1. Klystron Cluster System Development
.…………………….
Klystron Cluster System Development
Christopher Nantista
SLAC
LCWS11
Granada, Spain
September 27, 2011

2. Klystron Cluster Scheme
Main linac rf power is produced in surface buildings and brought down to and along the tunnel in low-loss circular waveguide.
Many modulators and klystrons are “clustered” to minimize surface presence and number of required shafts.
Power from a cluster is combined and then tapped off in equal amounts at 3-cryomodule (RDR rf unit) intervals.
ADVANTAGES
equipment accessible for maintenance
tunnel size smaller than for other options
underground electrical power and heat load greatly reduced
KLYSTRON CLUSTER BUILDING
CTO
ACCELERATOR TUNNEL
2.05 km of linac powered per 2-cluster shaft. 12 shafts total for both linacs.

3. KCS Surface Buildings and Main Waveguide
Building Concepts by Holabird/Root
(from Vic)
Two clusters can be housed in one building (feeding upstream and downstream).
10 MW, 1.6 ms, 1.3 GHz multi-beam klystrons (like RDR) powered by 120 kV (Marx?) modulators.
0.480 m
Main KCS Waveguide
(Aluminum WC1890)
TE01 mode:
evacuated
pressurized
No surface electric field → high power handling capacity
Attenuation falls quickly with radius → low loss achievable (~13%/km)
Extensive experience from NLC pulse compression, power distribution R&D E
= 6.77110-5 m-1 (6061-T6 Al)

4. Combining and Dividing Power
A novel waveguide device was developed for coupling into and out of the circular TE01 mode waveguide without creating large surface fields.
CTO (Coaxial Tap-Off)
Tunnel Cross-Section
(initial concept)
CTO
determines coupling
Couplings ranging from ~1 to 1/33 are required.
D =5m
For combining, the tap-offs are used in reverse.
Proper phase and relative amplitude needed for match (mismatched power goes to circulators).
Replace vacuum windows w/ smaller pressure windows.
Eliminate combiner and add two high-power circulators.
CTO’s of increasing coupling every ~35.3 m
~1/29
~1/28
C = ~1/2
C = ~1 …
bend

5. CTO (Coaxial Tap-Off)
A pair of 3-dB CTO’s.
A CTO connecting WR650 waveguide to 0.48 m-diameter circular waveguide.
Inner view showing wrap-around slots.
launcher
coupler
Shorting one circular port of a 3dB CTO at the proper distance, converts it into a mode launcher (or partial coupler).
for transmission test
for resonant test
Initial cold test of CTO’s back-to-back showed >99% transmission with unoptimized shorting plane (best match ~-28 dB). Transmission shorts have been remachined in anticipation of the immanent “big pipe” transmission test.

6. Four-Channel Bend |Es| |Hs| 0.835 m inspired by: 0.5154 m 34.925 cm
“Quadrupus” power divider m cm
@ 300 MW:
|Es|max = ~3.688 MV/m
|Hs|max = ~ kA/m
C. Nantista ’03

7. Step-Taper Bend |Es| |Hs| TE20 TE01 TE01 inspired by:
@ 300 MW:
|Es|max = ~3.228 MV/m
|Hs|max = ~13.79 kA/m
TE01
TE01
|Hs|
inspired by:
smooth-taper X-band bend
cm × cm cm
1.148 m
V. Dolgashev,
(S. Tantawi,
C. Nantista) ’06

8. Bend Design Comparison
4 Channel Bend
Step-Taper Bend
axis-face distance (Reff) m m
simulated transmission
99.994% %
|Es|max MW)
3.69 MV/m
3.23 MV/m
|Hs|max MW)
10.65 kA/m
13.79 kA/m
maximum parasitic mode
-51.7 dB
-43.1 dB
mismatch (TE01 reflection)
-45.7 dB
-42.6 dB
three assemblies connected w/ custom pressure/rf gaskets
Step Taper Bend chosen based on fabricatability.
Currently in mechanical drawing stage.

9. 1st Local Power Distribution System for NML (PDS1)
VTO’s
Utilizes VTO (Variable Tap-Off) manually adjustable power dividers to customize split among cavity pairs.
Currently in use on Fermilab’s first NML cryomodule.

