Download PROGRESS TOWARD THE EXTERNAL POWERED HIGH GRADIENT DLA STRUCTURES

PROGRESS TOWARD THE EXTERNAL
POWERED HIGH GRADIENT DLA
STRUCTURES
C. Jing, A. Kanareykin, Euclid Techlabs, LLC
R. Konecny, W. Gai, J. Power, W. Liu, HEP, ANL
S. Gold, A. Kinkead, NRL
S. Tantawi, V. Dolgashev, SLAC
S. Kasakov, KEK
US High Gradient Research Collaboration Workshop, May 24th, 2007
Outline
• Introduction
• Report on the Recent High Power RF
Testing of Quartz Based DLA Structure
• New Design: A Gapless DLA Structure
• New Design: Transverse Modes Damping
• Summary
2
Introduction
3
External Powered Dielectric-Loaded Accelerating
(DLA) Structure
ÆANL, Euclid, NRL, SLAC Collaboration
ÆProgram Goals
ÆDLA Structure Development
ÆDLA Structure High Power Testing
Geometry
Advantages of DLA
Electric Field Vectors
• Simple geometry
• No field enhancements on irises
• High gradient potential
• Comparable shunt impedance
• Easy to damp HOM
4
DLA Structures Tested To Date
Material
Al2O3 g
MgxCa1-xTiO3
SiO2 g
Dielectric constant
9.4
20
3.78
Loss tangent
2x10-4
3x10-4
2x10-5
Inner radius
5 mm
3 mm
8.97 mm
Outer radius
7.19 mm
4.57 mm
12.08 mm
R/Q
6.9 kΩ/m
8.8 kΩ/m
3.6 kΩ/m
Group velocity
0.13c
0.057c
0.38c
RF power for 1MV/m
gradient
80 kW
27 kW
439 kW
Demonstrated
Gradient
8 MV/m
5.7 MV/m
9 MV/m
Principal Problem
Multipactor
Breakdown at joints
Multipactor
g Both non-coated and TiN coated
5
Multipactor
2a
90
80
70
(%)
60
δmax
Pt
:Copper
δ=1
structure
attenuation
S21
e1
multipactor
loading
50
40
30
20
10
0
0.001
S11
0.01
0.1
Pinc (MW)
Å Multiplication Region Æ
1
10
Ki
e2
Incident Power (MW)
Light Intensity (arb. units)
2b
:Alumina
100
SEE Yield Curve
Multipactor Power (MW)
Pi
Pr
54 mm
200 mm
δ
54 mm
6
Dielectric joint breakdown
Port I
Port II
20 μm vacuum gap
‰ Dielectric joint breakdown
due to the local E-field
enhancement at the vacuum
gap.
‰ >100MV/m E-field is
expected in the gap ( ε=20,
Eacc=5.7MV/m).
7
The Recent High Power Testing of
Quartz Based DLA Structure @
ASTA Facility, SLAC (winter,
2006) & Magnicon Facility, NRL
(spring, 2007)
8
Quartz based DLA structure
High power testing setup
@ SLAC
Copper
Quartz
9
High power testing results (I)
‰ No signature of the bulk dielectric breakdown up to 36MW,
20ns rf input, which is equivalent to 9MV/m of the initial
gradient on axis.
‰ Multipactor was observed (with a saturation stage).
ΔS21 ~ 5%
10
High power testing results (III)
Multipactor with different pulse lengths
Transmission_400ns
Transmission_300ns
Transmission_200ns
Transmission_100ns
Reflection_400ns
Reflection_300ns
Reflection_200ns
Reflection_100ns
80
70
60
%
50
40
30
20
10
0
0
1000
2000
3000
4000
5000
Pinc (KW)
11
Suppressing multipactor
‰ TiN Coating with ALD technique (more evenly coatingÆ
good multipactor suppressing as expected.)
‰ More experiment to test its capability of handling high
power rf.
No light
Bright light
1.3nm coating
12
Summary of High power testing on Quartz
DLA structure
‰ No signature of the bulk dielectric breakdown
up to 36MW (Eqv. Eacc=9MV/m), 20ns rf input.
‰ Multipactor can be suppressed by coating.
‰ More experiments are needed to test if the
coating can survive for higher rf power (what’s
the optimal coating thickness?)
13
New Designs
14
New Design (gapless structure)
Gapless DLA structure based on coaxial type rf coupler*.
Coaxial type rf coupling scheme:
•No taper matching section
•No gaps
•Shorter rf coupling section
•Better hybrid mode suppression
* Supported by DoE SBIR Phase I
15
New Design (gapless structure)
The highest electric field in the new coupler
appears at the inner conductor tip. For 20 MW
rf input, the electric field at the tip will reach 24
MV/m, much less than the copper breakdown
voltage threshold.
16
New Design (gapless structure)
•Alumina based gapless DLA has been fabricated.
•Bench measurements have been done; a high power test has
been scheduled at NRL.
•MCT-20 based gapless DLA structure has been simulated.
