Measurement O1: Formation of a lead oxide blockage

At a pressure of 1×10⁻⁵mbar the oven was operated in a similar way as during measure-ment N2, presented in figure 5.11. The power was ramped up until a sufficient deposition rate was seen and then only increased when the rate seemed too low. Photos were taken to document the changes in the oven tip appearance, i.e. the formation process of the blockage.

Results

Figure 5.14 shows the resulting measurement of the evaporation rate together with selected photos of the oven tip.

The evaporation rate rose steeply and then fluctuated around a value of around 3.5 mg h⁻¹ until it dropped to nearly zero after 440 h. The photos show a material built up at the oven tip that grows into a complete blockage. After around 14 days the oven was completely

MeasurementO1O2O3PurposeEffectofoxygenEffectofoxygenatlowerpressureInfluenceofleadqualityDocumentleadoxideblockageformationSetupNochangesonovenSameasO1SameasO1ThermocouplesattachedChemicallypureleadPowersetaccordingtoevaporationrateAtmosphereOxygenwith1×10 −5mbarOxygenwith1×10 −6mbarOxygenwith1×10 −5mbar∆M[g]1.62±0.011.49±0.20.71±0.01A[mgÅ −1]×10 −37.4±0.184.66±0.66.07±0.6

Table5.3.:ExperimentsattheOTSusedtoassestheformationprocessofaleadoxideblockageattheoventip.

5.2. Active gas injection

0 100 200 300 400 500

0 2 4 6

0 5 10 15 20 25

A B C D

t [h]

R[mgh−1] P[W]

Evaporation rate,R Oven power Moment of photo A - D

Figure 5.14.:Effect of the oxygen atmosphere of 1×10⁻⁵mbar on the oven. The plot shows the evaporation rate and the oven power during the measurement O1. The dashed lines within the plot depict the moments at which the photos on the right where taken.

blocked. It was tested if the blockage can be removed by heating the oven with 20 W but that was not the case.

Calibration

The usual calibration from a deposition to an evaporation rate via the weight change of the crucible would lead to a calibration factor ofA=7.4[10×10⁻³mg Å⁻¹]. However after the oven was blocked the evaporation continued depositing the lead onto the inner part of the blockage. Hence the deposition sensor showed no signal, while the evaporation was still ongoing, making the integral of the detector readout an underestimation.

To still get an estimation of the evaporation rate during the run, the calibration factor of a former run, N2, was used (shown in table 5.2). The scattering of the measured calibration factors for different runs however shows, that this estimation has an uncertainty that exceeds the calculated uncertainties of the individual calibration factors for the respective run.

To take this into account, the standard deviation of all individual calibration factors throughout the study, which were assessed to not be flawed by blockages or larger lead spillage, was calculated and used as the uncertainty of the calibration in this case:σA,comb.= 1.4[10⁻³mg Å⁻¹].

Blockage formation process

The photos in figure 5.14 give insight into the formation process of the lead oxide blockage.

It is not growing from the crucible but, as it can be seen in photo ’A’, the starting point is a formation at the outer oven cover. This shows that the lead forming the blockage is oxidizing, after a fraction of the gas jet re-condenses on the front of the cover. By oxidizing the condensate becomes solid and the blockage can grow, as more lead is being deposited onto the already existing formation. For this to happen several factors play a role:

A significant amount of lead coming from the crucible is deposited onto the oven cover.

Secondly the oven cover is cold compared to the crucible and lowers the local vapour pressure of lead on the covers surface enough to form a condensate. As a third factor the oxygen pressure needs to be high enough to have a sufficient amount of reactions to form the blockage.

The fraction of gas being deposited onto the oven cover can be estimated using Molflow+ simulations (see section 3.4). For this purpose the properties of the facets that represent the inner oven cover are adapted to count and absorb all particles impinging on them. The simulation was performed for different positions of the lead surface within the crucible (the same as presented in figure 3.6) and also for two crucible positions.

At the GTS-LHC the crucible is usually placed as far in front within the oven cover as possible. To asses if this is already a countermeasure for the deposition onto the oven cover, the Molflow+simulations were performed for one case were the crucible is further within the oven and another one, where it is almost touching the oven cover in its front.

The two crucible positions are depicted in figure 5.15.

In the photo ’A’ of figure 5.14 it can be seen that the initial area of growth for the lead oxide blockage is the inner part of the oven cover opening. To distinguish, where the lead is actually deposited, two fractions of the lead gas jet were calculated,fcompl.is the one that hits the inside of the oven cover anywhere (the red area in figure 5.15) and the fraction,

f_opening, that is hitting the cover exclusively at its opening (shown as cyan in figure 5.15).

Table 5.4 presents the results of the simulations. The position of the emitting lead surface within the crucible is not changing the results significantly, so the presented table shows only the outcome of the simulations for the lead surface at the front of the crucible (case C in figure 3.6) and states an uncertainty that takes into account the result from the other surface positions.

The simulated fractions show that if the crucible is placed deep inside the oven more than half of the emitted lead is interacting with the oven cover. This does not imply that also half of the lead is re-condensed, as the gas atoms may be reflected off the walls of the

5.2. Active gas injection

Complete cover tip

Only cover opening Crucible in front

Crucible inside

Figure 5.15.:The two crucible positions that were simulated with Molflow+to derive the fractions of the gas jet that hit the inner walls of the oven cover. Fractionf_compl.is impinging the cover anywhere in the red region,f_openingis the fraction that impinges the opening of the oven cover, depicted in cyan.

Crucible position f_compl.[%] f_opening[%]

Inside 54±2 28±1

In front 18±1 18±1

Table 5.4.:Simulated fractions of the lead gas jet that hit the oven cover. f_compl.is the part that impinges the cover anywhere, whilef_openingis the fraction hitting the oven cover opening.

oven cover but it shows that the deposition is a significant fraction of the gas jet. When the crucible is moved to the front this value drops to around 20 %.

But the simulations also show that while the total value drops significantly the impinge-ment on the inside of the cover opening is only reduced from 28 % to 18 % by moving the crucible to the front. So in both cases it can be expected that enough lead is deposited onto the part of the cover that was identified as the starting point of the blockage.

5.2.5. Measurement O2: Does the blockage form at a lower oxygen