Sir, FWHM is the crude method to calculate the ion beam diameter. Is there any other way to calculate the ion beam diameter?
No Sir, I am looking for the profile density of ions extracted from the plasma source.
This is not easy to calculate and requires dimensional knowledge of the source and grids. The standard is to model each grid hole as a "beamlet." Each beamlet is assumed to have the same shape which could be cos^2(theta) as a first approximation. Finding the beam shape is then taking a finite section of the target plane and summing up the contribution of each beamlet to that point. The grid curvature needs to be taken in account since it part of what defines theta from each beamlet. Once you have the total shape against the target plane, the shape can then be normalized against the total beam current, which is known. Using this method we were able to predict uniformity vs pointing angle with good accuracy. Refinements can be in beamlet shape, variation in intensity vs source radius due to nonuniform plasma in the bowl, and others, and that quickly becomes a complicated research project requiring careful beam shape measurements and multiphysics modeling.
ion source front grid diameter+2*grid and sample distance*tan theta
theta is the beam divergence (0-25 deg max)
in the above, formula what does 2*grid here grid mean? Basanta Kumar Parida
At what distance do you want to measure the beam diameter?. Usually a single Faraday Cup can scan across the plume to scan the entire beam width and you can fit a gaussian function to estimate the FWHM and multiply it by 2 to get the diameter. You can also use a Faraday Cup array to avoid the complex moving mechanism. Each channel of the Faraday cup array can give you current density at a particular location which can be later used to fit a Gaussian function for estimation of the beam profile. However if the beam has some focusing effects due to bending of grids or its concave shape, then a single gaussian may not give the correct beam width. You need to incorporate the focal length. For multiaperture sources, assuming each aperture as a point source and summing up their intensities,you can estimate the beam profile using analytical methods as well as discussed by @KyleGodin above.
Grid means through which the ion are coming outside. Its diameter is around 4 cm in our case.
In my system, I am not using Faradays cup. We are measuring the current which is drawn by the sample holder with varying vertical height and fitting the Gaussian and followed the same method as you explained.
One more question I have, Like in the RF plasma ion source, I noticed with increasing beam current. The current drawn by the sample holder first increases achieves its maximum value, and then decreases. Why this phenomenon is happening? Because of the divergence of the beam?
Bharat Singh Rawat
Using a sample holder to measure a beam current may not be an accurate method because of secondary electron losses and you may measure extra current than the actual. I did not fully understand the second question, it would be better if you could share any plot of the current profile!?
I have attached the graph, in which with an increase in beam current, the current drawn by the sample holder first increases attains its maximum value, and then decreases. fixed parameters of the experiment are energy =400eV, acc voltage is 100eV.
Simply, we can say that with an increase in beam current, the ion flux first increases to attain its maximum value and then decreases why this is happening?
I believe the decrease in sample holder current may be attributed to an increase in beam divergence.Because increased beam diameter leads to decreased current density in the beam which causes the reduction in sample holder current. To address this, I recommend generating a plot that displays the sample holder current values across various perveance (Perveance=I/V^3/2) values, where 'I' represents your beam current, and 'V' is the total extraction voltage. This plot will offer insights into the most favorable operational conditions.
I have attached some perveance curves from one of the ion sources I've developed, which could be beneficial for your analysis. Additionally, you can employ simulation software like CST or OPERA to predict the optimal perveance limit for your ion source operation. This will help ensure that your ion source operates with minimal divergence.
Thank you so much. Sure I will look after the software too@Bharat Singh Rawat
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