Simulation of fields around spring and cathode for photogun

D. Lipka, MDI, DESY Hamburg, July 2012

Simulation of fields around spring and cathode for

photogunD. Lipka, MDI, DESY Hamburg


Simulate fields in gun (here gun design version 5 is used)Monitor fields at spring and cathode region to find unexpected high fields

Goal


Setup• Gun version 5• Frequency domain

solver around 1.3 GHz• Fields are scaled to

60 MV/m at cathode surface

• Body copper


Present cathode region setupMaterials:• Cathode: Molybdenum• Spring: CuBe

(conductivity 25∙106 S/m) or Silver

• Backplane (blue): stainless steel

Blue line: on this line field strength will be monitoredHolder at backplane is simplified to increase the field resolution (more mesh cells possible)


Setup• 85 lamella around

cathode• Contact verified on

inner spring to cathode and lamella to the other

• Contact of spring to copper body too


Mesh viewUse tetrahedral meshLarge cells in resonator, very small cells at cathode region for high resolution


Boundary and symmetry• Symmetry on yz-

plane with magnetic Ht=0

• Magnetic boundary at transverse coordinates

• Electric boundary at z=max

• Open boundary at z=min


Reflection resultEach frequency domain simulation produces reflection like shown here,p-mode is at 1299.82 MHz


Scaled E-field • The fields are scaled

such that E=60 MV/m at the cathode surface is generated,

• here the absolute E-field along the z-axis is shown,

• almost perfect field balance visible


Scaled E-field• Here monitored the E-

field along the x-axis at the surface of the cathode and further

• Higher field of almost 90 MV/m found at the corner of the cathode, smaller at the corner of the copper wall


Scaled H-field distribution

• The maximum of H-field is at the rounding of the resonators, Hmax=133 kA/m

• At the cathode it is much smaller, details will be given for different cases


Different cases

Beside the default setup different cases are investigated:

One missing lamella 11 missing lamella 11 shifted lamella cut sphere

(no contact to (in addition to cathode) shifted lamella)


E-field on curveAbsolute E-field along the cathode line shown; unit: V/m1. Default: ends at about

13.5 mm2. Missing lamella: slightly

higher field 3. 11 missing lamella: more

field toward other end of cathode

4. Shifted lamella and cut sphere (similar): more field behind cathode

Similar for silver and CuBe


E-field distributionShown for default and cut sphere caseResults: no significant higher field at the additional peak for the last case


H-field on curveAbsolute H-field along the cathode line shown; unit A/m1. Default: high field spike at

connection to spring2. 1 missing lamella: field

smeared out3. 11 missing lamella: more

smearing4. Shifted lamella and cut

sphere (similar): longer behind cathode

Similar to silver and CuBeField distributions in the following slides


H-field distribution default caseH-field strength at cathode is smaller compared to cavity, but at the junction between spring to cathode or holder higher local fields visible


H-field distribution default caseHere the local field spikes are visible at the cathode and spring


H-field distribution: missing lamellaAt the monitoring line the field strength is reduced but at the other contacts the field strength is slightly higher


H-field distribution: 11 missing lamellaA larger area get a reduced field, but at the starting spring the field is higher compared to the default case


H-field distribution: 11 shifted lamellaLike it is shown along the curve on the slide before the field strength is more smeared, but still higher field at the first contact


H-field maximum at cathode

Taking the maximum field strength on the cathode the surface current can be calculated with:Surface current I=H∙2pr/√2, with r the radius

For the different cases the maximum current are:• Default: 1207 A• Missing lamella: 1268 A• 11 missing lamella: 1634 A• 11 shifted lamella: 1926 A


SummarySimulation of gun 5 with present cathode design and different casesE-field: at corner higher field, behind is lowerH-field: strong difference between different cases, higher field and therefore higher surface current when spring does not have a good contact to cathode


I got the new holder and spring design, see 3D model on right,

Simulation: same settings

New spring and holder design


View around spring, 45 lamellasNew spring and holder design


On the right side the mesh view, low mesh sizes for cathode and spring and holder around defined

Mesh view


E-field on right plot in V/mSimilar field along z-axis, similar field on cathode surface in resonator compared to present design, a little bit longer field to the backside of cathode due to spring position is more behind, amplitude on cathode side few MV/m and lower

E-field along line


H-field on right plot in A/mSeems to be no spike at cathode, but see next slide …

H-field along line


Before the contact with spring high field: 28 kA/m (line was just between two lamellas) Corresponds to 1061 A surface current, lower than present design (1207 A)

But …

H-field on cathode


Introducing a bad contact of spring, I removed 6 lamellas, see design on right

Missing lamella


A strong H-field at the first contact on the cathode is produced with 67.6 kA, which corresponds to 2552 A surface current (higher compared to the failure setting with present design 1926 A)

H-field on cathode with missing lamellas

Documents

Simulation of fields around spring and cathode for photogun