1. Update on SLAC experiments with High Gradient Accelerators and RF Components
M.Franzi, V.A. Dolgashev, S. Tantawi
June 6, 2016

2. RF polarizer and new SLED cavity commissioned at LCLS
Outline
RF polarizer and new SLED cavity commissioned at LCLS
Breakdown studies
Cryogenic temperature X and S-band experiments
Full structure experiments
mm-wave accelerating structures
100 GHz wakefield accelerator (E204 at FACET)
110 GHz standing wave accelerator cavity (MIT/SLAC)

3. Motivation for new SLED design
RF polarizer and new SLED cavity
Motivation:
Replace SLED-I device with a single compact resonator
Commission new SLED on X-band deflector at LCLS

4. High power RF polarizer for use in new SLED cavity at SLAC
RF polarizer and single High-Q spherical resonator for new X-band SLED system :
Input TE10 at WR 90 input waveguide
Equal coupling to the TE10 and TE20 modes
Isolation of the opposite port
Each rectangular waveguide mode excite a TE11 mode
Excitation in quadrature produces quasi circularly polarized wave
TE11 modes excite degenerate TE114 modes of spherical resonator
TE20 and TE10 emitted/reflected from the cavity cancel at input port

5. Formulation of Scattering Matrix for RF Polarizer
The Polarizer RF network is broken down into 4 sub-networks.
Two 3-port networks (n=1,2)
A shorted tuning stub
𝑠 𝑠ℎ𝑜𝑟𝑡 = 𝑒 −2𝑖 𝜓 𝑒 −2𝑖 𝜓 2
A single 4 port matched input network
𝑆 𝑖𝑛𝑝𝑢𝑡 = 𝑒 𝑖 𝜓 𝑎 𝑒 𝑖 𝜓 𝑏 𝑒 𝑖 𝜓 𝑎 𝑒 𝑖 (𝜓 𝑏 +𝜋) 𝑒 𝑖 𝜓 𝑎 𝑒 𝑖 𝜓 𝑏 𝑒 𝑖 𝜓 𝑎 𝑒 𝑖( 𝜓 𝑏 +𝜋)
Cascaded scattering matrix defines viable ranges for subnetwork parameters
𝑆 𝑛 = − cos 𝜃 𝑛 − 𝑒 −𝑖 𝜙 𝑛 −cos 𝜃 𝑛 + 𝑒 −𝑖 𝜙 𝑛 sin 𝜃 𝑛 −cos 𝜃 𝑛 − 𝑒 −𝑖 𝜙 𝑛 −cos 𝜃 𝑛 − 𝑒 −𝑖 𝜙 𝑛 sin 𝜃 𝑛 sin 𝜃 𝑛 sin 𝜃 𝑛 cos⁡( 𝜃 𝑛 )
TE10
TE20
Perfect-H Boundary
Perfect-E
Boundary
Port-3
Mode-1
Port-2
Port-1
Mode-2
Operating point of each mode in final design

6. Testing of the RF Polarizer and SLED Assembly
Cold Test: Polarizer
Cold Test: Polarizer with SLED
High Power Test
-45 dB match between port:1 and port:2
TE114 @
TE014 @
Input power 35 MW
Peak power 207 MW
M. Franzi, J. Wang, V. Dolgashev, S. Tantawi et al., Physical Review Accelerators and Beams Accepted (June-2016).

7. XTCAV with SLED @ 85 MV (4-12-2016)
Commissioning of SLED at LCLS and doubled timing resolution of the X-band deflector (XTCAV)
XTCAV no 44 MV
( )
Bunch Charge: 175 pC
Bunch Energy: GeV
X-ray energy: mJ
XTCAV with 85 MV ( )
Bunch Charge: 186 pC
Bunch Energy: GeV
X-ray energy: mJ
Timothy Maxwell, Yountao Ding, Valery Dolgasev, Juwen Wang

8. Breakdown studies of cryogenic temperature X and S-band experiments
Motivation for breakdown studies of copper structures at cryogenic temperatures
Breakdown studies of cryogenic temperature X and S-band experiments
Motivation:
Characterize accelerating gradient and breakdown rate of single cell cold copper accelerating cavities