10. Phase Shifter for Local Power Tailoring
Assembly incorporating
2 phase shifters + 2 hybrids can replace VTO.
Phase shifter range of 0°–90°, the above arrangement allows full range power division.
If 1 and 2 are moved equally in opposite senses, the output phases are unaffected.
Pressurizable U-Bend Trombone Phase Shifter
combine with
Folded Magic-T
cold tests:
transmission: -.03.01 dB reflection: -5036 dB
high-power test:
No breakdown in 8 hours at ~2 MW in 1bar N2.
Mega Industries
BENEFITS:
more compact longitudinally than VTO (for individual cavity feeding)
motorizable for remote adjustment (crucial with cavity degradation)

11. 2nd Local Power Distribution System for NML (PDS2)
Efficiency Test
Linear fit achieved by taking losses of ~0.8% for through power and ~2.45% for extracted power.
Difference largely attributable to isolators loss of ~1.6%.
Fractional Transmitted Power
By moving phase shifters in opposite directions, extracted fraction can be varied over full range without affecting phase.
Two of four 2-feed assemblies have thus far been high-power tested and are ready to be shipped to Fermilab when needed.

12. Fully Adjustable PDS for ILC
For ILC, w/ large gradient limit spread, individual cavity adjustment of the power division can be achieved via the below PDS layout.
Cost and power losses will be increased, however, over pair-wise division due to doubling the number of power division units and including circulators.

13. Local RF Power Distribution Scheme
Power from each CTO is distributed along a 3-CM, 26 cavity rf unit through a local PDS. Distribution is tailored to accommodate gradient limits of cavities.
CTO
adjustable coupler
every ~36 m
feed 1
feed 2
load
hybrid
quad
Original VTO (Variable Tap-Off) Pair-Feeding Concept
manually adjustable by pairs
requires pair sorting
circulators can be eliminated
Alternate Scheme w/ Folded Magic-T’s and Motorized U-Bend Phase Shifters
remotely adjustable by pairs
requires pair sorting
circulators can be eliminated
Folded Magic-T’s and Motorized U-Bend s for Each Cavity
remotely adjustable by cavity
no pair sorting required
circulators required

14. KCS “Big Pipe” Waveguide Tests
0.48 m diameter, pressurized aluminum pipe resonantly powered to ~300 MW TE01 mode field equivalent in 1 ms 5 Hz.

15. Test Assembly Theoretical Attenuation: TE01:
Although KCS is TW, by resonating waveguide in SW mode, we can achieve peak fields equivalent to 300 MW TW with a limited rf source.
Four 8 ft. sections of WC1890 (0.48m diam.) circular waveguide, rolled and welded from 6061 aluminum by Keller, were assembled as follows.
step taper
pumpout section*
CTO
1/4-wave spacer to adjust resonance frequency
shorting hat
WC1890: ” + 2.8” + ~16.7”/2 = ~404.45” = ~ m
WC1375: ” – ~16.7”/2 = ~48.814” = ~1.240 m
Theoretical Attenuation:
= 107 W-1m-1 (6061-T6 Al)
f = 1.3 GHz (k0 = m-1)
ds = (m0pfs)-1/2
Rs = 1/(sds) = m0pf/s =
a = 0.24 m
c0,1 =
TE01:
= 6.77110-5 m-1
a = m:
= 2.40410-4 m-1
* We had originally planned to run under vacuum.

16. Test Power and Coupling (6061-T6 Al)
a(a=0.24m) = 6.77110-5 nepers/m, a(a=0.1746m) = 2.40410-4 nepers/m
10.273m (a=.24m) m (a=.1746m) storage line rnd trp transmission:
field , power → % loss
1.24310-4
9.09110-5
end loss:
round trip loss: % % = %, field atten. factor:
, c
round trip delay time: trt = 2(10.273/ /.593)/c = ns
exp(-trt/Tc0) =
→ Tc0 = ms
→ Q0 = wTc0/2 = 192,100
82.5 MW traveling power → energy density per length ul = 2Pt/vg = J/m, J/m
stored energy: U = .6795J/m10.273m J/m1.240m = J
forward + backward wave
= 2(L1/vg1 + L2/vg2) Pt
= 9.856×10-8J/W Pt
dissipated power: Pd = wU/Q0 = 346 kW
= needed input power to support 82.5 MW resonant power (330MW TW eqiv.), critically coupled w/ theor. Q0.
Now, for the emitted field to cancel the reflected field, we need:
C  82.5MW = (1-C)  346kW
critical coupling: → C = 346/82,846 = = dB
QL = Q0/2 = 96,050 → Tc = 2QL/w = ms
DfrC FWHM = fr/QL = kHz

17. High Power Test History of “Big Pipe”
12 psig
15 psig
18 psig
courtesy of Faya Wang

18. Resonance Measurement of “Big Pipe”
cold test, unpressurized
high power, pressurized
Q circle fit
fr = GHz
QL = 33,787
b =
Q0 = 146,028
τc = 7.37 μs
QL = 30,114
Q0 = 133,712
f = 1300 ~ MHz
→ DfrFWHM = 38.5 kHz,
tc = 2QL/w = 8.27 ms
Measured Q0 is only ~70-76% of theoretical estimate (192,100).

