Power Distribution System (PDS) Design and R&D Status

Christopher NantistaSLAC

SCRF Meeting @ FermilabApril 24, 2008

BCD: linear

ACD: semi-branched w/ VTO’s & no circulators?

•Fewer types of hybrids/splitters (3 vs. 9)

•Power division adjustable by pairs

•Elimination of circulators possible

Basic Distribution Scheme

Variable Tap-Off (VTO) Design

C = Pc / Pi =1/2 sin-1C

0 0.00°

1/4 15.00°

1/3 17.63°

1/2 22.50°

1 45.00°

length = 64.924”

machined aluminum; dip-brazed

rotatable flanges Coupling is a function of center rotation angle .

load

feed

mode rotator

010

001

100

010

CC

CC

CC

CC

S1

2 3

4

S21 = -3.11 dB (48.85%) - extractedS31 = -2.95 dB (50.70%) - through

S21 = -0.019 dB (99.55%) - extractedS31 = -32.54 dB (0.056%) - through

VTO Cold Test Results

Loss: 0.446% (-0.0194 dB)normalized coupling: -3.0917 dB(estimated ideal: ~-3.050 dB)

Loss: 0.390% (-0.0170 dB)

~45°

~22.5°

89.46°-3.0232 dB

-3.0221 dB

Loss: 0.277%

-45.5 dB-48.2 dB

-44.1 dB-42.8 dB-42.5 dB-41.0 dB

3 dB HybridTypical cold test measurements:

20.1

37”

1

2 3

4

HFSS model

dip brazed aluminum

0 50 100 150 200 250 300 350 4000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Time: miutes

Po

we

r in

to H

yb

rid

: M

W

Modulator tripped off

Hybrid has been tested at 4MW for few hours at the pressure of 0psig. No

breakdown during the test.

VTO -75Degrees test Jul-12-2007

The width of RF is 1ms and the repeat frequency is 1Hz

0 5 10 15 20 25 30 350

1

2

3

4

5

6

Time: minutes

MW

Average power of klystronPeak power of klystron

High Power Tests

VTO Hybrid

Faya Wang

Isolator (Circulator w/ Load)

Loss: ~1.6%

S.P.A. Ferrite Ltd.St. Petersburg, Russia

Typical cold test measurements:

1MW Load

Typical load match < -35 dB

104.9° over 25mm

Mean loss over range: ~1.22%.

Phase Shifter

S.P.A. Ferrite Ltd.St. Petersburg, Russia

DESY design

motorized moving side wall

Typical cold test measurements:

12

34 circulator

load

load

VTO

hybrid

window

phase shifter

turned for visibility

RF PDS Module Cold Test

The first 2 (of 4) 2-cavity modules of our RF power distribution system for Fermilab’s first NML cryomodule are assembled. The first is cold tested and ready for high-power testing. It incorporates:

SLAC VTO and hybridIbfm window (for pressurization of high-power volume)S.P.A. Ferrite isolators and loadsMega bends and flex guides (and dir. cplrs. while awaiting S.P.A. pieces)

C. Nantista

S31 = -6.318 dB (23.35%)

S41 = -6.276 dB (23.57%) S21 = -2.948 dB (50.72%) S11 = -43.0 dB (0.005%)

Bends: 0.41%VTO: 0.446%Window: 0.4930.088% = 0.043%Hybrid: 0.4930.42% = 0.207%Circulators: 0.4931.78% = 0.878%Phase shifters: 0.4930.55% = 0.271%Flex guides: 0.62% + 0.4930.62% = 0.926%

~3.18%

Phases of S31 and S41 initially within 1.7° of each other (adjustable with phase shifter).

Module through phase error = ~-6.7° (easily absorbed in next modules phase shifters).

Feed spacing measures ~1.3827m, compared to 1.3837m coupler spacing.

Module length measures ~2.7674m, exact to measurement resolution.

VTO set for 2nd to last cavity pair (~3 dB).COLD TEST RESULTS:

2.36% of power missing (-0.104 dB)

Pair power division equal to within 1%.

Slightly more than ½ power sent through to allow for downstream losses.

