Chromaticity Measurement

This tutorial shows how to measure the chromaticity.

Prerequisites

The tutorial requires an existing pyAML configuration file which includes configuration of the chromaticity monitor.

The example uses the configuration provided by the pyaml-test-lattice package.

from pyaml.common.constants import Action
from pyaml_test_lattice import configurations

Load the Accelerator

from pyaml.accelerator import Accelerator
accelerator = Accelerator.load(configurations["pyaml/tango/pyaml-cs-oa/fodo_1gev_6d_pyaml-oa.yaml"])

# Print the string representation of the accelerator
print(accelerator)

Accelerator(facility='Synchrotron SOLEIL', machine='sr', simulators=['design'], controls=['live'])

Control Mode Choice

• accelerator.design — runs pyAT locally, no control system needed. Set wait_time = 0.0.

• accelerator.live — connects to the real machine or virtual twin. Set wait_time to allow readback settling (typically 1.5–2 s).

For live control mode you need a running control system or virtual accelerator. If you want to skip this, run the notebook in design` mode only.

# Use the design mode
SR = accelerator.design

# For live mode
# SR = accelerator.live

wait_time = 0.0 if SR == accelerator.design else 2.0

# Print the string representation of the simulator
print(SR)

Simulator(name='design', lattice='/opt/hostedtoolcache/Python/3.11.16/x64/lib/python3.11/site-packages/pyaml_test_lattice/data/lattice/fodo_1gev_6d.json', mat_key=None, n_elements=193)

Momentum Compaction Factor

The momentum compaction factor is needed to convert RF frequency deviation to momentum deviation. It is computed from the lattice model (design mode).

accelerator.design.get_lattice().disable_6d()
alphac = accelerator.design.get_lattice().get_mcf()
accelerator.design.get_lattice().enable_6d()
print(f"Momentum compaction factor: αc = {alphac:.6e}")

Momentum compaction factor: αc = 9.187045e-02

Chromaticity Measurement

The chromaticity monitor is named CHROMATICITY_MONITOR in the configuration. The measurement sweeps the RF frequency and fits the resulting tune shift.

Parameters:

• alphac — momentum compaction factor (from lattice above)

• fit_order — polynomial fit order (2 = quadratic)

• n_step — number of RF frequency steps

• sleep_between_meas / sleep_between_step — settling times (set to wait_time for live mode)

• do_plot=True — show the tune vs. δp fit

def chroma_callback(action: int, cb_data: dict):
    if action == Action.MEASURE:
        print(f"Chromaticity: #{cb_data['step']} RF={cb_data['rf']:.2f} Hz, Tune={cb_data['tune']}")
    return True

chroma_monitor = SR.get_chromaticity_monitor("CHROMATICITY_MONITOR")

# Measure the chromaticity
chroma_monitor.measure(
    callback=chroma_callback,
    do_plot=True,
    alphac=alphac,
    fit_order=2,
    n_step=5,
    sleep_between_meas=wait_time,
    sleep_between_step=wait_time,
)

# Get the measured chromaticity
ksi = chroma_monitor.chromaticity.get()
print(f"Measured chromaticity: ξx = {ksi[0]:.3f}, ξy = {ksi[1]:.3f}")

Chromaticity: #0 RF=351642045.31 Hz, Tune=[0.43995006 0.28491609]
Chromaticity: #0 RF=351642045.31 Hz, Tune=[0.43995006 0.28491609]
Chromaticity: #0 RF=351642045.31 Hz, Tune=[0.43995006 0.28491609]
Chromaticity: #1 RF=351480666.02 Hz, Tune=[0.43994623 0.28482897]
Chromaticity: #1 RF=351480666.02 Hz, Tune=[0.43994623 0.28482897]
Chromaticity: #1 RF=351480666.02 Hz, Tune=[0.43994623 0.28482897]
Chromaticity: #2 RF=351319286.72 Hz, Tune=[0.43994507 0.28480055]
Chromaticity: #2 RF=351319286.72 Hz, Tune=[0.43994507 0.28480055]
Chromaticity: #2 RF=351319286.72 Hz, Tune=[0.43994507 0.28480055]
Chromaticity: #3 RF=351157907.42 Hz, Tune=[0.43994594 0.28482808]
Chromaticity: #3 RF=351157907.42 Hz, Tune=[0.43994594 0.28482808]
Chromaticity: #3 RF=351157907.42 Hz, Tune=[0.43994594 0.28482808]
Chromaticity: #4 RF=350996528.12 Hz, Tune=[0.43994825 0.28490893]
Chromaticity: #4 RF=350996528.12 Hz, Tune=[0.43994825 0.28490893]
Chromaticity: #4 RF=350996528.12 Hz, Tune=[0.43994825 0.28490893]
Measured chromaticity: ξx = -0.000, ξy = -0.000