PyMEOP

Data acquisition for optical polarimetry of helium-3 polarized by metastability exchange optical pumping (MEOP). PyMEOP scans a probe laser across two absorption lines of the metastable atoms in the discharge, fits the two peaks, and reports the nuclear polarization from the ratio of their heights. It runs as a PyQt5 desktop application on the polarizer's control computer.

Running it

pip install -r requirements.txt
python main.py

On Ubuntu, a Wayland error at startup means qtwayland5 needs installing.

Instruments

Each instrument is reached over the network at the address set in config.yaml:

Setting	Instrument
probe_ip	Toptica DLC probe laser controller (current and temperature)
meter_ip	wavelength meter
lockin_ip	lock-in amplifier reading the probe absorption
siggen_ip	SRS SG380 signal generator driving the discharge

The same file sets the scan timing, the scan x axis (current or wavelength), the baseline under the peaks (baseline_degree: 1 straight, 2 quadratic), and the data and log directories.

Using it

The window has tabs for Run, Find Peaks, Larmor Drive, Relaxation, Discharge Power and Slow Controls.

Find Peaks sweeps the probe laser's temperature or current over a wide range to locate the absorption lines and choose the scan range.

Run does the measurement. Each scan steps the laser current over the peaks while reading the lock-in, then fits two peaks on a baseline (app/scanfit.py). The line shape is selectable: Gaussian for Doppler broadened lines at low pressure, or Voigt once pressure broadening is comparable, around 100 mbar. With peak height ratio r and the ratio r0 measured with the gas unpolarized, the polarization is

P = (r/r0 - 1) / (r/r0 + 1)

The zero amplitudes are taken from the latest scan, or averaged over a range of scans selected on the polarization history plot. The Run tab also controls the discharge signal generator, including:

• a Larmor drive, which drives the discharge coil at the helium-3 Larmor frequency for the field entered, optionally swept in frequency, for a set time, then restores the discharge and resumes scanning;
• a discharge-off relaxation cycle, which switches the discharge off and on for set times to measure relaxation.

Discharge Power measures the RF power delivered to the discharge cell. A dual directional coupler between the Tomco amplifier and the matching box sends the forward wave to the PWR-8GHS power sensor and to the SR844 lock-in's reference, and the reflected wave to the lock-in's signal input. The tab reads both through their IOCs (discharge section of config.yaml) and computes

P_fwd    = P_sensor · K_fwd
M        = (X + iY) / √(P_sensor / (1 + m²/2))     reflected over forward, m = AM depth
Γ        = (M/g − E_d) / E_r                       g = lock-in range gain
P_net    = P_fwd (1 − |Γ|²)
P_plasma = P_net (1 − Q_L/Q_U)

A reading is made at each lock-in poll and is not used when the lock-in is unlocked, overloaded or has been touched, when the sensor is out of range or stale, or while the drive level is settling. The sensor polls more slowly than the lock-in, so after a step in drive or reflection, readings wait for the sensor to catch up, about a second. Warnings show when the drive frequency or AM depth has moved from the calibration's, when the sensor's compensation frequency doesn't match the drive (Match to Drive fixes it), or when the sensor is far from its −15 dBm target.

Calibrations are kept in app/discharge_cal.yaml, with a history of every one saved:

• Γ calibration, at about 0.5 W: connect a 50 Ω load, then an open, at the end of the cable that normally goes to the matching box, Measure each, then Save Calibration. Then measure the 25 Ω standard (two terminators on a tee, with their DMM resistances) on the same sensitivity to check Γ. Measured again on another sensitivity, it gives that range's gain relative to the calibration's, which then corrects Γ there, so calibrate at 0.5 W and operate at tens of W on another range. Pin the lock-in's reference phase in the IOC settings, since a changed phase invalidates E_d and E_r.
• Forward calibration: enter the reference meter reading and the attenuation to it and Record K_fwd, or type K_fwd in dB.
• Tank Q: type Q_U and Q_L, or take them from a resonance sweep, which steps the SG380, fits the absorbed fraction 1 − |Γ|² (width f₀/Q_L), and puts the SG380 back where it was. From the Γ locus it also finds the coupling β, so a sweep of the unlit tank gives Q_U = Q_L(1 + β). Adopting a sweep also records which sign of Im Γ means the drive is above resonance, shown with the readings. Off resonance most of the drive is reflected, so sweep at low power.

Sensor AM Check turns the SG380's AM off and on and compares the sensor's ratio with 1 + m²/2, since the sensor is specified for CW only. It interrupts the probe lock-in's reference, so it refuses to run during scans.

The reflection interlock acts when |Γ| stays over its limit for interlock_samples valid readings in a row: it sets the SG380 to the safe amplitude, or turns its output off, then stays tripped until Reset. It only acts on calibrated, valid readings, so it adds to the amplifier's own protection rather than replacing it.

Slow Controls shows EPICS process variables listed under epics: pvs: in config.yaml, grouped by their group, colored by alarm severity and greyed when disconnected. A PV marked control: true gets a setpoint box and Set button, and must have limits; a value outside them is refused. A readback PV can be shown beside a control's setpoint. If the IOCs aren't reached by broadcast, set ca_addr_list. This uses pyepics, which brings its own Channel Access libraries.

Data

Each scan is written as one JSON line to an event file in data/, holding the raw scan, the fit parameters and uncertainties, the polarization, and under pvs the last value of every slow controls PV when the scan ended (null if disconnected), and under discharge the latest discharge power reading (null if there is none). The file in use is named current_<start>.txt and is renamed to <start>__<end>.txt when closed (times in UTC). Logs go to log/.

Slow controls are also logged on their own, in slowlog/ (slow_dir), named the same way with a new file each UTC day. Each line is a JSON record with a kind:

• snapshot: every PV's last value, IOC timestamp and alarm severity, every log_interval seconds;
• set: a value changed from PyMEOP, with the old and new value and its source (user, scan, larmor, relaxation, range). This covers EPICS controls and PyMEOP's own settings: siggen.freq, siggen.amp, siggen.output, siggen.fm and zero, and the discharge power calibrations (discharge.gamma_cal, discharge.k_fwd, discharge.tank, ...) and interlock trips (source interlock);
• discharge: the latest discharge power reading, every log_interval seconds: X, Y, sensor dBm, M, Γ, the four powers, and the calibration they used;
• connect / disconnect: a PV came or went.

Data browser

webapp/ holds a separate, read-only web app for browsing and refitting the event files, locally or on a server that mirrors the data (start_server.sh and stop_server.sh here, deploy/mirror_data.sh for the mirroring). See webapp/README.md.

References

• https://doi.org/10.1016/j.nima.2019.02.019
• https://doi.org/10.1016/j.nima.2021.165590
• https://doi.org/10.1016/j.nima.2023.168792
• https://doi.org/10.1016/j.nima.2025.170870