High power rf measurements
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High Power RF Measurements. Ben Woolley, Amos Dexter , Igor Syratchev . Jan Kovermann, Joseph Tagg HG2013, Trieste June 2013. Outline. Phase Stabilisation of CLIC crab cavities RF distribution to the cavities. Accurate phase measurements and stabilisation system.

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High power rf measurements

High Power RF Measurements

Ben Woolley,

Amos Dexter, Igor Syratchev.

Jan Kovermann, Joseph Tagg

HG2013, Trieste

June 2013


Outline

Outline

Phase Stabilisation of CLIC crab cavities

RF distribution to the cavities.

Accurate phase measurements and stabilisation system.

LLRF for future/current X-Band test stands

Production of vector modulated signals for type II SLED pulse compressor.

Phase and power measurements of RF signals.

Working example using TWT and SLED II pulse compressor.

Easily transferable to other test stands (e.g. TERA C-band test)


Clic synchronisation requirement

CLIC SynchronisationRequirement

Cavity to Cavity Phase synchronisation requirement (excluding bunch attraction)

Estimate RF to beam synchronisation ~ 100 fs (0.43 degrees)


Integration

Integration

1)Use over-moded waveguide from klystron to the tee

2)Two klystrons with low-level/optical signal distribution.

1)35m of waveguide from the Tee to the cavities,

Scale: 20.3m from cavities to IP


Rf distribution

RF Distribution

Option 2:

A klystron for each cavity synchronised using LLRF/optical distribution.

  • Femtosecond level stabilized optical distribution systems have been demonstrated (XFELs).

  • Requires klystron output with integrated phase jitter <4.4 fs.

Option 1:

A single klystron with high level RF distribution to the two cavities.

  • Klystron phase jitter gets sent to both cavities for identical path length. Δφ=0.

  • Will require RF path lengths to be stabilised to within 1 micron over 40m.


Expected klystron stability

Expected Klystron Stability

  • For the Scandinova modulator with measured voltage stability of 10-4phase and amplitude stability should be 0.12° and 0.013%.

  • Active feed-forward or feedback would be needed to gain the required stability of 0.02°.

From kinematic model of klystron, phase and amplitude stability depend on gun voltage, V:


Waveguide choice

WaveguideChoice

Rectangular invar is the best choice as it offers much better temperature stability->

Expands 2.3 microns for 35 m of waveguide per 0.1 °C.


Rf path length measurement

RF path length measurement

RF path length is continuously measured and adjusted

4kW5ms pulsed

11.8 GHz Klystron/TWT repetition 5kHz

Cavity coupler 0dB or -40dB

Cavity coupler 0dB or -40dB

Waveguide path length phase and amplitude measurement and control

Forward power main pulse 12 MW

Single moded copper plated Invar waveguide losses over 35m ~ 3dB

-30 dB coupler

-30 dB coupler

Expansion joint

Expansion joint

LLRF

Magic Tee

LLRF

Reflected power main pulse ~ 600 W

Reflected power main pulse ~ 500 W

Phase shifter trombone

Phase shifter trombone

(High power joint has been tested at SLAC)

Waveguide from high power Klystron to magic tee can be over moded

Phase Shifter

Main beam outward pick up

Main beam outward pick up

From oscillator

48MW200ns pulsed

11.994 GHz Klystron repetition 50Hz

Vector modulation

12 GHz Oscillator

Control


Llrf hardware layout low bw

LLRF Hardware Layout (Low BW)

  • Fast phase measurements during the pulse (20-30 ns).

  • Full scale linear phase measurements to centre mixers and for calibration.

  • High accuracy differential phase measurements of RF path length difference (5 μs, 5 kHz).

  • DSP control of phase shifters.

