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API: chopcal.nicos

BIFROST's chopper settings in the terms NICOS sets them.

nicos

BIFROST chopper settings as NICOS sets them.

An ESS chopper controller does not take chopcal's delay. It locks its top-dead-centre sensor to a total, which the EPICS module evr_seq_calc forms from three parts, in ns::

TotDly = MechDly / 360 / |Spd_S| * 1e9 + BeamPosDly + ChopDly

MechDly-S is measured for each disc, so that with the other two at zero its first opening edge -- NeXus's slit_edges[0] -- is at the beam at the timing reference. BeamPosDly-S (the pulse delay) moves every disc together and is fixed by the instrument team. ChopDly-S (the experiment delay) is what an experiment sets, and the only delay NICOS writes, through <disc>_chopper_delay. So the first opening edge is at the beam pulse_delay + experiment_delay after the reference, and nothing about the real disc's mounting enters what an experiment has to set.

chopcal's delay is when the disc point at beam is on the beam, which for every chopper chopcal.bifrost describes is the centre of its one opening. Both are measured from the proton pulse, so the experiment delay follows from chopcal's own geometry alone: the time slit_edges[0] reaches the beam, less the pulse delay.

Functions:

  • bifrost –

    BIFROST's chopper settings, keyed by the NICOS device that takes each one.

  • experiment_delay –

    ESS ChopDly-S for one chopper, in seconds, in [0, 1/|speed|).

Attributes:

NICOS_NAMES module-attribute 

NICOS_NAMES = {'ps1': 'psc1', 'ps2': 'psc2', 'fo1': 'foc1', 'fo2': 'foc2', 'bw1': 'bwc1', 'bw2': 'bwc2'}

bifrost 

bifrost(pulse_delay, energy_min=None, wavelength_max=None, shaping_time=0.0002, apertures=True, pulse_shaping_speed_max=PULSE_SHAPING_SPEED_MAX)

BIFROST's chopper settings, keyed by the NICOS device that takes each one.

The arguments after pulse_delay are :func:chopcal.bifrost's. pulse_delay is the BeamPosDly-S the chopper controllers hold, in ns as they hold it; it has no default because no value is right for every installation. pulse_shaping_speed_max is the fastest the installed pulse shaping disks may turn, which NICOS knows as the largest speed its psc1_chopper_speed device maps. Each disc gets its <disc>_chopper_speed in Hz, signed -- bwc2 counter-rotates -- and its <disc>_chopper_delay, the experiment delay ChopDly-S, in ns::

>>> bifrost(pulse_delay=10_000, energy_min=7.0)     # doctest: +SKIP
{'psc1_chopper_speed': 196.0, 'psc1_chopper_delay': ..., ...}

This takes chopcal's timing reference, the proton pulse, to be the one the choppers and the detectors are timed from (NiSync), and slit_edges[0] to be the edge MechDly-S is measured to.

experiment_delay 

experiment_delay(chopper, pulse_delay=0.0)

ESS ChopDly-S for one chopper, in seconds, in [0, 1/|speed|).

pulse_delay is the instrument's BeamPosDly-S, in seconds. An edge at angle a is on the beam at delay + (beam - a) / (360 speed), as chopper-lib defines it, and the openings recur every turn, so the result is folded into one turn of the disc. A disc that is not turning has no delay; zero stands in for it.

Only a disc of one opening has a first opening edge that does not depend on where its top-dead-centre mark is, which chopcal does not know; any other is refused.