FIG. 6: (color online) Schematic of the E-field electrodes as- sembly, and a result of the E-field modeling showing an X-Z slice of the amplitude of the E-field z-component, e z , in a midplane (Y=0) of the assembly normalized by the total E- field amplitude, E. The voltage is applied to electrode 1, and electrode 2 and the correction electrodes are grounded. The laser is locked to the PBC using the FM-sideband technique [16]. In order to remove frequency excursions of the PBC in the acoustic frequency range, the cavity is locked to a more stable confocal Fabry-P´ erot ´ etalon, once again using the FM-sideband technique. This stable scannable cavity provides the master frequency, with the power-build-up cavity serving as the secondary master for the laser. A schematic of the optical system is presented in Fig. 5. The magnetic field is generated by a pair of rectangu- lar coils designed to produce a magnetic field up to 100 G with a 1% non-uniformity over the volume with the di- mensions of 1 × 1 × 1 cm 3 in the interaction region. Addi- tional coils placed outside of the vacuum chamber com- pensate for the external magnetic fields down to 10 mG at the interaction region. The residual B-field of this magnitude does not have an impact on the PV measure- ments since its contribution is discriminated using the field reversals. The electric field is generated with two wire-frame elec- trodes separated by 2.1 cm (see Fig. 6). The copper elec- trode frames support arrays of 0.2-mm diameter gold- The signals from the PMT and PD are fed into lock- in amplifiers for frequency discrimination and averaging. A typical time of a single spectral-profile acquisition is 20 s. The signals at the first and the second harmonic of the electric-field modulation frequency are registered si- multaneously, which reduces the influence of slow drifts, such as instabilities of the atomic-beam flux. The mod- ulation frequency is limited by several factors. Thermal Light at 556 nm emitted from the interaction region is collected with a light guide and detected with a pho- tomultiplier tube. This signal is used for initial selec- tion of the atomic resonance and for monitoring pur- poses. Fluorescent light from the probe region is col- lected onto a light guide using two optically polished curved aluminum reflectors and registered with a cooled photodetector (PD). The PD consists of a large-area (1 × 1 cm $^{2}$) Hamamatsu photodiode connected to a 1-GΩ transimpedance pre-amplifier, both contained in a cooled housing (temperatures down to − 15 $^{◦}$C). The pre-amp’s bandwidth is 1 kHz and the output noise is ∼ 1 mV (rms). The 649-nm excitation light is derived from a single-frequency diode laser (New Focus Vortex) produc- ing ≈ 1 . 2 mW of cw output, high enough to saturate the 6s6p $^{3}$P$_{0}$ → 6s7s $^{3}$S$_{1}$ transition. Due to the sat- uration of this transition, ∼ 3 fluorescence photons per atom exited to the $^{3}$P$_{0}$ state are emitted at the probe region. The natural width of the 649-nm transition is 70 MHz, thus, its profile covers all transverse velocity groups ( v$_{x}$ ) in the atomic beam ( ≈ 8 MHz Doppler width at 649 nm). A drift of the laser frequency ( ∼ 100 MHz per minute) is eliminated by locking the diode laser to a frequency-stabilized He-Ne laser using a scanning Fabry- P´ erot ´ etalon with the scanning rate of 25 Hz. The spec- tral distance between the ´ etalon transmission peaks from the two lasers is measured during each scan and main- tained constant within an accuracy of ± 3 MHz, good enough to eliminate any degradation of the probe-region signal. The result of the electric field non-uniformity calcula- tions is shown in Fig. 6. These calculations demonstrate that errors in the centering of the light beam with respect to the E-field plates may induce substantial parasitic components as large as, for example, | e$_{z}$ | ∼ 5 × 10 − 3 ˜ E$_{0}$ , producing parasitic effects comparable to the PV asym- metry. In order to measure and/or compensate the impact of the parasitic fields, additional electrodes de- signed to simulate stray E-field components were added to the interaction region. By applying high-voltage to these electrodes (“correction electrodes” in Fig. 6), the parasitic-field components may be exaggerated and ac- curately measured as described in the following sections. plated wires. This design allows us to reduce the stray charges accumulated on the electrodes by minimizing the surface area facing the atomic beam, thereby minimizing stray electric fields. AC voltage of up to 10 kV at a frequency of 76.2 Hz is supplied to the electrodes via a high-voltage amplifier. An additional DC bias voltage of up to 100 V can be added. 7