The main parameters of technological ion-plasma beam are the following:
Physicists use more parameters to describe beams but I suppose that these ones are the most important and in general sufficient for technological purposes.
The ion current density my be most easy measured by the probe as shown in Fig.1a (so called Langmuir probe). It is a small piece of stainless steel foil with known dimensions (usually about 3 x 3 mm or something like this) with screened or insulated wire lead. The main sources of errors are plasma beam electrons and secondary electron emission. When you are measuring the ion current density of plasma beam the probe reacts to both ions and electrons of plasma. The electron and ion currents flow in opposite directions and you register the total current that is more than ion current you need to register. To avoid it you have to bias the probe to approximately - (5 - 20) V. Usually in low energy plasma beam electrons have very low energy (< 1 eV, so called thermoelectrons) so you don't need a high bias voltage. In my PAEZA ion current density measurements I did not obtain any difference between the measurements with biased probe and grounded one. But I strongly recommend to get the "probe current bias voltage" dependence (Fig.2) and to figure out if you need of bias or you don't before starting your measurements.
Fig.1
Fig.2
Second problem is the secondary electron emission from the probe surface due to ion bombardment. These electrons increase current density reading. You can ignore this error when you measure the current density of ions with the energy of several hundred eV or less, but when you measure the ions with the energy of several keV, the error may be significant. It can be accounted for with the screened positive biased probe (Fig.1b). However the grid may decrease the ions flow on probe. This problem may be solved with Faraday cup shown on Fig.3.
Fig.3
Faraday cup consists of two coaxial isolated cylinders: grounded outer screening cylinder and inner collecting one (collector). It works like "black body" in optic absorbs all particles which pass into collector and significantly decrease the output of secondary particles created due to interaction of primary particles with the surface of collector. The ratio of the input aperture diameter to the length of the collector should be not less than 1 : 5.
You can measure the distribution of ion current density over the beam cross-section with the help of a number of probes placed in the different points of the beam. It is easy but not too accurate method. The better way is to use the probe moving across the beam and connected to the chart recorder. I used a set of five probes fixed on a moving common holder on the different distance from the ion source and connected to the 12 channels chart recorder. To the rest seven channels the signals from power supplies (discharge voltage and current, solenoid current) and pressure meter have been led. So during the couple of days I could obtain a lot of information about the space distribution of ion current density depending on the ion source parameters.
The ion energy can be measured with an electrostatic energy analyzer with retarding field r retarding grid analyzer (Fig.4).
Fig.4
It consists of a Faraday cup with at least three grids. The two negatively biased grids are used to eliminate the influence of plasma electrons and secondary emission electrons. If you are working with low energy plasma source with thermocathode you can ground these grids. The positive retarding voltage VR increasing from zero to the value about discharge voltage VD is supplied to the middle grid and the collector current jS is recorded. The collector current jS is created only by ions with energy more than eVR. The ions with less energies will be retarded in the field with potential difference VR between the retarding and screening grids. The derivative djS(VR)/dVR shows the ion energy distribution. The energy distribution of the ions generated by PAEZA is shown in Fig.5.
Fig.5
It is not enough to measure the ion energy at one point of beam, for example, on its axis. The matter of fact is that there is the drift of particles out of beam volume. This drift is a result of different energy, momentum and charge exchange interactions between the ions and gas molecules. So in the first approximation we may suggest that there are two kinds of plasma in chamber: directed beam plasma containing the ions with relatively high energy generated by ion source and secondary plasma diffusing from the beam in vacuum chamber volume. The energy of ions in the secondary plasma is a few times less than in the beam plasma. The current probes are non selectable to the energy of ions so if you have got any ion current distribution by these probes, you can't be sure that the tails of distribution will work, for example, in the process of sputtering, like the center of beam. I can't say that this process is significant for all kinds of ion and plasma sources. However I had done these measurements on PAEZA and obtained that when, for instance, the ion energy in the center of beam is equal to 200 eV, the ion energy on the periphery of beam is only 40 50 eV. Therefore the peripheral part of the beam doesn't take part in the sputtering because of very low ions energies. This conclusion has been proven in experiments.
A second useful method to measure the energy of ions is the calorimetric technique. In this method you exchange the Faraday cup to a calorimeter and measure the energy of ions passing through the grids system. You should better use this method if you suggest that your beam contains a significant part of accelerated neutral particles (atoms or molecules). The main problem is calibrating calorimeter well.
In the first geometric approximation the divergence of the ion beam is defined as:
q = arctg [(D1 D2)/dH],
where D1 and D2 - the beam half widths on the different distances H1 and H2 from ion source (H1 > H2 ) and dH = H1 - H2. The first question is what is the width of beam? In general it is your own selection but you have to indicate it when you are talking about the divergence. For the beams with axial symmetry you can define the width like the distance between the points of ion current density distribution where the ion current density is 1/10 or 1/2 of maximum value on the axis of beam. If the distribution is close to Gaussian one better to measure the width on the level where the ion current density is e-1 = 0.368 of the value in the center of beam.
The second question is: can we really suppose that ions propagate along the straight trajectories? I dont know. May be yes, may be no. Please measure the divergence in the range of distances from ion source that you are interesting for (for example, 5 6 measurements in the distances from 100 mm to 200 mm from ion source) and take a look on a result. If the divergence is the same you can use this approximation. If not you at least can figure out the dependence q(H) but only for the given parameters of your experiment! The divergence can vary if the parameters of ion source are varied.
This is easy to measure. Just put the Langmuir probe into your beam and measure potential with voltmeter. Of course, use only voltmeter with big input resistance for accurate measurements. You can measure the potential distribution over the volume of the beam, it can give you a lot of useful information.
Obtain these parameters like functions of discharge (anode) voltage, gas consumption, gas pressure, solenoid current. Measure them at different distances from the ion source. This work takes some time but it is the only way to know out the information about a real possibilities of your ion source.
Of course, you can find the values of these parameters in Manual but you have to take them only for reference bevause the work of ion source strongly depends on the parameters of vacuum equipment where it is installed. And I am not sure that your vacuum chamber is better than the vendors chamber is (but may be your equipment is better so you can get the parameters that are better than in Manual. So if you would like to use all advantages and to know about all limitations of your ion source you have to be able to measure all the main parameters of the ion beam.