Measurement T1: Additional insulation at the front

resistance was not sufficient to keep the evaporation rate stable. This can be interpreted as such that the process in the oven, that changes the thermal distribution within it, affects the crucible stronger than the filament temperature and thereby its resistance.

It is not understood why this measurement showed more resistance changes than the measurement before (5.22 compared to 5.25). It is possible that structural changes of the oven, like aging of the heat reflective foil, or difference in the tightness of the screws holding the oven cover caused this behaviour.

5.4. Oven insulation modifications

stability of the lead evaporation by making the temperature profile of the crucible more homogeneous.

Following the results presented in section 5.2 it has to be considered that additional parts exposed to the lead gas jet can create more condensate and thereby also lead oxide formations.

The presented measurement was conducted with rings of the inner opening radius rTa=3.6 mm which are shown in figure 5.28.

Tantalum foil rings as additional heat reflection

Figure 5.28.:Tantalum insulation rings at the front of the oven to enhance the heat reflection at the oven tip. The rings outer diameter is adapted to the oven covers inner size so that their central opening hole is in the middle of the ovens front face without the need of any attachment. In measurement T1 three of these rings were inserted into the oven.

During this measurement the oven was again controlled via a setpoint for the filament resistance which was then realized by a feedback loop that changes the filament current.

This decision was made as the feedback loop showed no negative effect on the stability of the oven in the previous tests. Like in the measurement I2 the setpoint was manually controlled with the aim of reaching a sufficiently high evaporation rate and keeping it over a time comparable to the oven operation at the GTS-LHC ion source.

Results

The evaporation rate over the complete run together with the applied power are shown in figure 5.29.

At the beginning of the run the setpoint of the filament resistance was corrected down-wards as the evaporation rate grew larger than intended. The evaporation rate could then be held around a median of approximately 3 mg h⁻¹over the duration of the complete run, which lasted 350 h. Initially the evaporation rate stayed comparably stable for around 24 h

0 50 100 150 200 250 300 350 0

2 4 6

0 5 10 15 20

t [h]

R[mgh−1 ] P[W]

Evaporation rate,R Oven power,P

Figure 5.29.:Evaporation rate and oven power over the time of the measurement T1. The power is not actively controlled but a result of the feedback loop changing the current in the filament to achieve a preset filament resistance. The photo shows the oven tip after the measurement.

and then started to show fluctuations.

In its first change the evaporation rate dropped around 0.5 mg h⁻¹and then recovered to its previous value again several times. This happened without any outer changes, and might be due to redistribution of lead within the crucible. After around 90 h of evaporation also drops occurred that would not recover. Like in the measurement I2 the feedback loop here reacted and raised the filament current as a reaction of a decreasing filament resistance.

Again the evaporation rate only recovered after the resistance setpoint was raised manu-ally. In comparison to e.g. measurement I2 the evaporation rate underwent smaller drops but fluctuations happened more frequent. It is notably how the oven power throughout the run only had to be raised in a rather small range from 8 W to 10 W. The photo in figure 5.29 shows the oven tip after the run. A big droplet of lead has gathered in the opening of the oven cover. Smaller droplets can be seen on the tantalum foil rings.

Figure 5.30 shows several parameters during a selected drop of the evaporation rate. In the upper plot the evaporation rate and the filament resistance are depicted. To emphasize the resistances behaviour a line, showing the resistance where a Gaussian filter was applied, is included. In the presented time window the evaporation rate started stable at around 2.6 mg h⁻¹ and then dropped to 2.1 mg h⁻¹ in less than an hour. At the same time the resistance dropped too from initially 2.119Ωto 2.116Ω. After dropping the resistance rose again as the control loop reacted on the drop and raised the current. The current and the power are shown in the second plot from above. The two lower plots show the signals

5.4. Oven insulation modifications

2.116 2.118 2.12

1.8 2 2.2 2.4 2.6 2.8

8.6 8.65 8.7

2.01 2.02 2.02

323 324 325

98 99 100 101 102 103 104 105 106

161 161.5 162

r[Ω]

R[mgh−1] P[W]

I[A]T[°C]

t [h]

T[°C]

Fil. res.,r Ev. rate,R

Power,P Current,I

TC1

TC2

Figure 5.30.:Several parameters during an evaporation rate drop that where recorded in measure-ment T1.

of the two thermocouples, TC1 and TC2, that where attached to the outside of the oven during the measurement. Both thermocouples showed a slight increase in temperature at the time of the evaporation rate drop.

Discussion

Without oxygen injection the oven could be operated for two weeks, without raising the oven power over 10.5 W. But the evaporation rate showed unpredictable fluctuations and even in combination with the feedback loop reacting on the filaments resistance, the oven needed active tuning to maintain its output. From the formation of condensate at the tantalum foil rings it can be deduced that they would be a new starting point for a lead oxide blockage in an oxygen atmosphere. From this behaviour it can not be deduced that the additional shielding increases the oven stability. Even if the rings might even out the crucible temperature distribution this is overshadowed by the instabilities that where seen before and seem to affect the overall crucible temperature.