1. Demonstration of Complete Multipactor Suppression in Externally Powered Dielectric Loaded Accelerators
Joint efforts from Euclid (SBIR grant DE-SC ), AWA, NRL, SLAC.
Chunguang Jing
Euclid Techlabs LLC, / AWA, Argonne
Steve Gold
Naval Research Laboratory (retired)
AAC2016, Aug, 2016

2. Electric Field Vectors
External Powered Dielectric-Loaded Accelerating (DLA) Structure
Euclid, ANL, NRL, SLAC Collaboration
Program Goals: Demonstration of high gradient, practical dielectric accelerators
Geometry
Advantages
Simple geometry
No field enhancements on irises
High gradient potential
Comparable shunt impedance
Easy to damp HOM
Electric Field Vectors
Major concerns
Multipactor
Breakdown
External rf powered or beam wakefield driven

3. Multipactor in Dielectric Accelerators
SEE Yield Curve
dmax d
d = 1
 Multiplication Region 
structure
attenuation e1 e2 Ki
S21
multipactor
loading
Light Intensity (arb. units)
Multipactor Power (MW)
S11
Incident Power (MW)
J. G. Power, et al, Phys. Rev. Lett. 92, (2004).

4. Multipactor Mitigation by TiN Coating
25% improvement
1.3nm ALD coating
C. Jing, et al., IEEE Trans. Plasma Sci. 38(6), 1354–1360 (2010).

5. Multipactor Suppression by surface grooves
4 different grooved DLA Structures were fabricated.
Unfortunately no high power test being performed.
S. Antipov, et al, Proc. PAC2011, New York, NY, USA, pp

6. Multipactor Suppression by Solenoid field
Principle: the introduced Bz can effectively alter the transit time  of secondary electrons. A proper strength of Bz makes  in the range of (T/2,T) so that Er is always pushing electrons back to the dielectric surface, leading to a very small impact energy, then SEE<1.
0.2T~0.6T for Xband
C. Chang, J. Verboncoeur, S. Tantawi, and C. Jing, J. Appl. Phys. 110, (2011).

7. Impact energy and particle numbers for different strength of the solenoid field
=0.25
=0.5
=
=2

8. The First Test (Dec.2012) Limited solenoid strength  limited success
C. Jing, et al, Appl. Phys. Lett. 103, (2013).

9. The Latest Test(Oct.2015) 1T solenoid Standing wave DLA
RF probe to monitor the field
Rf probe
C. Jing, S. H. Gold, Richard Fischer, and W. Gai, Appl. Phys. Lett. 108, (2016).

10. Experimental Setup

11. Results(I)
Bifurcation
Enhancement
Mitigation
Blocking

12. Results(II)
Prf ~42kW
Bz ~2.4KG
Bz ~9.4KG
Bz ~0KG
Prf ~207kW
Time (ns)

13. Simple circuit model for a SW DLA w/ multipactor:
est. electron energy ~1keV ave. if assuming all missing power absorbed by electrons
Turn-on time~15ns

14. Simple circuit model for a SW DLA w/ multipactor:
est. electron energy ~9keV ave. if assuming all missing power absorbed by electrons
Turn-on time~6ns

15. Long pulse length (FWHM~400ns)

16. Summary
Successfully demonstrate the complete multipactor suppression with an external solenoid field.
Theoretical studies are needed to improve the understanding of the experimental results (like accel. gradient, freq. dependences, etc).
Studies of the combination of different techniques are needed to reduce the requirement of solenoid strength.