Damping materials* and WFMs # measurements

G. De Michele: HG2013, 3-6 June, ICTP Trieste

Damping materials* and WFMs# measurements*G. De Michele1,2,3, A. Grudiev1 ;

# M. Dehler2, S. Bettoni2, G. De Michele1,2,3 ;1CERN, 2PSI, 3EPFL

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• The study of EM properties at microwave (μw) frequencies is full of academic importance (materials property research)

• μw communications and engineering (military, industrial, civil)

• Clock speeds of electronic devices at μw frequencies require knowledge of permittivity and permeability

• EM interference (EMI) and EM compatibility (EMC)• Various fields of science and technology could profit:

agriculture, food engineering, medical treatment, bioengineering

• High-quality design of the HOM dampers in CLIC accelerating structures (but also collimators, kickers)

Motivations

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• Non-resonant techniques: Reflection method: open-circuited reflection; short-circuited reflection (S11) Transmission/reflection methods: (S11,S12,S21,S22)

• Resonator techniques: (the sample forms a resonator or a key part of resonator) Dielectric resonator Coaxial surface-wave resonator Split resonator (dielectric sheet sample)

• Resonant perturbation method: The sample under test is introduced into the resonator and the EM

properties are deduced from the changes in resonance frequency and quality factor of the cavity

• Planar circuit methods (both resonant and non-resonant methods): Stripline Microstripe Coplanar line

Different techniques for materials characterization (1/2)

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In a more general view we can split the techniques in two big families:

• Non-resonant techniques : possibility to measure EM properties in a wide range of frequencies

• Resonator techniques: Measurements possible at single or several discrete frequencies Higher sensitivity and higher accuracy

The presentation will focus on the non-resonant technique e.g. a coaxial line filled with the material under test (MUT).In the past waveguide method at CERN have been used and EM properties have been measured at some discrete frequency points. Measurements in a wide range of frequencies were done for the material AlN:R. Fandos, W. Wuensch, Estimation of the RF Characteristics of Absorbing Materials in Broad RF Frequency Ranges, CERN-OPEN-2008-019, CLIC-Note-766, 2008.

Different techniques for materials characterization (2/2)

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The Coaxial Technique

Simulations

Measurements

AND Material properties

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• The measurements system is simulated with 3D EM code using the material properties as free parameters

• From simulations one obtains the complex transmission parameter S21 as function of real(epsr) and imag(epsr) at a given frequency

• The possible solutions are calculated from the contour plots that result from the intersections between the 3D surfaces, obtained numerically, and the measurements (real and imag)

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The Measurements Setup• The reflection method gave some limitations on the maximum measurable frequency

range. For this reason transmission method has been evaluated• The setup consists on a SMA adapter filled by the material with a hollow coaxial shape.

The dimensions of the samples are:o Length = 15.00±0.02mmo Outer diameter = 4.07±0.01mmo Inner diameter = 1.30±0.02mm

• The samples were measured in two frequency ranges: from 2GHz to 10GHz and from 10GHz to 27GHz in order to have a better calibration

• The calibrated planes are directly the 3.5mm adapters. No transition between the calibrated planes and the DUT reduces source of errors

• The measured materials are CerasicB1, EkasicF, EkasicP

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Results

The data are filtered in order to delete multiple solutions.Below fits of the measurement with the Cole-Cole Model:

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1%

Error Analysis• By mounting and dismounting the setup we found an error within 5% for the real part of the

permittivity and 10% for the imaginary part• Furthermore measurements on 4 different samples for EkasicP were performed. The magnitude

and phase variation of S21 is within 5% and this gives a maximum variation of 6% for the real part of permittivity and 9% for imaginary part

• Repeated calibrations gave error within 1%

5%

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Remarks• The reflection method has been deeply investigated as well; a

transmission line model was also used in order to crosscheck the simulation results

• The air-gap correction has been investigated and an error of few percent has been found on final results (air-gap~20-30um) in the frequency range of interest

• Does not exist a unique setup: it depends on the frequency range of interest and on the MUT

• For details on reflection method, TL modeling and EM simulations please see:G. De Michele, A. Grudiev, EM Characterization of damping Materials for CLIC RF Accelerating Structures, CLIC Note 2013, to be published

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• X-band structure (common development of PSI, CERN, ELETTRA) in order to linearize the longitudinal phase space for high efficiency bunch compression

• Constant gradient design: 72 cells, active length 750 mm• Long cells with large mean aperture of 9.1 mm: small transverse wake• Wake field monitors to ensure optimum structure alignment• Average gradient 40 MV/m with 29 MW input power• Group velocity variation: 1.6-3.7%• Fill time: 100 nsec• Average Q: 7150• For details: M. Dehler et al., X-band structure with integrated alignment monitors, PRST

2009

WFM measurements at PSI Swiss FEL injector

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beam

The x-band structure

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WFMupstream

WFMdownstream


Simulated output signal spectra

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M. Dehler

There is a strong coupling to the beam in the regionfrom 15.3 to 16.2GHz .

Transverse wake impedance

Zt


Installation

beam

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yawpitch

roll

z

yx

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Measurements

•The structure was moved w.r.t. the beam by using the mechanical mover system in order to have a clear picture of the emittance dilution.

•Signals were measured outside the tunnel with a LeCroy SDA816zi digital scope (18 GHz BW, 40 GS/sec). The measured attenuation of the connecting cable was 25 dB at 16 GHz.

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upstream

downstream

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Typical output from the WFMsSignal from theupstream WFM

Signal from thedownstream WFM


WFM monitors signals vs. horizontal and vertical offsets of the X-band structure

•Averaged peak-to-peak over several hundreds measurements•Next time take the standard deviation of the peak and combine it with the noise floor•Next time take an external stable trigger•WFMs sensitivity: ≈10V/mm •A systematic study on shifts and tilts of the structure to be done

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Emittance dilution vs. vertical shift• The quadratic fit gave a minimal emittance for a vertical offset y = -75 um (WFM

predicts minimum at -100 um) • The same measurement was not possible in the horizontal plane since we lost the

reference of the mechanical movers during the run

S. Bettoni

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Tilt (yaw) in the structure after alignment in horizontal plane

The "hole" in the signal means that the 15.7GHz dipolar mode (that is strongly coupled to the beam in the central cell of the structure) is not excited i.e. the beam is passing in the center of the cell in the middle of the RF structure (cell 36).

TILT OFFSET

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Conclusions

• We were able to align the structure to the golden orbit in the vertical plane. In the horizontal plane was not possible only because of the mechanical movers problem and available time

• Even with a preliminary setup the alignment of the structure was much faster than measuring the emittance growth

• With a dedicated RF front end we hope to see also bends, random misalignments in the structure

• Next step will be the high power operation and in this phase effects of RF fields on beam emittance could be study as well if any

• Comparison with Trieste WFMs measurements could be interesting as well

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G. De Michele: HG2013, 3-6 June, ICTP Trieste 20 of 194/6/2013

THANK YOU FOR YOUR ATTENTION

Background: Naples, Italy

Damping materials* and WFMs # measurements

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