19. High Power RF in Pressurized N2
Could breakdowns in experiment be simple TE01 gas breakdowns? What level of field can we expect to hold off in N2 at 18 psig?
For 300 MW in TE01 mode in WC1890: |Emax| = MV/m
WC1375: |Emax| = MV/m
Paschen Curve*
18 psig = psi = 1,691 Torr
1.3 GHz: ½ period = ns
 deff < cm 4 5
d = cm: 1,691 Torr × cm = 19,498 Torr cm  VB = ~600 kV  |EB|= ~5.2 MV/m
1 cm: |EB|= ~7 MV/m, 1 mm: |EB|= ~10 MV/m
*plot taken from Wikepedia and extended.
 Gas shouldn’t break down until P > ~1GW.

20. “Scorched” Flange Faces
Upon opening the waveguide, we found considerable damage on the inner region of the flange faces.
upper photos provided by Sam Chu
aluminum crumbs
Images and SEM analysis provided by Lisa Laurent

21. opposite flange is flat
Flange Design
The “big pipe” mating flanges are designed with two grooves on one side, one for a vacuum/pressure seal and one for an rf spring gasket.
18.90”
opposite flange is flat
0.275”
rubber O-ring pressure seal
canted coil rf seal

22. Flange Gap Fields
With no longitudinal wall currents, fields of TE01 mode should not extend into a gap between flanges.
However, fields of parasitic modes will.
Gap modes near operating frequency can be strongly excited by even low levels of parasitic modes.
O-ring & rf seal grooves bring flange modes down into range.
TM11
|E|
|H|
TM ” gap mode: GHz
location of observed damage

23. Flange Mode Gap Dependence
E field
½ of rf seal groove
Rubber O-ring
gap
bc: E-plane H-plane
Though they needn’t for TE01, flanges were designed to touch, but for vacuum.
Switch to pressurization could have introduced/widened gaps.
Gap mode resonances are functions of gap width, and at proper spacing, they cross 1.3 GHz.
SOLUTION: Clean and remachine flanges w/ a .015” bevel so as to pre-stress i.d..
Retest at high-power and confirm solution before finalizing 80m pipe order.

24. Retest After Flange Repair/Modification
cold test:
fr = GHz (unpress.)
QL = 33,362
b = 4.043
Q0 = 168,258
high power test:
Q0 15% up over previous assembly, perhaps due to flange fix, but still ~12% below theoretical.
After some initial trouble with processing/ frequency tracking code (new operator), “big pipe” ran many hours without breakdown.
No further flange damage was observed upon opening up after this run.

25. Transmission Test
The next test to be performed next month is a transmission test.
For this, the second CTO is being installed at the other end of the assembly.
Both CTO’s will be shorted with end caps designed for optimal (full) coupling.
On a longer time scale, when the 90 bend is ready, we plan to test it by incorporating it into the “big pipe” assembly in both the resonant and the transmission mode.
Further future R&D plans and proposals will be described on Thursday.

26. Initial Probe Antenna
The “big pipe” field level is derived from resonator characterization and the input and reflected power signals.
As an afterthought, a cluged antenna was inserted into a hole of the vacuum pump-out section, to provide a direct, though uncalibrated, monitoring signal.
It was considered that this would be useful for an interlock or in understanding breakdown events.
It did not work terribly well.
-64 dB coupling seen in cold test.
For 566 kW input, it should see ~225 mW.

27. New Antenna Design
0.500”
A new inductively coupling probe antenna has been designed to directly monitor resonant “big pipe” fields.
It will use spring stock to contact the pipe on one side of a hole ridge, as shown.
It will be incorporated into it’s own 1/4-wave spacer to be inserted between pipe section flanges.
Coupling to standing wave will depend on location. The signal is expected to be 100’s of milliwatts, to be attenuated as necessary.
MDC SMA feedthrough
0.400”
coupling toTE01: dB mode scattering loss: < -40 dB

28. Water Cooling
In order to reduce thermal effects, so as to make it easier for the high-power operation program to track the resonant frequency, water-cooling channels are being epoxied to the “big pipe” sections.