9. Testing of normal conducting X-band (3-Cell) structure in ASTA at cryogenic tempertures
For low breakdown rates gradients and peak electric fields significantly larger than at room temperature.
V.A. Dolgashev, A. Cahill

10. Cryogenic normal conducting copper tests: understanding Q0 at low and high power
Measured total Q of 8,150, and coupling β=1.5. Q0=20,500
According to Cold test should be Q0=36,075
Measurements made with 400 ns 500 kW pulses.
Forward
Reconstructed Reflected
Reflected
Reflected
Reconstructed Reflected
V.A. Dolgashev, A. Cahill

11. Reproduction of low power CW measurements with sub-MW Klystron Pulses
Performed sweep of input power to characterize influence of pulse heating.
Scan klystron power from kW
Measured total Q remains unchanged (~8150)
Performed frequency scans from GHz to GHz to measure S-parameters
Reflected power measured using input pulse 800 ns at 7 K.
Frequencies match network analyzer data, but coupling does not.
In process of building precise model of whole circuit
Down Mixers
PPM data
Cold Test
PPM data
V.A. Dolgashev, A. Cahill

12. Breakdown properties S-Band normal conducting copper structures at cryogenic temperatures
Now expanding our work into S-Band
Conceptual design of RF circuit is completed
Fabrication anticipated to begin Fall 2016
SUPERFISH simulation of “single” cell accelerating mode
HFSS simulation of Rect. WG to Cyl. WG quadrupole-free mode launcher. Fields for 50 MW input rf power.
V.A. Dolgashev, A. Cahill

13. Breakdown studies of room temperature full accelerating structures
Motivation for breakdown studies of copper structures at cryogenic temperatures
Breakdown studies of room temperature full accelerating structures
Motivation:
Characterize conditioning of CLIC TD 24 accelerating structure using real time controller
Demonstrate high gradient operation of new Distributed Coupling accelerating structure

14. Testing of Traveling Wave X-Band Structure: CLIC TD24
CERN-SLAC
Gradient (MV/m)
# of BD (x10)
Power (MW)
ΔT (C)
Pulse Length (10 ns)
Scaled Gradient (E0 τ(1/6) BDR(-1/30) (250ns : 10E-7 bpp/m))
Gradient (MV/m)
PXI Controller
Based on xbox-II architecture
CLIC Accelerating Structure TD 24
Installed at ASTA
M.Franzi, V. Dolgashev

15. Distributed Coupling Accelerator Enables Doubling RF to Beam Efficiency and Ultra-High-Gradient Operation.
Optimize individual cell shape for maximum gradient and shunt impedance without cell-to-cell coupling constraint (Patent filed by Stanford)
Requires only 66 MW/m for 100 MV/m gradient compared to 200 MW/m for a typical X-band structure
2016
2015
2017
High power RF testing in ASTA
Breakdown and conditioning studies
Structure moved to NLCTA
High power tests with beam
Wakefield characterization
Applications
Waste-water treatment accelerator
Incorporate into an energy recovery system.
Fold into future TeV collider
S. Tantawi

16. Distributed coupling standing wave accelerating structure
-S11(dB)
Field amplitude (a.u.)
Phase (degrees)
-30
11.3
Frequency (GHz)
11.41
Position (cm)
Position (cm)
Reflected signal from accelerating structure has one single resonance instead of 20 individual resonances
S.Tantawi

17. S. Tantawi, C. Limborg, A. Cahill, M. Nasr
Split Structures accelerates beam and operates at high demonstrate high gradient while demonstrating the predicted shunt impedance
87.5MeV
85.6MeV
89.4MeV
26 cm structure installed at XTA
The structure added 24 MeV energy gain by beam
16.5 MW of input power with 300 ns pulse length
filling time is 140 ns (expected)
The structure is being processed at XTA to go beyond 120 MV/m
S. Tantawi, C. Limborg, A. Cahill, M. Nasr

18. Goals of the E204 experiment
mm-wave accelerating structures
Motivation:
Determine statistical properties of rf breakdown in metal structures, accelerating gradient and pulse length at 100 GHz and 200 GHz frequencies

