On the RF system of the ILC

Sami G. Tantawi

Chris NantistaValery Dolgashev

Jiquan Guo

SLAC

Outline• This talk is a collection of thoughts about the rf

system based on our experience with X-band system!

• Review of the X-band RF systems• The main linac division into separate system, A

bad idea• Examples from a system point:

– The distribution system and relations to structure spacing

– Rf source developments and relations to modulator and couplers

• Some thoughts on couplers

dual-moded resonant delay lines ~30mQint=106

Qext=2 x 104

RF inputs to the four 50 MW klystrons

single mode waveguide input to the pulse compression system; ~100 MW/line for 1.6 µs

dual-mode waveguide carrying ~200 MW

compressed output ~600 MW, 400 ns.

output load trees

Experimental RF Pulse Compression System

The Heart of the Pulse Compression System

dual-mode combiner

two fundamental mode inputs

TE10/TE20

TE01/TE11

dual-mode directional couplerpumpout

super hybrid

TE01 to delay lines

TE01/TE11 to loads

Dual-Mode Rectangular-to-Circular Taper Mode Converter

TE01

139.8 mm

40.64 mm

36.45 mm

30.58 mm

WC160

Section#1

Section#2

Section#3

width taper

36.63 mm

TE11

TE20

TE10

Dual-Mode Vacuum Pumpout

The hole pattern is designed to cancel any coupling or reflection for the TE01 and TE11 modes.

Dual-Mode CombinerTE10

TE20

Magic H Hybrid

2.3821”0.900”

C. Nantista ‘02

Head

TE10

straight through

TE20

through SLED-II

to and from delay lines

The input power, carried by the TE01 mode, is split 4 ways to be absorbed at the loads

Load Tree

High-Power Load

Magnetic stainless steel carrying circularly polarized TE11 past matched pairs of

partial choke grooves.

Dual-Moded Delay Line

TE01

TE02

TE02

TE01

17.08 cm delay line ~29 m longinput taper

end taper

mode converter / tuning short

Dual-moding the delay lines cuts their required length approximately in half.

21

21

2 gg

gg

vvvvTL+

=

Input Taper

-1

-0.8

-0.6

-0.4

-0.2

0

11.3 11.35 11.4 11.45 11.5 11.55

TE01 Transmitted TE02 Reflection

dB

Frequency [GHz]

System Layout

Flattened Pulse

0

100

200

300

400

500

600

0 0.5 1 1.5 2

InputOutput

Pow

er (M

W)

Time (µs)Sami Tantawi (1/27/2004)

176

178

180

182

184

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1

500

505

510

515

520

525

530

1.4 1.5 1.6 1.7 1.8 1.9

Pow

er (M

W)

Time (µs)Sami Tantawi (1/2

Multimoded SLED-II Output With Feedback

Phas

e (d

egre

es)

8-Pack Phase 2a 8-Pack Phase 2b

Power Distribution6 dB directional coupler

4.8 dB directional coupler 3 dB

directional coupler

hybrids

structures

Highlights from the X-Band System• We have reliably produced and manipulated flat

400 ns rf pulses carrying over 500 MW.• We have developed waveguide components

capable of manipulating hundreds of megawatts.• We have utilized dual-moding, both for power

direction and for shortening delay lines.• The circular TE01 mode is a miracle mode solves all

problems• The charging and discharging of the delay lines

could be improved with active elements• We could not relay on circulators, they do not exist

at these power levels.

TESLA TDR RF DISTRIBUTION

TESLA Design:

Alternate Scheme:

2:1

1:1

17 different directional couplers

Less waveguide. More compact.

Fitting may be difficult where klystron feeds meet.

Only 2 different dir. couplers (-1.76 dB and -3.01 dB)Same path lengths as above.Power traverses on avg. 4.33 rather than 9.44 dir. couplers (fewer flanges and coupling regions → less loss)More waveguide. More space.

With the proper choice of directional coupler port orientations and spacing (same for all), all 90° bends can be eliminated.

This configuration requires no more pieces of waveguide than the TESLA scheme.

Circulators could be eliminated by cancellation of reflections from each pair.