17
New Design (gapless structure)
S21=-0.6dB, S11=-18dB @11.43GHz
18
New Design (gapless structure)
E-field profile
@11.424GHz
Bead pull
Pulsed rf
bench test
19
New Design (hybrid mode damping structure)
Conventional Quartz DLA
HEM11 mode Ez
TM01 mode Ez
10.18GHz
11.424GHz
20
New Design (hybrid mode damping structure)
Surface current
TM01 mode (axial current only)
HEM11 mode (azimuthal current included)
21
New Design (hybrid mode damping structure)
Hybrid mode damping DLA
SiC(εr ~13; tanδ ~0.22 @ 11GHz)
Copper
Dielectrics
Vacuum
22
New Design (hybrid mode damping structure)
E_TM01(0,1mm,z)
E field (TM01 mode)
E_HEM11(0,1mm,z)
E-field (HEM11 mode)
23
New Design (hybrid mode damping structure)
X-band DLA
structures
Accelerating Mode
(TM01 mode)
Freq
(GHz)
Parasitic Mode
(HEM11 mode)
Q
Q
(conventio
nal DLA)
(damping
DLA)
Freq
(GHz)
Q
Q
(conventio
nal DLA)
(damping
DLA)
Quartz based (εr
=3.8; ID=17.9mm;
OD=24.16mm)
11.424
9825*
7812*
10.18
10877*
49*
MCT-20 based
(εr =20; ID=6mm;
OD=9.13mm)
11.424
2864*
2440*
9.915
2546*
100*
*SiC (εr ~13; tanδ ~0.22 @ 11GHz) used in the calculation of the damping DLA structures; dielectric losses
of the loaded materials are not included in the Q calculation; Slots / circumference = 22%.
24
Summary
• Investigating characteristics of multipactor.
• Studying multipactor suppressing technique.
• Understood the mechanism of the dielectric
joint breakdown.
• Developed a gapless DLA structure, and bench
tested.
• Designed hybrid mode damping DLA structure.
• More High power rf tests are planned.
25
Experimental Studies of Dielectric Loaded Accelerator
(1948-1958)
Major
Research
Group
Scientists
Atomic Energy R.B. R.ShersbyResearch
Establishment Harvie, etc.
(AERE), UK
References
Research Scope
1. R.B. R.-ShersbyHarvie, Nature, 162
(1948) 890.
2. R.B. R.-ShersbyHarvie, L. Mullett, and
W. Walkingshow, Proc.
I.E.E. B., 104 (1957)
273.
Constructed and Tested a 4MeV
Traveling wave Dielectric Disk Loaded
Accelerator.
Beam Acceleration.
Discovered Multipactor in DLA.
Queen Mary
College, Uni.
of London, UK
G.B.
Walker,
etc.
1. G.B.Walker and
N.D.West, Proc. I.E.E.
C., 104 (1957) 381.
2. G.B.Walker and
E.L.Lewis, Nature, 181
(1958) 38.
Constructed and Tested Dielectric Disk
Loaded Wavguide Cavities.
Tested Dielectric Breakdown.
IIT, USA
G.I. Cohn
and G.T.
Flesher
1. G.T.Flesher and
G.I.Cohn, A.I.E.E. C.,
70 (1951) 887.
2. G.I.Cohn and
G.T.Flesher, IIT Report,
2 (1952).
Constructed and Tested a Quartz Tube
Based Dielectric Loaded Accelerator.
26
Major Achievements from the Group at AERE, UK
Coaxial type
coupler!
Structure: 3GHz,TiO2,
2.6MV/m per 1MW at v=c
9Complete theoretical study of the Disk DLA.
9Beam accelerated from 45KeV,0.4c to 1.48MeV, 0.966c,
with rf: 1MW, 2μs, 50p/s.
9Observed anomalous power loss; identified it’s multipactor;
varying pulse repetition rate from 50 to 500Hz, No ATTN
change being observed (rf Heating Independent); varying
vacuum from 1~2×10-5 to backing pressure, No ATTN
change being observed (Pressure Independent);
9Multipactor cured by degreasing structure and replacing oil
pump with mercury pump (?).
27
Major Achievements from the Group at Queen Mary College, UK
Vacuum,10-5 Torr
Ceramic,TiO2, ε=90, tgδ=3×10-4
λ/2
Dielectric Disk Loaded Cavity,
Powered by 2MW,3GHz, Magnetron.
9 Observed dielectric breakdown at
Ez≤10MV/m.
9Outer edge of dielectric disk being
silvered, but on influence on breakdown
voltage.
9Coating with a layer of glaze (green
resistor enamel consisting mainly of
lead borate, a few thousandths of inch),
NO BREAKDOWN at 30MV/m
(available power source at that time).
28
Major Achievements from the Group at IIT, US
9Complete theoretical study of the Tube DLA.
9Built a Quartz based DLA
Unfortunately, NO Further References Available.
29
30
Faint Light (can be
conditioned away)
Bright Light
31