Expect roughly:

C. Nantista

POWER

PHASE

SPACING

The safety administrative threshold for human exposure to our pulsed rf flux is 1.08 kW/m2 (see IEEE C95.1 2005 Table8, p. 24 and Note f, p. 26).

To avoid human exposure to unacceptable RF field levels during monitoring and the difficulty of measuring pulsed flux, we will test waveguide systems at low power, using a CW signal generator and a loop antenna prior to running with klystron power.

With our 1” diameter loop antenna, 1.08 kW/m2 should read 15.4 dBm on the spectrum analyzer. With a 15 dBm signal generator replacing rf power of up to 5 MW (an 82 dB difference) we need to set our threshold at ~-67 dBm.

With the uncertainties in our calibration, let us add a safety margin of 20 dB and try to keep leakage below ~-87 dBm.

Of course, in addition to being swept around the sytem, the loop antenna must be turned and twisted in different orientations to assure coupling to any radiated fields.

RF Leak Checking

50

D

d

50

La

Voc

Rl

Rr

)W/m(V 04015.0 cos424 22/10

2/

0

2200 SSdxxkxDk

ZZ

ZV

D

aL

LL

~0.108 (dominated by loop inductance)

beam

RF

Alternative RF Distribution Layout

with circulators:

without circulators:

VTO’s allow pair-wise adjustment of power distribution.

Hybrid feeding of equal-Q cavity pairs directs reflected power into hybrid loads.

RF

beam

loads

hybrid

VTO

circulators

flex guide load

directional couplers

H-plane bends

No CirculatorsWill elimination of circulators result in inter-cavity coupling problems due to insufficient cavity isolation?

* Justin Keung of University of Pennsylvania obtained similar results with his Penn Virtual Cavity (PVC) program.

Typical L-band Hybrid port isolation measurements:

-42–48 dB

To get a feel for the effect of coupling alone, assume a pair of identical cavities and a lossless, symmetric coupling network with equal coupling but imperfect port isolation. Let be the phase length from the hybrid ports to the cavities (with S23 set to 0). A MATLAB program was used to integrate the coupled differential equations for the fields with various |S23| amplitudes and values, producing the following results*.

0 0.5 1 1.5

0

5

10

15

20

25

30

35

Time (ms)

Acc

eler

atin

g G

radi

ent

(MV

/m)

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.530

30.2

30.4

30.6

30.8

31

31.2

31.4

31.6

31.8

32

Time (ms)

Acc

eler

atin

g (b

lue)

and

On-

Cre

st (

mag

enta

) G

radi

ents

(M

V/m

)

cavity field profile

333

222

1

1

EiEEdt

dE

EEEdt

dE

ebe

e

ebe

e

332

23224223

2233

222

23334223

2232

1

1

1

1

EiEEeSEEEeS

eS

dt

dE

EEEeSEiEEeS

eS

dt

dE

ebei

ebe

i

i

ebei

ebe

i

i

Emitted Fields

Incident Fields

0/121

2QQL

accelerating gradient (with 5° beam phase)

cavity gradient

No Coupling (S23=0):

Coupled Field Equations

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50.98

0.985

0.99

0.995

1

1.005

Time (ms)

Nor

mal

ized

Gra

dien

ts in

Cav

ity P

air

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50.9

0.92

0.94

0.96

0.98

1

1.02

1.04

Time (ms)

Nor

mal

ized

Gra

dien

ts in

Cav

ity P

air

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50.993

0.994

0.995

0.996

0.997

0.998

0.999

1

1.001

1.002

Time (ms)

Nor

mal

ized

Acc

eler

atio

n T

hrou

gh P

air

=/4

|S23| = 0: black

-40 dB: blue

-30 dB: cyan

-20 dB: green

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

0.986

0.988

0.99

0.992

0.994

0.996

0.998

1

1.002

Time (ms)