-1.76 dB

-3.01 dB

load

BRANCHING DISTRIBUTION

Effect of Random Hybrid Coupling ErrorsRed: branching distributionBlue: series distribution± 0.1 dB ± 0.2 dB

1000 runs

RUPAC ’02

Binary Branching

• one splitter design

• same path lengths → same thermal phase change

Lf f Æ -w L f

c

0 2φφ (n-1)φ3φ

Vi(2) Vi(3) Vi(4) Vi(n-1)

Vo(1) Vo(2) Vo(n-1)

Current RF unit designCirculator(~20000 of them)

1

2

3

4

si =

i

k

jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjj

0 1è!!!!!!!!!!!!!!!-i+n+1

0 $%%%%%%%%%%%%%%%%%%%1 + 1i-n-1

1è!!!!!!!!!!!!!!!-i+n+1

0 $%%%%%%%%%%%%%%%%%%%1 + 1i-n-1

0

0 $%%%%%%%%%%%%%%%%%%%1 + 1i-n-1

0 - 1è!!!!!!!!!!!!!!!-i+n+1

$%%%%%%%%%%%%%%%%%%%1 + 1i-n-1

0 - 1è!!!!!!!!!!!!!!!-i+n+1

0

y

{

zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

i

k

jjjjjjjjjjjjjjjjjjjjjjjj

0 ‰Â IHi-1L f-⁄k=1i-1 qHkLM

è!!!!!!!!!!!!!!!-i+n+1

0 ‰Â qHiL "##################1 + 1i-n-1

‰Â IHi-1L f-⁄k=1i-1 qHkLM

è!!!!!!!!!!!!!!!-i+n+1

0 ‰Â IHi-1L f-⁄k=1i-1 qHkLM "##################1+ 1

i-n-10

0 ‰Â IHi-1L f-⁄k=1i-1 qHkLM "##################1+ 1

i-n-10 - ‰Â qHiLè!!!!!!!!!!!!!!!

-i+n+1

‰Â qHiL "##################1 + 1i-n-1

0 - ‰Â qHiLè!!!!!!!!!!!!!!!-i+n+10

y

{

zzzzzzzzzzzzzzzzzzzzzzzz

qHiL

yHiL = Hi - 1L f - ‚k=1

i-1

qHkL

R =1n

„‰ Hn-1Lf ‚i=1

ncosHH-2 i + n + 1L fL =

sinHn fL „‰ Hn-1Lf

n sinHfL

f Æi p

n

Lf

0 2φφ (n-1)φ3φ

Vi(2) Vi(3) Vi(4) Vi(n-1)

Vo(2) Vo(n-1)Vo(1)

L f = m l -i l

2 n

l

2 x 8= 1.441 cm

0 2 4 6 8 10Delta SeparationHcmL

-25

-20

-15

-10

-5

0noitcelfeR

oteht

ecruosHBd

L

For a solution i of an n structure problemthe amplitude of the signal at load j is given by

k=1

j„

2 ‰ i k pn - j

2 k=1j k n

Distribution of power over the loads for different distribution solutions for 8 accelerator structures

1 2 3 4 5 6 7Load Number

0.05

0.1

0.15

0.2

0.25evitaleR

rewoP

HYBRID COUPLERS

TESLA Baseline:• Manufactured by RFT SPINNER• -12.6 to -3.0 dB coupling (1/18 to 1/2)• coupling accuracy: ±0.2 dB• return loss 35 dB

Improvement:• tighten coupling accuracy to ±0.1dB• adjustable coupling?

(being investigated at DESY)

Dual-Mode CombinerTE10

TE20

Adjustable Coupling?squeeze

Distribution Without Directional Couplers

With a circulator at each cavity, are directional couplers necessary?

What about 3-port tap-offs or multi-port dividers?

1

2 3

4

5

67

8

TE20TE10

TE30 TE40

With This we can construct a 16-port precisedirectional divider; one input, 8-output and 7 loads

2ln22ln

R/Q ,IL,,E:Given

0

2

0bacc

ωτ L

ci

b

L

b

acc

Qt

QRI

PQ

VIPLEV

==

=

==

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛−−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛−=

−=−

−−L

i

L Qtt

LbQ

t

L

brf

eQQRIeQ

QRP

tVtVtV

2)(

0

2

0114

)()()(ωω

For optimal coupling of power to beam:

Basic Equations

Va(Ib)Va(Prf)

Va(C)

Superconducting Cavity Behavior*

*Infinite Q0 assumed.

C∝Ee/Va∝Qe-1/2 assumed large enough that

Qe«Q0 but small enough that the initial reflection ≅ 1.

Dashed lines are for continually optimized coupling.

Ti(Prf)Ti(Ib)

Ti(C)

( )

( ) 0200

00

00

12)(

2)(

12)(

aa

ab

bba

arfrfrfa

VCCCC

CV

VIIIV

VPPPV

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−=

( )( )[ ]

( ) ( )( )

00

2

30

20

00

00

2lnln112)(

ln1)(

ln2ln2

11)(

ii

ibbbi

irfrfrfi

TCCCCCC

CT

TIIIT

TPPPT

⎟⎠⎞

⎜⎝⎛ +⎟

⎟⎠

⎞⎜⎜⎝

⎛−=

−=

⎥⎦⎤

⎢⎣⎡ +=

For more than -0.26 dB (5.8%) below nominal power, steady state cannot be achieved at nominal injection time.

