The Monochromator: MOVE/MC

After the monochromator has been aligned, energy scans (scan/energy) or energy changes (energy() = ...) are possible. The alignment is done by an expert who knows the mechanical details of the beamline. Apart from scans which are used to adjust the motors, there is a menu that supports the alignment:

ONLINE> move/mc

ONLINE uses the symbol beamline to select the setup of the experiment. There are differences between the experiments but the concept is the same. Let's look at our example: W1.

Table 7.5: Example for move/mc

 

              The ROEWI Monochromator


     Energy:          8005           [eV]        Move: |                 |
     Wavelength:      1.548835       [Angstr.]   Move:
     Bragg Angle:     14.29849       [Deg.]
     Lattice Spacing: 3.13563        [Angstr.]
     Asym.            0              [Deg.]
     Offset_MC1       0              [mm]
     Offset_MC2       0              [mm]
     MC1 Height:      5.223813       [mm]        Move:
     MC1 Theta:       14.29849       [Deg.]      Move:
     MC1 Z:           456            [mm]        Move:

     MC2 Height:      -152           [mm]        Move:
     MC2 Theta:       14.29849       [Deg.]      Move:
     MC2 Z:           744.405        [mm]        Move:
     SLIT2 Y:         -152           [mm]        Move:

     "Const. Exit":   -152           [mm]

------------------------------------------------------------------------------
     PF1 - COLL. MOVE, PF2 - SINGLE MOVE, PF3 - CALIBRATE, PF4 - EXIT

The energy is displayed, relevant motor positions and other parameters.

The lattice spacing is given by the symbol d_crystal.

Asym. is the asymmetry angle of the second crystal, symbol PHI_A.

Offset_mc1 is the distance between the rotation axis and the crystal surface.

MC1 stands for monochromator crystal 1.

Each crystal has three degrees of freedom. There are two conditions that have to be satisfied when the energy is changed:

1. The beam has to leave the monochromator at a constant exit height. This fixes the height of the second crystal.
2. The first crystal is rotated but not translated.

In general: Movements that satisfy such requirements are called collective moves. The requirements may change from experiment to experiment. But a collective move does not change the alignment of the setup.

In order to change the energy, enter the new value and press PF1. ONLINE displays the final positions and prompts for an answer “Start movement? [Y]”. Reply with 'Y' to execute the movements.

Movements may be interrupted by pressing the space bar. If this occurred, ONLINE asks whether it should drive back to the old positions.

Single moves (PF2) are executed during the alignment of the monochromator, e.g.: One crystal is rotated the other is fixed in order to find a reflection. After a single move has been executed, the positions of the monochromator motors are no longer consistent.

W1: If you change the height of the second crystal with a single move, the constant exit condition is changed as well.

If a reflection has been found and the energy is known, the energy has to be calibrated: Enter the value in the “Move:” field and press PF3. ONLINE displays the changes that are involved by an energy calibration and prompts for a confirmation. At W1 the angles are calibrated but the z-position of the second crystal not. Instead the crystal are moved because the z-positions are well known from direct measurements. The way how the energy is calibrated differs from beamline to beamline. Read the confirmation message carefully before you respond.

ONLINE uses this coordinate frame: The z-direction points downstream in the direction of the photons, y upwards and x left looking from the source towards the experiment.

Note that the symbols bem_command and aem_command are executed before and after the energy is changed. They are intended to be used to disable the MOSTAB during monochromator moves.