![Page 1: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/1.jpg)
Christopher Mayes – July 23, 2014
LCLS-II dark current tracks from cathode edge and possible collimation
![Page 2: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/2.jpg)
Christopher Mayes – July 23, 2014
Charge assignment from FN parameters
http://www-physics.lbl.gov/seminars/old/Sannibale.pdf
• Used the parameters from measurement to assign charges to the dark current particles based on their initial longitudinal E-field.
• Particles are uniformly distributed in time and at radius 5mm (the edge of the cathode puck)
• The sum of all charges is normalized to 1(Coulomb)
![Page 3: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/3.jpg)
Christopher Mayes – July 23, 2014
Particles from r = 5 mm on cathode colored by charge
(red is higher charge)
![Page 4: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/4.jpg)
Christopher Mayes – July 23, 2014
Particles lost colored by charge (red is higher)
![Page 5: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/5.jpg)
Christopher Mayes – July 23, 2014
Particles escaping colored by charge (red is higher)
![Page 6: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/6.jpg)
Christopher Mayes – July 23, 2014
Particles lost backwards
0.25% of the current turns around and hits the cathode
![Page 7: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/7.jpg)
Christopher Mayes – July 23, 2014
Current lost per meter
![Page 8: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/8.jpg)
Christopher Mayes – July 23, 2014
Forward moving particles in frontend
![Page 9: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/9.jpg)
Christopher Mayes – July 23, 2014
Collimator at z = 2 m
These are the particles passing z=2m in the forward direction.
The fraction captured current for a given collimator radius. Note that some current was already lost prior to this location
![Page 10: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/10.jpg)
Christopher Mayes – July 23, 2014
Collimator placement after buncher summary
This extends the previous plot to all possible z locations for collimation.
For example:
A collimator with radius 5 mm placed at z=1.5 m would capture 80% of the dark current
A collimator with radius 10 mm placed at z = 2.0 m would capture 70% of the dark current
![Page 11: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/11.jpg)
Christopher Mayes – July 23, 2014
Tracks including HTR, Col0 sections
![Page 12: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/12.jpg)
Christopher Mayes – July 23, 2014
Tracks including HTR, Col0 sections
.02% survive
![Page 13: LCLS-II dark current tracks from cathode edge and possible collimation](https://reader030.vdocuments.us/reader030/viewer/2022032607/56813025550346895d95aa87/html5/thumbnails/13.jpg)
Christopher Mayes – July 23, 2014
Tracks including HTR, Col0 sections