We need to measure the efficiency of an electrode-less magnetic plasma thruster.
We compressed neutral ionized gas with 30 Megawatt injection during 150 microseconds. In the Pulsotron-2 May test campaign we measured plasma speed over 33 km/s.
We would like to know: speed and kg particles thrown and also the plasma thruster specific impulse.
We should use a gauge to measure directly the specific impulse by measuring the "weight" of the plasma thruster but we prefer to measure at the exhaust.
A problem is that during the test that about 1-2 megawatts of electromagnetic interference is generated.
Another problem is the high output power: in the second test that we made one of the probes was dismantled and thrown two meters away
I will have to respond myself. I will use coil attached to a distance sensor. As long as my plasma thrusters emitts too much light pulse and during short times I will use a linear variable resistor.
To measure particle speed I will try using high accuracy pointing optics and electric dipole measuring plasma resistance at high speed. The results will be written in the next pulsotron test campaign report
Dear Javier,
I stumbled upon this old question by mistake. However, the question is pretty good and should even be generalized a bit since it consists of a dilemma that many researchers in the field face.
First of all, it does not matter if the thruster is electrode-less or of different kind. In addition, the question can be generalized to all types of electric thrusters.
Second, measuring Isp is not the same as measuring the exhaust velocity, although the former is an average (over the exit plane) of the latter, multiplied by g0. Normally, the Isp should be measured by a direct measurement of thrust (or impulse in your case) divided by the mass flow rate (ejected mass per pulse in your case).
I would not try to measure the Isp by measuring the exhaust velocity since you would have to measure the velocity at different locations on the exit plane. This process is tedious and will introduce many possible sources of errors.
Since the thrust efficiency is what you are after use the impulse and ejected mass per pulse to determine the axially-directed kinetic energy and divide it by the energy invested per pulse.
Measuring the exhaust velocity is usually done by measuring the axially-directed ion energy distribution. Using a Retarding Potential Analyzer (RPA) is an easy way to do it. There are commercial off-the-shelf RPAs you could purchase if you don't want to design one. Now using an RPA will not necessarily give you all the data as your plasma might be partially or doubly (or higher order) ionized in which case you would need to determine the ionization level.
You can also avoid the above by using an optical tool such as LIF. Note that, although very useful, it is expensive and complex to install and handle.
Lastly, do not attempt to start all of the above without making sure you do not have any significant EM noise.
- Dan Lev
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