In laser produced plasma. Do we see any difference in the the source size vertically and horizontally and why?
Thanks,
Rad
In very rough sense, if your metal filter or collimator / focusing magnetic field are absolutely uniform, then source size in vertical and horizontal direction will be same. Do you think earth's magnetic field (North-South) may cause some disturbance? In our ion probe microbeam beam size and focusing is highly effect by Earth's magnetic filed.
In our plasma source, the source is measured for both vertically and horizontally. We have actually observed some differences between both, attributed to the expansion of the plasma or even the observation angle with respect to the geometry of the source.
Cheers,
Rad
The vertikal and horizontal size of the source depends on your geometry, e.g. the incident angle of the laser, the observation angle and also the roughness of your targetmaterial.
The size of the x-ray source will be dependent on various parameters. Mainly it depends on time duration and intensity of lasers, which will decide the speed and the geometry of plasma expansion. In ns time duration plasma expansion will be nearly semi spherical, whereas for pico second time duration laser, plasma expansion will be cylindrical. In the case of femto second laser plasma expansion will be very small. These expansion will be different in horizontal and vertical direction. Intensity of the laser will decide the speed of plasma expansion. Finally a rough estimate can be as x= v.t, where v is the plasma acoustic speed and t is the time duration of laser.
Hi Virendra,
Yes, this maybe be true. The intensity, pulse length, and plasma acoustic speed plays the role here. Also the stability of the target should be taken into consideration.
However, with our measurement so for the smallest laser focus (19.5 um), and lens focal length 6 cm, the horizontal source size appeared to be slightly bigger than the vertical one (1-5 microns), possibly attributed to either the slight tilt in the source with respect to the IP and the resulting geometric broadening effect, or can be related to the plume expansion of the generated plasma.
Measurements were also taken with 12.5 µm Al filtering and without (direct exposure), and no difference was seen in the source size.
For longer focal length lenses (7.5 cm, 10 cm), and larger laser focal spots 24.4 and 32.4 microns respectively , the x-ray source was seen to be highly symmetric. Yet, for the smallest focus spot there is a little bit of difference but this is within a percent or less therefore to the accuracy of measuring they are thought to be identical as there is a slight asymmetry which is probably statistically significant for the smallest spots, but for the biggest ones, no significantly different numbers were seen, and by taking error bars into consideration they are near identical.
In a conclusion, it looks like there is a slight significant difference in the smallest focal spots and actually it’s a nearly perfect symmetric spot as the aspect ratio is very nearly 1.
Best,
Rad
Dear Radhwan
I agree that instabilities in the plasma can generate either asymmetry or some other changes in the shapes of the plasma plume. Small focal spot of laser can produce unstable plasma. Because it will produce higher density and temperature along with change in density and temperature gradient of plasma.
Hi Mohammad,
The target we used is a thin tape drive (Mylar (C₁₀H₈O₄) as a carrier film and a fine magnetic powder of both Fe2O3 and chromium dioxide CrO2). This is an entirely different geometry than the gas you are using.
The source is primarily used as LPP x-ray, and in a lateral stage as particles accelerator (Protons), which is dependent on the focused spot size, intensity, and pulse length, target thickness, and Z number.
In your case i.e. gas/cluster target, the focusing of the heating beam has to be close to the gas nozzle to maintain the necessary high density which in turn gives rise to some instability of your focused point, on top of that the expansion of the gas, so I am not surprised if you happen to observe significant change in both horizontal and vertical shape of the zero point.
Best,
Rad
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