CST Particle studio. Analysis of
“STEP-OUT”
Olav Berrig
Thanks to Benoit Salvant
Device under analysis
• Graphite(S=30000, er=1, mr=1), r_exterior=5
Device under analysis
• Vacuum inside: r_small=1 cm, r_big=2 cm. Radius gets bigger: STEP-OUT
Device under analysis
Gausian Beam: longitudinal sigma=1.5cm, charge=1*10^-9C. Max beam freq.
=11.8231 GHz, Beta=1, measuring particle at x=0,y=0.5. Beam direction=z.
Background consists of PEC (Perfect conductor).
CST Particle studio, can calculate (post-processing) the wakefield according to three methods:
Direct
The direct method is used if the particle beam is not
ultra relativistic, or if an indirect method is not
applicable.
The wake potential is computed by an integration
along an axis parallel to the beam axis.
Indirect testbeams
This is the most accurate integration method. It can be
used if the beam has an ultrarelativistic velocity and the
beam tubes cross section at the entry boundary equals the
cross section at the exit boundary.
The wake potential is computed via recording the
longitudinal field values on the extruded shell of the beam
tube in the discontinuous region (blue lines).
Indirect interfaces
If the cross section of the beam tube varies at the entry and the
exit boundary, one can choose this method for ultra relativistic
beams.
The wake integration in the discontinuous region is done directly on
an axis parallel to the beam (dotted blue line). For the tube regions
the wake potential share is considered by a computation on the
interface areas (thick blue lines).
If the solver recognizes than an indirect integration scheme cannot
be applied, the direct method is used an a warning will be printed.
Other configurations calculated for comparisons:
PEC(S=,e=undefined, mu=1), r_exterior_small=4, r_exterior_large=5. Normal
background. With vacuum between small cylinder and background.
Graphite, r_exterior_small=4, r_exterior_large=5. Normal background. With vacuum
between small cylinder and background.
Other configurations calculated for comparisons:
Beam direction=-z. (Beam going away from us)
Graphite, r_exterior=10.
Q1: Why are electro-magnetic fields introduced in the vacuum between the graphite
and the background, and not in the vacuum between the PEC and the background?
Graphite
PEC
Q2: Why does the wakefields change, when the background changes, because the fields does not penetrate out to the background?
The exact same effect was seen with 10 cm exterior radius:
Graphite, r_exterior=10cm
Graphite(S=30000, er=1, mr=1).
PEC background.
Graphite(S=30000, er=1,
mr=1).
Normal background.
Q3:
Why does none of the three wakefield integration methods give the correct solution?
DIRECT:
Notice the
scale: 10-6
Graphite
INDIRECT TESTBEAM:
Notice the scale:
1000
INDIRECT INTERFACES
Addition to Q2
Changing the background from “Normal” to “PEC” has
no effect, when metal plates are installed at the ends