angle of this second crystal is optimized when the total flux coming out of the monochromator is maximized.

Chi2

Chi2 is a slight twist in the monochromator crystal. This setting affects a couple of things at once. The most obvious (but, it turns out, not the most important) effect is that it affects the beam shape. This effect is more easily illustrated than described. These images are snapshots of the beam image on the back of the shutter for several values of Chi2 near its optimum. All values of Chi2 are in degrees.

A more important effect of Chi2 is that it affects how the horizontal position of the beam responds to changes in monochromator energy. We don't want the horizontal position of the beam to respond at all to changes in monochromator energy, so we'll adjust Chi2 to that end.

The standard way we do this is to run a two-motor scan using the LabView interface.

For every value of chi2, we will scan the mono energy over a broad range while monitoring the horizontal position of the x-ray beam (actually the "Horizontal Centroid"). The value of chi2 for which the horizontal position is most nearly constant is the Chi2 setting we want. On beamline 12.3.1, Chi2 will always be very close to 0.50°; 0.49 or 0.51 would be a large deviation, and we are essentially always in the 0.501-0.505 range.

In this scan, we are scanning across Chi2 in the x axis, across energy in the y axis, and depicting the horizontal centroid by color (the "z axis"). (Chi2 is on the x axis because it is faster to change chi2 than to change energy, and the x value is scanned more often than the y value.) In this plot it's clear that the optimum value for chi2 is in the range 0.500 - 0.502.

By changing the color scale for the z axis, we can get some more resolution. The limits of the color scale, circled in white on the scan below, can be changed by clicking and typing on them. This scan indicates that 0.501 is a bit better than 0.500 or 0.502, because the horizontal centroid (color) at chi2=0.501 varies the least with energy.

M2 Mirror

There are more settings for the M2 mirror than for any other element of the beamline optics. (Actually there are lots of settings for the monochromator too, but mostly we don't need to worry about them.) We'll go through them in an order that is fairly efficient for beamline tuning.

M2 Tilt

M2 Tilt does two things at once. Like M1 Tilt, it pitches the beam up or down. M2 Tilt also shifts the focus of the beam "upstream" or "downstream". In practice, we usually set M2 Tilt to focus the beam where we want it (almost always at the sample position), then set all the other beamline parameters to follow M2 Tilt. At the end, we can go back to M2 Tilt if we need to make small corrections in the vertical position of the beam, because the beam size is actually not very sensitive to small variations of M2 Tilt when it is close to the focus.

One important consideration in adjusting the M2 tilt is where you want the beam to be focused. The easiest setting is to focus the beam on the experimental shutter, but there is really no reason to focus it there. Most of the time, the desired focus will be at the sample.

M2 Yaw

The most distinctive thing about M2 Yaw is that it doesn't have a motor attached to it. M2 Yaw is adjusted by hand.

These images depict the M2 mirror tank from downstream and left and right, respectively. The knob for manually adjusting M2 Yaw is circled in white. There are two bolts, indicated below, that should be loosened before M2 Yaw is adjusted.

Once these two bolts are loosened, the knob should turn easily and the effect on spot shape should be obvious. At this point the question arises of how to see the spot shape while turning the M2-Yaw knob.

This will be under-explained, mostly because the details are likely to change with time. There is a video patch cable hanging down from above the M2 Mirror tank, labeled "31". The other end of this cable, also labeled "31", can be found in the 12.3.1 user area in the "BL1255" rack. Cable 31 can be used to patch the PX BPM signal from rack BL1255 to the monitor which (at this writing) is on the Beamline 12.2.2 M2 tank. With the beam on and the experimental shutter closed, the PX BPM signal should look something like this:

At the left, M2 Yaw is mis-set, and the right side of the beam spot is out-of-line with (above) the left side of the beam spot. At right, M2Yaw is adjusted correctly. The spot has shifted down somewhat because I adjusted M2 Tilt between these two images, but that doesn't noticeably affect the spot shape that we see here.

Adjust M2 Yaw so that the beam is left-to-right symmetrical, retighten the set screws, and you're done with M2 Yaw. It should be noted that Chi2 will affect the spot shape in a similar way, so any large adjustments to Chi2 should be made before setting M2 Yaw.

M2 Bend Up & M2 Bend Down

These two settings are generally handled together. They are so closely related that we don't usually care which is which, but the names refer to the "upstream" and "downstream" bending motors.

The job of the M2 Bend motors is to focus the beam vertically --- in other words, to make the spot size as small as possible. The Labview 2-motor scan can find the best M2 Bend settings mostly automatically.

Shown here is an M2 Bend scan in progress. (Try to ignore the "glitches" in the right center of the scan.) The z-axis (dependent variable) is "Vertical FWHM", which is the vertical full width at half maximum of the beam spot on the back of the shutter, as seen by the video monitor. (The experimental shutter has to be closed for this scan.) The scan shown is pretty typical, in that the minimum is a broad valley running from the upper left to the lower right of the screen. Within that valley, the beam size is fairly insensitive to the M2 Bend. The usual practice is to pick a setting in that broad minimum that has "M2 Bend Up" about equal to "M2 Bend Down".

The Optical Table: Stand Tilt

The PX Endstation itself is able to move somewhat to follow the beam. Since the beam can be shifted up and down by Theta2 or M2 Tilt, we can move the last part of the beamline to make sure the beam is still passing through the shutter aperture. This also lets us position the pinhole to "catch" the beam after we make a large change in M2 Tilt (such as when we refocus the beam hundreds of mm downstream from the sample position.)