Nor

mal

ized

Acc

eler

atio

n T

hrou

gh P

air

=0

|S23| = 0: black

-40 dB: blue & red

-30 dB: cyan & magenta

-20 dB: green & yellow

|S23| = 0: black

-40 dB: blue

-30 dB: cyan

-20 dB: green|S23| = 0: black

-40 dB: blue & red

-30 dB: cyan & magenta

-20 dB: green & yellow

C. Nantista

Simulation of Cavity Pair Coupling Through Hybrid w/o Circulators

Typical measured isolation: -42—48 dB → gradient variation << 0.1%

Individual Cavity Gradients Net Acceleration

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

-6

-4

-2

0

2

4

6

time (ms)

Pha

se E

rror

of

Cav

ity F

ield

s (d

egre

es)

=

|S23| = 0: black

-40 dB: blue & red

-30 dB: cyan & magenta

-20 dB: green & yellow

For = /4 and 3/4, the cavity field phases are flat, and for /2, they’re reversed.

Cavity Phase:

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50.9

0.92

0.94

0.96

0.98

1

1.02

1.04

Time (ms)

Nor

mal

ized

Gra

dien

ts in

Cav

ity P

air

=2.5°

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50.9

0.92

0.94

0.96

0.98

1

1.02

1.04

Time (ms)

Nor

mal

ized

Gra

dien

ts in

Cav

ity P

air

=/4 + 2.5°

-40 dB, best phase (47.5°): Mean acceleration = 0.999949, Spread = 8.7610-6

-40 dB, worst phase (2.5°): Mean acceleration = 0.999911, Spread = 6.4710-5

Ee (extracted)

Et (transmitted)

010

001

100

010

2

1

i

i

i

i

S

1 2

34

1

2

11

ieE

2

2

12

ieE

Tailoring Power Distribution With Spacers and 3 dB Hybrids

42cos

2

1

22

42cos

2

122

1

22

21422/21

12422/2/2/2/2/2/

2/2/21

21

21

21

1212

121

12

1

21

iii

t

iii

ii

ii

iie

eeeEE

iE

eeeee

ee

eeE

iE

E

If 1 and 2 are changed in opposite senses by half the desired , the coupled and through phases are unaffected as the amplitudes are adjusted.

3 dB

equal amplitude inputs

20.137”

52.205”

Flange spacers: T0 ± T, T: -0.793” — 0.793” (T = 0.207”—1.793”)

replaces VTO

terminated fourth port absorbs some reflected power, avoids resonances

All hybrids -3dB.

Alternative to VTO

L/4 (2 U-bends)

± 0.7928”

± 0.2643”

± 0.1715”

± 0.000”

- ± 0.7928”

42

sin42

cos ,42

cos 22

2

2

2

i

t

i

e

E

ET

E

EC

"1712.32/,1

"00,2/1

"6860.0471.19,3/1

"0572.1/6,4/1

"1712.32/,0

L

L

L

L

L

C

.00881”/degree in thickness of spacers

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

DeltaT (DeltaL/4) (inch)

Cou

plin

g

Nominal Set (2 each)

1.7928” 0.2072”

1.2643” 0.7357”

1.1715” 0.8285”

1.0000” 1.0000”

T = 1.000” ± L/4

Adjust for system losses and for specific desired relative power levels.

Insert between flanges and connect with single set of long bolts or threaded rods. (Extra gasket required if gaskets used)

Configuration With Cavity Pair Power TailoringRequires circulators, 7 different hybrids, and 7 different waveguide connections.

Requires circulators, 8 3dB hybrids, and 8 waveguide T’s.

Requires 8 3dB hybrids, 4 waveguide T’s, and pairing of like cavities.

Configuration With Fixed Cavity Power (BCD)

Configuration With Individual Cavity Power Tailoring

Conclusions & Plans

•We have all parts for the Fermilab NML RF distribution system (except for overdue dir. cplrs.). 2 of 4 modules are assembled and awaiting high-power testing. After testing, the modules will be shipped assembled to Fermilab for installation.

•Simulations suggest that with pairing of cavities to allow identical QL’s, elimination of circulators should not pose a problem to field stabilization. This will be experimentally demonstrated when we run at NML in the circulatorless configuration.

•We are exploring other options to reduce the system cost, including eliminating phase shifters, simplifying the VTO fabrication or replacing the latter with alternate assemblies. A balance will have to be struck between system flexibility and cost (see Chris Adolphsen’s talk later).