Linear Phase Detector (1MHz BW)

Amp

LPF

10.7GHz Oscillator

DBM

DBM

ADC

Amp + LPF

ADC

DBM

Power Meter

To DSP

DSP

DAC2

Wilkinson splitters

DAC1

From DAC2

Calibration Stage

To Phase Shifter

-30 dB coupler

-30 dB coupler

Magic

Tee

To Cavity

To Cavity

Piezoelectric phase shifter

Piezoelectric Phase Shifter


Board development and cw tests

Board Development and CW tests

Front end electronics to enable phase to be measure during the short pulses to an accuracy of 2 milli-degrees has been prototyped and dedicated boards are being developed.

MCU

PLL controller

10.7 GHz VCO

Wilkinson splitter

Digital phase detector

400 ns span:

RMS: 1.8 mdeg

Pk-Pk: 8.5 mdeg

DBMs

90 s span:

Drift rate : 8.7 mdeg/10s

Total drift: 80 mdeg

Power Meters

Inputs


Crab cavity stabilisation next steps

Crab cavity stabilisation: Next steps

LLRF board revision:

Some problems with the PLL and tolerances on the Wilkinson splitters. (Non equal power splitting).

Investigation into pulsed power operation, effect of amplitude instabilities and higher order mixing products (not visible in CW tests).

RA joining Lancaster in September to increase/broaden the effort on this project.


Future llrf generation and acquisition for x band test stands

Future LLRF Generation and Acquisition for X-band test stands

12GHz vector modulated signal to DUT

2.4GHz vector modulated signal

12 GHz BPF

IF

RF

Vector Modulator

RFout

LOinLOout

2.4 GHz Oscillator

LO

9.6GHz BPF

X4 freq.

Amp

12GHz CW reference signal

LO

3dB hybrid

12 GHz BPF

RF

IF

2.4GHz CW reference signal

Oscillators should be phase locked

IF

LO

RF Input 1

IF

LO

1.6 GSPS

12-bit ADCs

RF Input 2

400 MHzLPFs

IF Amps

11.6 GHz BPF

X4 freq.

IF

Amp

2.9 GHz Oscillator

LO

RF Input 3

Digital IQ demodulation

IF

12GHz CW reference signal

LO

RF_Referance


System testing sled ii pc

System Testing: SLED II PC

RF Input 1

RF Input 2

RF Input 3

20 dB attenuator

20 dB attenuator

-40 dB coupler

-40 dB coupler

Phase modulated pulse input

TWT

LOAD

SLED II pulse compressor


System testing sled ii pc1

System Testing: SLED II PC

  • National Instruments PXI crate containing:

  • 2 CW generators for the LOs.

  • Vector modulator (up to 6.6GHz) with 200 MSPS I/Q generator

  • 5Chs 1.6GSPS 12-bit and 4Chs 250MSPS 14-bit ADC each connected to FPGAs.

  • 200 MHz digital I/O board for interlock and triggering signals.

Up/down-mixing components and cabling

Power Meter

TWT: 3kW

10-12GHz

RF Load

SLED II Pulse compressor

Igor Syratchev


Llrf system test sled ii pc

LLRF System Test: SLED II PC

Power

Phase

400MHz raw signals

Trans.

Refl.

Inc.


Llrf system test dynamic range

LLRF System Test: Dynamic Range

Power

Phase

400MHz raw signals

40 dB below

TWT saturation!


Pulse compressor detuning

Pulse compressor Detuning

Power

Phase

400MHz raw signals


Llrf system accuracy i

LLRF system Accuracy I

Possible sources of error

  • Problem with locking between the two CW generators and the 10MHz system reference caused the signal to beat.  This affects acquisition AND vector generation. Proposed solution: Clock ADC’s with 400MHz reference.

  • Bit-noise on the ADCs limit accuracy to 0.1% (9.5ENOB).

  • All harmonics generated by the multipliers and/or mixers will be mixed down to 400MHz and be indistinguishable from the signal of interest.

400MHz Reference

Power accuracy: 0.4%rmsPhase accuracy: 0.9°rms


Llrf system accuracy ii

LLRF system Accuracy II

Calibrated Power Meter

Output:

Gain 2.9

Input

Input

Comparison with calibrated power meter

  • Transients at the start of the main pulse and phase flip are observed (input).

