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The temperature of the cell affects the final rotational and translational temperatures of the beam, its divergence, and its forward velocity. Lower cell temperatures produce lower (i.e., more favorable) values for all three of these quantities [106, 107]. We therefore typically run the cell at the lowest possible temperature at which the buffer gas has significant vapor pressure. For neon, this is approximately 16 K.

In the thermochemical source, we expect the ThO production to scale rapidly with the laser power applied to the target, as discussed in Chapter 4. Since the cell temperature also increases with this applied heat load, we wished to investigate the dependence of the single-quantum-state beam signal on the cell temperature in order to optimize the trade-off between keeping the cell temperature low and maximizing the beam yield.

Figure 5.3.1 shows a measurement of the per-pulse |X, J = 1⟩ beam signal (in units of time-integrated optical depth on the X → C Q(1) transition) as a function of cell tem- perature. All other settings are held constant. The cell temperature was controlled using a resistive heater bolted to the cell exterior.3 _{The |X, J = 1⟩ beam signal at the cell exit} shows an approximately linear dependence on temperature between 16 and 23 K, falling by a factor of 2 between 16 and 21 K. This decrease is likely caused by a combination of effects, including an increased rotational temperature spreading out the ThO population among moreJ levels, an increased translational temperature broadening the Doppler profile (typically ≈ 140 MHz FWHM at a cell temperature of 16 K, which is much larger than the absorption laser linewidth of _≈ 1 MHz) and thereby decreasing the optical depth on resonance, and a higher forward velocity decreasing the fly-through time of the molecules. Since all of these effects are detrimental to the eEDM sensitivity (see Eq. (6.2)), we take the integrated OD as a rough proxy for the beam figure of merit. We can therefore infer from Fig. 5.3.1 that lower cell temperatures are strictly preferable, down to 16 K.

Below a cell temperature of about 15.5 K, however, the beam signals are no longer stable: At these temperatures, neon gradually freezes inside the fill line, forming a plug of ice. When this happens, we have to halt the run in order to thaw it out. This effect sets a lower limit on the ideal cell temperature.

The data in Fig. 5.3.1 indicate that the signal degrades with increasing cell temperature when all other parameters are held constant. However, as shown in Chapter4, increasing the

3_{The temperatures of the cell and other cryogenic components in the source are measured using DT-670-} CU (uncalibrated) Lake Shore Cryotronics silicon diode thermometers.

15 16 17 18 19 20 21 22 23 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Figure 5.3.1: Beam signal v. cell temperature. Fiber laser firing at the 3rd _{upstream target}

(7 cm from the cell exit). Galvo programmed to spot hop randomly around most of the target surface with a frequency of 5 Hz. The lens has a 500 mm focal length and is positioned at

≈ 460 mm from the target. The fiber laser has its current set at 100% and is pulsing at a rep rate of 13.7 Hz with a 25 ms pulse width modulated at 400 Hz with a duty cycle of 70% (1.75 ms on, 0.75 ms off). The absorption signal is measured on the X →C Q(1) line just after the cell exit. Each point represents an average of 16 pulses. Over this range, the data shows a clear linear decrease in signal v. temperature with a rate of about 8% per K. The red line is a linear fit which serves as a guide to the eye. Data analysis by Adam West.

heat load from the fiber laser is expected to give higher production rates of ThO. In order to get a better sense for the trade-off between these effects, we monitored the cell temperature during a run in which many beam parameters were varied in an effort to optimize the signal. This data is shown in Fig. 5.3.2. For these running conditions, the molecule flux is roughly independent of temperature up to _≈ 19.5 K, at which point the signals start to drop. Although this measurement plainly has a number of confounding variables, it does suggest that at sufficiently high temperatures, the signal decrease with temperature observed in Fig. 5.3.1 is no longer compensated by the larger ThO yields effected by a higher time- averaged fiber laser power.

Cell Top Temperature (K) 17 17.5 18 18.5 19 19.5 20 M ol ec u le R at e at C el l E x it (s ! 1 ) #1013 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

Yield vs. Temperature for Run 2 All Data, Modulated and Unmodulated 100 ms Pulses at 50% Duty Cycle on 3rd Upstream Target

All traces

Smoothed with 50 pt. moving average

Figure 5.3.2: This data shows the correlation between the beam signal (population in

|X, J = 1⟩at the cell exit) and cell temperature during a run in which the cell temperature setpoint was kept at 17 K while the fiber laser power, modulation status, and modulation frequency were varied. This data and the conditions under which it was taken are presented more fully in Fig. 5.4.11. This plot illustrates that temperature correlations like the one shown in Fig. 5.3.1 can be compensated or more than compensated by other advantages of increasing the heat load on the cell. Nevertheless, even under these rather catholic running conditions, a universal fall-off in the signal appears around 19.5 K, suggesting a limit to these advantages.

Results: All else being equal, the single-quantum-state signal falls off monotonically with cell temperature above16K. Above 19.5 K, this fall-off is no longer overcome by most other advantages of increasing the heat load from the fiber laser.

Conclusion: Choose a time-averaged fiber laser power less than13W, such that the average cell temperature is at most 18 K when the entire heat load comes from the fiber laser (see Fig. 5.1.1). Fix the cell temperature setpoint at 16 K, and servo using a resistive heater. This will keep the cell warm enough to prevent neon ice from clogging the fill line during times when the fiber laser deposits too little power.