Gas pipes
4.4 Experimental problems
4.4.1 Debris
The results of a later series of experiments are shown in figure from 4.9 on the following page. In this case, the voltage was decreased to 19 kV, and therefore the energy to 180 J. This voltage reduction allowed us to reduce the turbulence as well. In this experiments we observed for the first time a channel after the collision, with the correct density profile for laser acceleration.
Figure 4.9: Image of a circle well before the collision of the shocks. The delay between the explosion and the diagnostic laser is 49.8 1lB. This picture is a good example of Mach addition, because the shocks are creating a uniform common shock with a circular shape. The uneven background is due to defects of the lens used to collimate the laser. Wbank = 180J.
Figure 4.10: Image of a gas pipe before the collision of the shocks in the centre. The delay between the explosion and the diagnostic laser is 52.8 ~.
The circle is not perfect, probably due to small errors in the position of the wire. It can be noted that there is no turbulence behind the shocks.
Wbank = 180 J.
Figure 4.11: Image of a big pipe before the collision of the shocks. The delay between the explosion and the diagnostic laser is 56.6 llS. The pipe has now a diameter of about 1 mm. Wbank = 180J.
Figure 4.12: An example of a quasi perfect pipe, with a delay of 61.2 ~.
Only two shots are arriving later, without affecting the shape of the pipe too much. The inset shows a schematic drawing of the shock waves. One can see that six shocks are almost perfectly on time. It is not clearly understandable what the two on the right are doing. Probably, distortions like this one are smoothed by Mach addition after collision. Indeed, we noted that the pictures after collision tend to show more regular shapes than the ones before.
Wbank = 180 J.
Figure 4.13: A fast shot. Despite the delay of 57.4 IlS the inner diameter of the pipe is now smaller. As the time between the explosion and the firing of the laser is measured on the oscilloscope, this can only mean that the shocks were faster. Indeed, from shot to shot the residual energy left in the capacitor, and therefore the energy gone into the shock, is different. Wbank
=
180J.Figure 4.14: An even faster shot. After 55.8 IlS the shocks have already merged through Mach addition. Despite the delay is smaller than in pic- ture vrefpict:s32, the circle is smaller, which means the shock waves travelled faster. The circle is now smooth and the single shocks cannot be recognised anymore. Wbank
=
180J.Figure 4.15: This picture with a delay of 57.0 IlS shows the smallest pipe we have observed before collision. It can be noted that the pipe is not perfectly circular but has an elongated shape. The line shows the path of the densitometry in graph 4.4 on the following page. Wbank = 180 J.
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Graph 4.4: Densitometries of the picture 4.15 on the preceding page. The top graph shows the densitometry along the longest side of the pipe, the other one along the shortest side. The density, obtained with a computer scanner, is in arbitrary units. Wbank = 180 J.
Figure 4.16: In this picture with a delay of 62.2 ~ the shock are colliding in the centre. In the centre there might be a solid column of gas or a very small and slightly misaligned pipe, so that no light is getting through. Wbank = 180 J.
Figure 4.17: This picture, obtained with a delay of 54.6 ~shows the smallest pipe we obtained after collision. The pipe is not perfect because of some turbulence, but the size is very small. The central black dot should be noted.
This corresponds to light refracted inward by the rebounding shock waves.
Is is the desired result for colliding shock lenses. Itmay also be the optimum time for colliding pipe formation. Wbank = 180J.
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Graph 4.5: A densitometry of the picture in figure 4.17 on the preceding page.
It can be noted that the bright spot in the centre has a size of rv 100 !lm (FWMH). In order to have a focus reasonably close to the lens, the laser beam was not parallel, but converging 5 m after the device, by means of a quartz lens. Wbank= 180 J.
Figure 4.18: Another shot immediately after the collision of the shock in the centre. The circle has already a diameter of about 5 mm. Unfortunately, the delay is unknown because the oscilloscope did not record the trace. The small dots close to the centre are foci created by the intersection of the shock waves. Wbank = 180 J.
Figure 4.19: In this fast shot after collision with a delay of 65.2 I..l.S the shocks have expanded more. Now the circle has expanded up to more than one centimetre in diameter and no focus is observed in the centre. Itis interesting to notice in any case that there is no light in the centre. Wbank
=
180J.Figure 4.20: SEM pictures of copper droplets from the melted wire on the lens surface.
damaging the lens we were using to focus the laser beam. The SEM pictures in figure 4.20 on the preceding page show tiny droplets (smaller than 100 ~m) on the lens. Since the debris are travelling slower than the shock wave, they should not have a bearing on particle acceleration experiments, in which the centre of the interaction chamber is empty.