  • Transients reduced when passed through the detuned system. Hybrid is narrow band  extra filtering of harmonics gives cleaner signal when mixing down.

Output


Tera c band c avity testing

TERA C-band Cavity Testing

PMT and faraday cup

Cavity under test

PXI crate

Directional coupler

Circulator

RF detector diodes, PMT and faraday cup inputs.

Timing board/ trigger generation

5.7GHz 4MWMagnetron

Pictures: Alberto Degiovanni


Screenshots from cbox1

Screenshots from ‘CBOX1’


X band llrf future developments

X-band LLRF: Future Developments

Characterisation of errors/transients in the system. Both on the generation and acquisition side.

Miniaturisation of LLRF system  components into crate.

Software: logging, interlock control, timing and triggers etc.

Integration into XBOX-2 test stand.

Scaling up and integration towards XBOX-3.


Thank you for your attention

Thank you for your attention!


Extra slides

Extra Slides


Observed klystron s tability

Observed Klystron Stability

  • Amplitude jitter reduced to ~1-2%.

  • Phase measurements will be performed in the coming weeks.

Observed ~5% amplitude jitter on the output of the klystron.  This was due to a mismatch in the pulse forming diode in the LLRF network and a triggering error in the ADC firmware.


Klystron phase noise

Klystron phase noise

  • Can use a standard method to measure the phase stability of the klystron.

  • Reference source is split such that its phase noise is correlated out by the mixer for both channels.

  • Phase shifter adjusted as to bring RF and LO inputs into quadrature.

  • Digital scope or ADC and FPGA/DSP preforms FFT analysis, to obtain phase noise curve.

  • Experiment can also be repeated for different lengths of waveguide to ascertain the effects of waveguide dispersion.

Waveguide

Klystron

TWT

To Cavity

PIN diode

switch

-57 dB coupler

12 GHz Oscillator

RF

Oscilloscope/ADC for FFT analysis

LO

Φ

IF

phase shifter

Wilkinson splitter

DBM

Measurement requires good amplitude stability as any AM will be present in the IF.


Waveguide stability model

Waveguide Stability Model

Use ANSYS to find “dangerous” modes of vibration for a 1 m length of waveguide fixed at both ends.

Fundamental mode 65.4 Hz


Planned clic crab high power tests

Planned CLIC crab high power tests

Travelling wave 11.9942 GHz

phase advance 2p/3

TM110h mode

Input power ~ 14 MW

Test 1:

Middle Cell Testing – Low field coupler, symmetrical cells. Develop UK manufacturing.

Test 2:

Coupler and cavity test – Final coupler design, polarised cells, no dampers.

Made with CERN to use proven techniques.

Test 3:

Damped Cell Testing – Full system prototype


Future

Future

Continued investigations into the phase stability of the 50 MW X-band klystron.

Development of feed-forward and/or feedback system to stabilise the klystron’s output.

Continued characterisation of electronics to obtain stand alone phase measurement/correction system.

Design/procurement of the waveguide components needed.

Demonstration RF distribution system, with phase stability measurements.

Perform phase stability measurements during the CTF dog-leg experiments.

Measure phase across the prototype cavity during a high power test.


Llrf hardware layout high bw

LLRF Hardware Layout (High BW)

  • Fast phase measurements during the pulse (50MHz).

  • 400MHz direct sampling to centre mixers and for calibration.

  • High accuracy differential phase measurements of RF path length difference (5 μs, 5 kHz).

  • DSP control of phase shifters.

400MHzLPF

Amp

11.6 GHz Oscillator

DBM

DBM

1.6 GSPS 12-bit ADC’s

Amp + LPF

ADC

DBM

Power Meter

To DSP

DAC2

DSP

Wilkinson splitters

DAC1

From DAC2

Calibration Stage

To Phase Shifter

-30 dB coupler

-30 dB coupler

Magic

Tee

To Cavity

To Cavity

Piezoelectric phase shifter

Piezoelectric Phase Shifter


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