Ib = 9.5 mA

Eacc = 23.4 MV/m

L = 1.038 m

V0 = 24.29 MV

P0 = 230.8 kW

QL0 = 2.47×106

ti = 418.9 µs

~69% of rf power (at cavity) goes into the beam.

31% of rf wasted: 22% filling cavity

9% to load during fill

Pin

Pload

Pacc

Ein

Ee

Eload

Eload

Ee

( )2/12ln −= cinL Pfill

τE

Field and Power Waveforms

Cavity heating: ~11.6% during fill~67.1% during beam~21.3% after beam

By reversing the rf phase after the beam and extending the pulse to 1.61ms, we can save 16.2% of cavity heating, but at a cost of 17.5% more rf power.

RF CAVITY HEATINGQ: Should we use rf phase flip to empty cavity faster?

A: No. But if we have switchable coupling, we could drop QL to dump the energy faster.

With agile coupling control, some or all of the 9% lost during filling can be saved,

reducing the fill time from 419 µs to τc/2 = ti/(2ln2) = 302 µs.

• Switch Array– Targeted for the implementation of sub-

gigawatt level X-band active pulse compression system for the Next Linear Collider (NLC)

– Working under TE01 mode in circular waveguide

– Implemented with array of silicon PIN diodes– Required switch time: ~10ns

Design of the switch arrays (cont.)

• Low loss– Use high purity silicon wafer– When diodes are off

• 3% loss from HFSS simulation– High carrier density when diodes are on

• 3% loss for carrier density of 5×1017/cm3

• 10% loss for carrier density of 5×1016/cm3

• carrier layer thickness 50µm.

Introduction (cont.)• Amplifier array– Potential megawatt level pulsed RF source– Similar configuration as the switch array– Implemented with silicon IMPATT diodes– Output of diodes are spatially combined– Other device/material options

Design of the switch arrays

• Working at TE01mode in circular waveguide

• Fabricated on one 4 inch floatzone silicon wafer

• Hundreds of PIN diodes integrated

• Two configurations with different number of diodes (960 and 576)

Design and Implementation of PIN/NIP DiodeArray Active Window

PIN diode array Active Window• All doping profile and metallic terminals on the window are

radial, i.e. perpendicular to electric field of the TE01 mode. Effect of doping and metal lines on RF signal is small when thediode is reverse biased.

• With forward bias, carriers are injected into I region and I regionbecomes conductor RF signal is reflected.

P

N N

side view(not to scale) metal line (1.5um thick)

I

~10um

• Base material: high resistivity (pure) silicon, <5000ohm-cm, n-type

• Diameter of active region: 1.3 inch• Thickness: 220um• Coverage (metal/doping line on the surface): ~10%

Metal terminal

Radial-line PIN diode arraystructure (400 lines)

220um

2 inch

A B

Section A--B

RF structure• DC isolation by Al2O3 ceramic ring• No RF choke is needed (TE01 mode)• Higher impedance (Zg / Z0 ~4, close to cutoff) for this

experimento Enhance the effect of window switching statuso Lower loss at the window during forward biaso Huge mismatch without bias

Ceramic ring for DC isolationActive PIN/NIP diode window

2”

Metal spring for DC contactbetween RF structure and theactive window

Layout of the switch arrays

Design of the switch arrays (cont.)

• High contrast between on and off states– Self matched (S11<0.1)

when diodes are off.– Full reflection

(S12<0.1) when diodes turn on.

Design of the PIN diode

• 2-D structure compatible with CMOS process• Diodes length 60-120micron

– Generate1016~1017/cm3 carrier density with moderate current

– With proper bias, on time <20ns, off time<200ns.

Factors limiting diode speed

• Need high voltage to compensate space charge field.

• Need inject/draw big charge (~50µC) in short time

• Non-uniformity in diodes:– Some diodes response faster than the others,

will draw more current, lower the voltage over other diodes, and burn themselves.

