1. Study of Beam Properties at SECRAL and The Solenoid Pre-focusing LEBT
Youjin Yuan
Institute of Modern Physics, CAS
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

2. The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China
Outline
Introduction to SECRAL
Test Experiment and analysis
Simulations and possible solutions
Conclusion
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

3. Superconducting ECRis at Lanzhou (SECRAL)
Magnetic confinement configuration:
SECRAL Superconducting magnet structure.
Solenoids-inside-sextupole
The magnetic confinement fields are typically achieved by the superposition of an axial mirror field and a radial sextupole field.
Achieved axial field: 3.7 T, 2.2 T
Sextupole at the wall: 2.0 T
S-Forward
SECRAL schematic view and the axial magnetic field distribution.
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

4. Expected Beam Properties From SECRAL
Asymmetric plasma distribution at extraction.
Inhomogeneous ion density distribution across the extraction aperture.
Particles are extracted and accelerated in a strong descending axial magnetic field.
Transversal coupled beam.
Sputter marks at the inner side of the
plasma electrode
Analyzing beam line
Layout of the multi-component ion beam transport line.
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

5. The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China
Testing Condition
Beam: 209Xe29+;
Extraction voltage: 25 kV;
Load current: I0=0.95 emA;
Beam current: I29+~19 emA
To study beam properties neglecting space charge effects:
Minimizing the total beam current in the mixing beam transmission line.
Using a low beam current.
Relatively high extraction voltage.
High transmission efficiency before emittance measurement
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

6. Transversal Coupling Property
Measured beam rms emittances by changing the focusing strength of the solenoid. (“H” horizontal, “V” vertical; “F” forward, “R” reverse.)
The difference of projection emittances product mean that the ion beam extracted from the ECR ion source is transverse coupled!
And the extracted beam is not a “round beam”! (same phase space distribution)
When the product of projection emittances reaches minimum value, equal to the determinant of the 4D sigma matrix, the beam can be considered as “decoupled”.
Assume decoupled
Product of projection emittances for different solenoid currents and polarities.
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

7. Projections of 4-D Phase Space (No SC effect)
Back-tracking from measured beam parameters under Is= -180 A, assuming no coupling. ex ey
Beam emittances from forward beam-tracking simulation by using the 4-D phase space distribution as the initial beam condition.
At SECRAL Exit
After Analyzing
Back-tracking
Measurements after Analyzing
Simulated projections of phase space distribution of beam from ECR at the location before the beam line solenoid.
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

8. Transfer Matrix of Solenoid
Symmetry for both x and y plane.
Focusing
Rotation
When
x,y plane exchange
One solenoid is not sufficient to provide required strengths for both focusing and coupling!
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

9. Periodic Property of The Coupling
Simulation using the coupled beam as the initial beam.
The ion beam transmits only through the solenoid lens, but not the analyzing magnet.
 the beam emittance values are somewhat larger.
-180A
-2870Gs
450A
7160Gs
Exchange of transverse emittances
The coupled beam from ECR ion source is decoupled periodically by the solenoid lens!
Brs= Tm
Sol-L=0.15 m
450 A+180 A ~1T*0.15 m
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

10. Layout of LEBT of SFC (Spiral Focusing Cyclotron)
Solenoids
Deficiency: 3 focusing solenoids used, but coupling effects are not considered!
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

11. Possible Solution to Overcome the Deficiency
For 1st solenoid section,
S1  S1 + F + D, or
S1 
to decouple the phase space and provide required focusing
For 2nd and 3rd solenoid sections, make the solenoids with different field direction but similar focusing strength
SBBSB 
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

12. Test Experiment for Upgrading of the LEBT
The first S section:
The strength is larger than required for decoupling
The rear part is modified:
Without consideration of SFC efficiency, LEBT efficiency can easily reach ~100%
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

13. Efficiency Increase of SFC
Overall efficiency of SFC is almost doubled, and for LEBT increased by ~20%.
After analyzing
Before injection
Extracted
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

14. Space Charge Induced Emittance Increasingz x
Is not for beam with skewed ellipse cross section!
For a symmetry “round” beam, space charge effect can be taken as defocusing lens, will keep emittance!
S.Y. Lee
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

15. Space Charge Induced Emittance Increasingex ey
Transfer of high intensity beam with coupled phase space will increase the emittances
The space charge effect is strong before analyzing at LEBT
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

16. Other Information about Efficiency of SFC
In addition to the solenoids mentioned above, the final solenoid-train and the spiral inflector before injection into SFC will be considered later
The voltage of pre-buncher is insufficient, it’s increase will be beneficial to efficiency of SFC.
I (emA)
Voltage of pre-buncher (V)
r300mm
r700mm
extraction
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

17. The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China
Conclusion
The beam with coupled transversal phase space and solenoid focusing makes transfer study complicated
To fully utilize the powerful SECRAL, careful and deep study of LEBT and matching to SFC is necessary
The modification of LEBT is easy and will contribute to dramatic increasing of SFC efficiency
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China

18. Thanks for Your Attention!
The 12th Symposium on Accelerator Physics, Yuzhong, Gansu, China