1. Puzzles of the VELA bucking solenoid v. 2.0
B.L. Militsyn
18 September 2015

2. Motivation Both measurements:
Recent emittance measurements
Beam rotation measurements
Show trend to very high (close to 10 A) compensation current in the bucking solenoid
But
Previous emittance measurements shows optimum current of 2.2 A
Ben’s measurements show current of 5.6 A
Still there was no calculation of the field in the cathode plane

3. VELA Photocathode Gun

5. Assumptions
Data of the main and bucking solenoids provided by Strathclyde corresponding to reality
Drawing corresponds to reality
Position of the main solenoid has been measured with a ruler
Property of the yoke steel is closed to steel 1010

6. Main solenoid field at a current of 170 A without presence of the bucking solenoid

7. Main solenoid field at a current of 170 A with bucking solenoid (I=0)

8. Compensated field of Main solenoid field with bucking solenoid (NI=-1875 A or 2.6 A if N=720)

9. Preliminary results
At N=720 and at current location of the main solenoid edge at 22 mm from the photocathode surface required compensating current is 2.6 A.
Measurements in the magnetic lab shows that compensation current should be 5.6 A. What is wrong?
Location of the main solenoid
Number of turns

10. Different location of the main solenoid at measurements in the magnetic lab: 6.7 mm instead of 22 mm. Compensating current NI=2685 or I=3.7 A at N=720. To have current 5.6 A N=475

11. To calibrate N the excitation curve of the bucking solenoid has been built.

12. Radial dependence of the bucking solenoid field and measured values.
-12 mT
37 mT
45 mT

13. Map of the compensated field in the gun

14. Field profile at z=-30 mm

15. Compensated field in the cathode plane

16. Compensated field of Main solenoid field with bucking solenoid (NI=-1875 A or A at N= )

17. Conclusions and Ideas
At current position of the main solenoid its field of the main solenoid with a current 170 A should be compensated by a bucking coil with a current of A
Ideas:
During winter shutdown introduced a strong magnet which induce additional beam coupling
More ideas are welcome