DRIFT

  Drift the particles to a certain time or to a certain s coordinate. DRIFT T=DT=S=, [RIGHT,] [LEFT,] [KIND=k(,),] [EXTERNALFIELD,] ;

Drift until Time= (meter).

Drift over time interval (meter).

Drift to s coordinate (meter).
In any of the three cases T, DT, and S, the particles may go backwards in time depending on the parameters.

RIGHT,LEFT

Drift right- or left-going particles only.

k

Drift only particles of kind k.

EXTERNALFILED

Take into account the external field.  

When there is only external field without beam interaction, DRIFT EXTERNAL is much better (more accurate and faster) than the PUSH command. The difference is that DRIFT EXTERNAL uses an exact solution in a constant field whereas PUSH carries out step-by-step integration, and that PUSH accepts only t as the independent variable while DRIFT also allows s (as in most accelerator program codes).

How to use DRIFT EXTERNAL may be understood by the following example. Suppose that the region is shined by a laser. An electron beam comes from the left and goes through the laser region to created back-scattered photons, and subsequently goes through a magnetic field region . If the interval is shorter than the bunch length, the bunch head is already in the field region when the tail gets out of the laser region. If you use PUSH command, you have to track the beam till the end of the magnetic field region. Instead, you can do more elegantly,