• Problem for turn off speed:– Local breakdown caused by high voltage

Fabrication of the diode array

• Fabricated at Stanford Nanofabrication Facility

• Using CMOS compatible technology

Diode array

Testing structure

Testing setup

Switch on time

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

54.00 56.00 58.00 60.00 62.00 64.00 66.00

time (us)

pow

er(d

bm)

reflected powertransmited power

The structure of the new thin wafer RF Switch

Port 2 (rf)

Port 1 (rf)

Port A (Bias)

Garnet Material

Wrap-around Mode converter

Coaxial TE01and TE02 mode

propagate in this coaxial structure

Port 3 (rf)

Port 2 (rf)

Port 1 (rf)

Port A (Bias)

Garnet Material

Wrap-around Mode converter

Coaxial TE01and TE02 mode

propagate in this coaxial structure

Port 3 (rf)

Port B (Bias)

Port 1 (rf) Port 2 (rf)

Port A (Bias)

Garnet Material

Wrap-around Mode

converter

Coaxial TE01 mode propagate in this coaxial

structure

Port B (Bias)

Port 1 (rf) Port 2 (rf)

Port A (Bias)

Garnet Material

Wrap-around Mode

converter

Coaxial TE01 mode propagate in this coaxial

structurePort 1 (rf) Port 2 (rf)

Port A (Bias)

Garnet Material

Wrap-around Mode

converter

Coaxial TE01 mode propagate in this coaxial

structure

Wrap-Around Mode Converter for Tap-off, and extraction, tested to 470 MW

rg

ri

ro

Garnet Material

r

x

y

z

φ

Biasing magnetic field

rg

ri

ro

Garnet Material

r

x

y

z

φ

Biasing magnetic field

Active nonreciprocal phase shifter prototype

100

120

140

160

180

200

220

240

11.1 11.2 11.3 11.4 11.5 11.6 11.7

Phase difference between forward and backward waves

Phas

e di

ffere

nce

betw

een

forw

ard

and

back

war

d w

aves

Freq GHz

0

0.2

0.4

0.6

0.8

1

11.1 11.2 11.3 11.4 11.5 11.6 11.7

Matched Nonreciprocal Overmoded Phase Shifter

S11

S12

S21

S22

Dat

a M

ag

Freq GHz

Cold test data for the first prototype of the overmoded nonreciprocal phase shifter. It indicates low losses and good match. The phase shift between forward and backward waves is approximately 180 degrees

15 cm diameter1 mm gap (vacuum)The waveguide is Al2O3 RodTE01 Mode

0.0485672 + 0.998832 = 1.00002

1 cm Gap

0.44062 + 0.8977052 = 1.0

-46.3 dB suppression of TE11 relative to TM01 at end of 10 cm beampipe.

D = 6.0 cm

a = 6.500” = 16.51cm b = 3.6275 cm

h = 8.4652 cm

WR650 to 6cm Beampipe Waveguide Coupler

Chock coupler for 1.3 GHz superconducting structureProperties

•Circular symmetric flange to superconducting structure •No current on the flange joint•Symmetric fields on axis•Takes little longitudinal space•Could be used in the middle of structure

Electric field lines calculated by 2D finite-element code SLANS 3D geometry with

coupling port

Beam pipe

Input coaxial port

Magnitude of electric fields on chock mid-plane

excited by coaxial port, 3D HFSS calculation

V.A. Dolgashev, SLAC, 28 January 04

Suggested Future activities

• RF Sources:– Small RF sources cabple of powering one or two cavities are

important to speed R&D efforts• We would like to develop a compact inexpensive rf crossed field device.

30 such devices can replace a klystron (In the current design the power from the klystrons are divided 30 ways!) This would be more efficient and, although not obvious, I believe less expensive. The development would capitalize on L-band commercial cooker magnetrons that have an efficiency of 90%.

• Solid state sources become a viable alternative at L-band. Our experience with these devices at the much more difficult X-band could help in the development of a completely solid state source.

• Through the SBIR program we would like to we could develop a newmulti-beam klystron. The output cavity and structures would rely on our experience in overmoded and multimoded rf systems. This will give the klystron designer the opportunity to separate the beams and hence increase the reliability of these klystrons. This klystron will operate at voltage lower than 30 kV and output power close to 1 MW.

• Accelerator Structure Couplers: The present design for these devices is very complicated and expensive. We could contribute to the modification of these devices to increase their reliability and decrease there cost.

•By Relating spacing of cavities to the RF distribution system it is possible to eliminate about 20000 Circulator ($20 M to $200 M) without any clear draw backs

•One should think about the main linac as a system. The separation of the development of the RF sources from the modulators from the distribution system from the power couplers from linac / cryogenic module development will result in lost opportunities

•Spatially combined RF devices can provide RF sources. It can also provide switches to enhance the charging and discharging of the cavities, i.e., active couplers.

Conclusion