API: chopcal¶

Chopper settings for BIFROST, from the slow edge of the band you want.

BIFROST passes a fixed bandwidth of about 1.91 angstrom, so one number decides where that band sits. Give it as either the longest wavelength or, equivalently, the lowest energy you want to reach the sample; the band runs from there to about 1.77 angstrom shorter::

wavelength_max=3.0  ->  1.15 to  3.05 angstrom  (   8.8 to  62.4 meV)
energy_min=7.0      ->  1.56 to  3.47 angstrom  (   6.8 to  33.5 meV)

Note the sense of it: the number you give is the slow end of the band. Asking for energy_min=7 does not cap the incident energy at 7 meV -- it puts 7 meV at the bottom and lets everything faster through, up to about 31 meV.

Parameters:

• energy_min (float, default: None ) –

  The lowest incident energy to pass, in meV.

• wavelength_max (float, default: None ) –

  The longest incident wavelength to pass, in angstrom. Overrides energy_min.

• shaping_time (float, default: 0.0002 ) –

  How long the pulse shaping pair should burst for, in seconds. They co-rotate at a harmonic of 14 Hz, one lagging the other, so the burst is one slit crossing less the lag; asking for longer than a crossing at pulse_shaping_speed_max drops them to a lower harmonic and says so. Defaults to 0.2 ms.

• apertures (bool, default: True ) –

  Whether to describe the beam at its real width, which widens every window in time and the band by about 0.13 angstrom. True by default. Pass False for the pencil beam every release before the disk geometry was known described, and the narrower numbers that went with it.

• pulse_shaping_speed_max (float, default: PULSE_SHAPING_SPEED_MAX ) –

  The fastest the installed pulse shaping disks may turn, in Hz. They turn at the highest harmonic of 14 Hz up to this that shaping_time allows. Defaults to the 196 Hz BIFROST was designed for; a substitute disk set rated lower needs its own limit, e.g. 126.

Returns:

• ChopperSet –

  The six choppers by name, in beam order -- ps1 and ps2 shaping the pulse, fo1 and fo2 suppressing frame overlap, bw1 and bw2 cutting the band -- each a :class:~chopcal.lib.Chopper of speed, delay, beam, edges, path and aperture. Feed .values() to :func:chopcal.lib.wavelength_limits to see what the train actually passes.

  Every one is a single opening centred on the beam, so beam is 0 and edges is [-width/2, +width/2] -- what the angle field of chopper-lib 2.x meant, said in the terms 4.x uses. aperture is 0, the point beam that field also described; chopcal holds no disk geometry to compute a real one from.

Examples:

>>> settings = bifrost(wavelength_max=3.0)
>>> settings
name  speed [Hz]  delay [ms]  beam [deg]  opening [deg]  open [ms]  aperture [deg]  path [m]
 ps1         196      3.8781           0            170      2.409           5.651     6.342
 ps2         196      6.0874           0            170      2.409           5.651     6.362
 fo1          14     6.13997           0          38.26      7.591           5.651      8.53
 fo2          14     9.54764           0          52.01      10.32           5.651    14.973
 bw1          14     42.8823           0            161      31.94           13.38        78
 bw2         -14     42.8823           0            161      31.94           13.38     78.02
>>> settings['bw1'].delay
0.04288227...
>>> settings['bw1'].edges
[-80.5, 80.5]