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Set the choppers from NICOS

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

TotDly = MechDly / 360 / |Spd_S| * 1e9 + BeamPosDly + ChopDly
PV NICOS what it is
MechDly-S — 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; fixed by the instrument team
ChopDly-S <disc>_chopper_delay the experiment delay: what an experiment sets

So the first opening edge is at the beam pulse_delay + experiment_delay after the reference, and how the real disc is mounted never enters what an experiment sets. chopcal.nicos turns chopcal's settings into those numbers:

>>> from chopcal.nicos import bifrost
>>> bifrost(pulse_delay=10_000, energy_min=7.0)
{'psc1_chopper_speed': 196.0, 'psc1_chopper_delay': 641654.8...,
 'psc2_chopper_speed': 196.0, 'psc2_chopper_delay': 2850951.9...,
 'foc1_chopper_speed': 14.0, 'foc1_chopper_delay': 10828029.4...,
 'foc2_chopper_speed': 14.0, 'foc2_chopper_delay': 16281415.9...,
 'bwc1_chopper_speed': 14.0, 'bwc1_chopper_delay': 67096439.9...,
 'bwc2_chopper_speed': -14.0, 'bwc2_chopper_delay': 35151995.4...}

Each key is the NICOS device that takes the value: a speed in Hz, signed (bwc2 counter-rotates), and an experiment delay in ns, folded into one turn of its disc. pulse_delay is the BeamPosDly-S the controllers hold, in ns. It has no default, because no value is right for every installation.

The pulse shaping pair turns at the fastest harmonic of 14 Hz that both the requested shaping_time and the installed disks allow. The disks BIFROST was designed for are rated to 196 Hz; a substitute set rated lower needs its own limit, which NICOS knows as the largest speed its psc1_chopper_speed device maps:

>>> bifrost(pulse_delay=10_000, energy_min=7.0, pulse_shaping_speed_max=126)['psc1_chopper_speed']
126.0

chopcal.nicos.experiment_delay(chopper, pulse_delay) does the same for one Chopper, in seconds. chopcal's delay is when the centre of a disc's one opening is on the beam, so the edge slit_edges[0] reaches the beam width / 2 of a turn later for a disc turning anticlockwise (it closes the slit) and earlier for one turning clockwise (it opens it). A disc of several openings is refused: which one comes first depends on where its top-dead-centre mark is, and chopcal does not know.

What this assumes

  • chopcal's times are measured from the proton pulse, and that is the reference the choppers and the detectors are timed from (NiSync rather than event14).
  • MechDly-S is measured to slit_edges[0], the geometric first opening edge, rather than to whichever edge reaches the beam first.
  • Each real disc has the one opening chopcal describes, of the same width. NICOS's BIFROST setups say so: 170, 170, 38.26, 52.01, 161 and 161 degrees.