19. Beam excited mm-wave structure - list of experiments
Solid model of the 100 GHz accelerating structure
Experimental setup with copper accelerating structure
TOP HALF
1 mm
Electron beam
Regular cell
BOTTOM HALF
Coupler iris #
Date
Type
Material
Frequency
Charge
Bunch Length 1
Jun
Standing wave Cu
100 GHz
2.7 nC
50 um 2
Mar
Travelling wave
3.2 nC 3
May
S. Steel
3 nC 4
Mar 5
Apr 25,
200 GHz 6
May
Cu-Ag
1.6 nC
25 um 7
Nov
Nov =
Nov
M. Dal Forno, et al., Physical Review Accelerators and Beams 19.1 (2016):
M. Dal Forno, et al., Physical Review Accelerators and Beams 19.5 (2016):

20. Results from the experiments at 100 GHz
Achieved gradients and pulse length: #
Eacc
Emax, (fund. Mode)
Emax, (all. Modes) τp
latt
[GV/m]
[ns]
[mm] 1
0.1
0.35
2.45 91 2
0.3
0.64
1.5
2.38 3
0.19
0.52
0.33
3.5
Standing wave copper
f = 100 GHz, q = 2.7 nC, σz = 50 um
Travelling wave copper
f = 100 GHz, q = 3.2 nC, σz = 50 um
Travelling wave S. Steel
f = 100 GHz, q = 3 nC, σz = 50 um 1 2 3
4,7
2012
2014
2015
On next slide
Low-transmission coupler ( -30 dB)
Filter external interference.
No RF breakdowns detected
Used crystal and pyro detector.
Installed motorized stages
Change the gap of the structure.
Matched coupler to improve SNR
Isolated each half of the structure
Used current monitor to detect breakdown
Automated scans of motorized stages
EPICS control

21. Results from the experiments at 100 GHz (2015)
M. Dal Forno, et al., Physical Review Accelerators and Beams 19.5 (2016):
Experiment # 4 : Travelling wave Cu, f = 100 GHz, q = 3.2 nC, σz = 50 um Experiment # 7: Travelling wave Cu-Ag, f = 100 GHz, q = 3.2 nC, σz = 25 um
Centroid MAX deflection: ~0.3 mm
Simulated with
CST studio
Measured
Measured the deflecting voltage by observing the displacement of the 20 GeV FACET electron beam on a diagnostic screen.
Confirmation of simulated values of the deflecting and accelerating gradients.
Gap f τp nC
Eacc 1 mode
@2.7 nC
Emax 1 mode
[mm]
[GHz]
[ns]
[MW]
[MV/m]
[GV/m]
0.3
136.27
2.36
0.77
224
0.51
0.5
130.3
2.37
2.48
169
0.47
0.7
126.01
4.03
4.23
127
0.43
0.9
122.66
2.35
5.47 97
0.41
1.1
119.93
1.57
6.42 76
0.32
1.3
117.59
1.15
6.85 59
0.30
200 GHz results will be in V. Dolgashev presentation…

22. Planned high power (110 GHz) structure testing at MIT
Demonstrate realizable mm-wave accelerating structure
Power with stand-alone RF source MIT)
Direct comparison with breakdown studies at x-band
Single Cell: Standing Wave Accelerating Structure
Iris Braze Test in Progress
Full RF Structure
110 GHz Gyrotron Pulse
Accelerating Gradient
E. Nanni, V. Dolgashev, J. Neilson

23. E. Nanni, V. Dolgashev, J. Neilson
Planned high power (110 GHz) structure testing at MIT
Large diameter window to reduce surface power density
Quasi-optical launcher converts guassian mode to TE01 feeding the cavity
W-band output coupler with crystal detector for power diagnostics
E. Nanni, V. Dolgashev, J. Neilson

24. Conclusion SLAC has numerous ongoing experiments: New SLED embodiment
Single spherical resonator with RF polarizer
Successful implementation at LCLS
Breakdown studies:
Cryogenic temperature single cell cavities
In progress of characterizing full RF circuit
Full accelerating structure
Conditioning of CLIC structure is ongoing
Distributed coupling accelerator is being tested with beam at XTA
mm-wave structure testing
Wakefield accelerator
First series of mm-wave breakdown experiments
Standing wave single cell accelerating structure
Conceptual design completed