CST Particle studio. Analysis of “STEP-OUT” Olav Berrig Thanks to Benoit Salvant

Device under analysis Graphite(S=30000, e r =1, m r =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. = 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? Graphite(S=30000, e r =1, m r =1). PEC background. Graphite(S=30000, e r =1, m r =1). Normal background. The exact same effect was seen with 10 cm exterior radius: Graphite, r_exterior=10cm

Notice the scale: Notice the scale: 1000 Q3: Why does none of the three wakefield integration methods give the correct solution? Graphite INDIRECT TESTBEAM: INDIRECT INTERFACES DIRECT:

Addition to Q2 Changing the background from “Normal” to “PEC” has no effect, when metal plates are installed at the ends