If we see a X ray generating tube the voltage applying to tube is in kV, but the tube current is in milliamps. Why this current is in mAs not in amps?
Electronic – Why the current in X ray tube is in milli amps
currenthigh voltagetube
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Regarding the first question, the reason is that the negatively charged space charge itself creates an electric field, which is in opposition to the electric field generated by the positive anode voltage. Increasing the number of negative charge carriers available for conduction is offset by the increase in the opposition electric field these charge carriers generate.
This puts a theoretical limit on the maximum anode current density for a given anode voltage when a space charge is present, and for a planar plate geometry is given by the Langmuir-Child equation:
$$1.67×10^{-3}\left(\frac{q}{mc^2}\right)\frac{V_a^{\frac{3}{2}}}{d^2} \frac{A}{m^2} $$
Where \$d\$ is the distance between the plates, \$V\$ is the anode voltage, \$q\$ is the particle charge, \$m\$ is the mass, and \$c\$ is the speed of light (all SI units).
I'd get a precision 0.01 ohm power resistor and measure the voltage across that. At 20 amps you'd only have to dissipate 4 watts at the resistor.
Alternately, you could find yourself a Hall Effect sensor. The resistor is simpler though.
Power resistors usually look like this:
Or this:
Best Answer
An X-ray tube might require 100kV. A current of only 1A would be 100kW, which is a fairly large amount of power even for a water cooled tube with rotating target.
The efficiency using a tungsten target would be less than 1%, so most of the power goes into heat.
There's no need to produce more X-rays than required, however CT scans may require something of the order of a 100kW power input. They also can expose the patient to a relatively large amount of radiation.
Along similar lines, a particle accelerator beam of just a few tens of nA can carry a great deal of power if the particles are energetic enough (say hundreds of MeV).