Choice of initial energy as kinetic or total.

The program offers the choice of initial energy as kinetic or total -accessed in /databuilder/sources of rays/particle type and form of energy/.

(See end for advanced option on disabling tests for low kinetic energy.)

Particle initial energies are usually entered as kinetic energies.

Total energy means the sum

T = K + q*P,

where K = 0.5*m*v**2 is the kinetic energy (corrected for relativistic effects if necessary), q is the charge of the particle and P is the potential. T is a constant of the motion (for time-independent fields).

Warning: The total energy option is only intended for use for beams in which all the rays should start with the same total energy even when their starting potentials are not exactly the same.

Remember that the calculated value of P at the starting point which might have a slight error. So if you know the value of T and the correct initial value of P then the 'total energy' can be chosen, in which case the user is asked to give the correct initial value of P. All the kinetic energies will then be corrected by adding the energy shift:

(correct_initial_potential - calculated_initial_potential)*(charge number),

where charge number = -1 for an electron.

For example, suppose that the user wants to simulate photoelectrons that start with a kinetic energy of 0.5eV from a photocathode at 0V, and that the rays are started at a distance of 0.1mm from the photocathode where in principle the potential should be 0.12V, say. So when the electrons reach this point they should have a kinetic energy of 0.62eV. Now suppose that the calculated potentials at the starting points of 2 of the rays are actually 0.11 and 0.13V. By using 'total', specifying the initial potential as 0, and entering the energies as 0.5, the formula given above shows that the program will add 0.11 and 0.13eV respectively to the kinetic energies of the 2 rays, making them 0.61 and 0.63eV. Extrapolating back to the photocathode, the initial kinetic energies there were both 0.5eV, as required.

In the total energy option it is assumed that all the rays start at the same potential.

For technical reasons, the choice is forced to be 'kinetic energy' by the program when a cathode is used. Also, when 'total energy' is used, you will not be allowed to change the applied potentials at run time.

In practice some users, even experienced users, have misused the 'total energy' option and have obtained confusing and misleading results. It is therefore very important when using the 'total energy' option to check the values of the initial kinetic energies of the rays, by looking in the information box or the output data file.

Disabling tests for low kinetic energy:

The program tests the kinetic energy during the ray tracing, and will stop a ray if the kinetic energy is about to become negative, and might stop the ray if the kinetic energy becomes too small. The threshold for this is the product:

(range of applied voltages)*(ray inaccuracy)*(charge number)*0.1

For example if the minimum applied voltage is 10 and the maximum is 100, and if the requested inaccuracy is 0.001 and the particles are electrons, then a ray will be stopped if the kinetic energy falls below 0.009 eV, because it is judged that the kinetic energy is comparable to the uncertainty in the calculated potential.

This test can be disabled, although in general it is not advisable to do this. When disabled the ray will of course still be stopped if the kinetic energy would become negative.

For users who are editing or constructing an 'input data file' without the use of the data-builder -that is, pre-processor:

But Manual editing is certainly not recommended -it is a relic from the time when the databuilder was not available All users are strongly encouraged to use the databuilder, which always gives the correct formats and which has many options for which the formats are not described or easily deduced.

Put the letter 'k' or 't' on the relevant line, and it the letter is 't' also put the exact value of the initial potential on the same line, in space 4 or later.

The letter 'x' in the third space will disable the tests that are made for low kinetic energies.