TABLA GRADO DE FIABILIDAD Y VALORACIÓN DE LAS ESTRATEGIAS

Another set of beam source parameters we sought to optimize are those relating to the laser spot on the target. These parameters include the fiber laser focal size, the amount of time spent firing at a single target spot, whether the laser spot position is rastered during a pulse, and the program for exploring the surface area of the target. We found that the behavior of the target surface was one of the most complicated things to understand about this source, especially because the beam properties change so much as the target surface ages. The data from the “stress-test” run in Section 5.7 shows the dramatic difference between signals from a fresh target surface and those from a used surface. New targets produce beam signals that are consistent, large, and relatively independent of running parameters, while older targets exhibit more shot-to-shot fluctuations, more dead and short-lived spots, and more stringent requirements for e.g. fiber laser power and focal size.

In order to better understand the target surface usage, Jacob Baron performed the analysis shown in Fig 5.5.1. For this analysis, the target surface was divided into an imaginary grid of 100 µm squares, and each time the galvo aimed the laser into one of the squares, it was

counted as a “visit.” The fiber laser focus was 140µm FWHM during this run. The galvo was programmed to hop randomly to a new location on the target whenever the signal fell below a fixed threshold (as in item 4 in Section 5.1), but it could return to the same spot later. Figure5.5.1shows that while the target spots do recover nearly to their previous signal levels on each sequential visit, the target spots become slightly shorter-lived and less productive each time. After 10 visits, the total yield produced from a spot has fallen by nearly 50%. If we take this as the criterion for “spot death,” we conclude that each(100µm)2 _{region on the}

3 cm2 _{target surface can be visited 10 times with an average of 25 shots before it is depleted.}

If the source is run with 20 ms pulses at 25 Hz, then we should be able to run for_∼80hours on a single target. This estimate is only slightly higher than the 70 hours of total run time achieved during the “stress-test” run described in Section 5.7.

Visit Number

0

Avg Number of Shots

22 24 26 28 30 32 34 36

Average number of shots spent at visit number

Visit Number 0 5 10 15 Mol/Sec (Mol/pulse × Rep Rate) ×1013 5 5.5 6 6.5 7 7.5 8 8.5 9

Mol flux in first pulse of visit

Total molecules produced on single spot Visit Number

0

Avg Mol/Sec (Mol/pulse

× Rep Rate) ×1013 4.5 5 5.5 6 6.5 7 7.5 8

Avg # Mol flux during visit

Visit Number

0

Tot # Mol Produced at visit

×1013 3 3.5 4 4.5 5 5.5 6

Total molecules produced on single spot during visit

Visit Number

0 5 10

Avg Mol/Sec (Mol/pulse

× Rep Rate) ×1013 4.5 5 5.5 6 6.5 7 7.5 8

Avg # Mol flux during visit

Visit Number

0 5 10 15

Tot # Mol Produced at visit

×1013 3 3.5 4 4.5 5 5.5 6

Total molecules produced on single spot during visit

15 5 10 15

5 10 15

Figure 5.5.1: Target spot depletion upon repeated visits. All molecule signals refer to the total population of _|X, J = 1⟩. Left: This plot shows that the target spots do recover between visits but deplete more rapidly with each sequential visit. The number of shots providing good signal decreases by 25% after 5 visits. Middle: This plot shows that the average molecule flux from a spot also decreases slightly with each subsequent visit and falls by 20% after 5 visits. Right: This plot shows the decrease in the total molecule production from a spot in each visit, which has fallen by 30% after 5 visits. Figure, data, and analysis from Jacob Baron.

Figure 5.5.2 shows the length scale over which the target surface is depleted by the laser pulse. As expected, this length scale is comparable to the beam diameter at the focus,

≈100 µm.

Results: Regarding spot positioning, we have found in data not shown here that rastering (i.e. continuously scanning the laser) tends to add noise to the signal on short timescales while smoothing out longer-timescale drifts. In some cases, adding a small “micro-raster” with a ∼ 1 mm amplitude and a ∼ 1 Hz frequency while spot hopping may help with the instantaneous signals and target region longevity on a depleted target (see Section 5.7), but on a fresh target face, spot hopping without rastering tends to give more consistent signals. In principle, spot hopping with a signal threshold should be strictly better than spot hopping without one; however, on a fresh target face, simply hopping to a new spot at

Binsize (µm) 50 100 150 200 250 300 350 400 Percent Decrease 0 2 4 6 8 10 12

Percent Decrease in Signal from First Visited Spots over 4 Hours of Run

Binsize

Figure 5.5.2: Target spot depletion correlation length scale. Left: The target surface is divided into a grid with regions of width “binsize,” and each bin is considered “first visited” the first time the center of the fiber laser spot falls into a bin. Right: The length scale of target surface depletion can be defined as the bin size below which spots are significantly less productive on their “first visit” as the run progresses. Such behavior indicates that the “first visit” is not truly the “first,” as previous laser spots must have overlapped significantly with these bins in order to deplete them. This plot shows that the relevant length scale for target depletion is ≈100µm, which is comparable to the diameter of the laser spot. Figure, data, and analysis from Jacob Baron.

every pulse gives the maximum signal levels, as we found during the long run discussed in Section5.7. As the target surface starts to age and deplete, turning on the signal threshold helps to find and stay on the remaining good spots.

Regarding the optimum spot size, the data is at present inconclusive. We performed several runs in which we translated lenses of different focal length (300 mm, 400 mm, and 500 mm) through their focal position and for several cm on either side. In some instances, the optimum signals occurred with the target at the focus, while at others, a spot size of up to approximately 1 mm appeared to give the best signals. When the beam size became much larger than 1 mm, the signals fell off rapidly.

Conclusion: On a fresh target, hop to a new target spot with every pulse in order to maximize the beam flux. Turn on the signal threshold spot-hopping algorithm as the target starts to age and the signals decrease. Turn on “micro-rastering” as desired to smooth out slow drifts.

Before running on a new target, optimize the lens position. Re-optimize periodically as the target ages. Expect that the target signals will fall by a factor of about 2 after 1 week of run time.