TWIICE 2014: Topical Workshop on Instabilities, Impedances, and Collective Effects

BEAM COUPLING IMPEDANCE STUDIES OF THE CLIC DAMPING

RING EXTRACTION KICKER

C. Belver-Aguilar (IFIC)

On behalf of: A. Faus-Golfe (IFIC), F. Toral (CIEMAT),

M.J. Barnes (CERN)

http://gap.ific.uv.es

KICKER PARAMETERS FOR CLIC DAMPING RINGS

Beam parametersCLIC DRs

1 GHz 2 GHz

Energy (GeV) 2.86

Circumference (m) 427.5

Bunch population [109] 4.1

Normalized emittance (nm) 500 (H)5 (V)

Bunches per train 156 312

Bunch length (mm) 1.6 1.8

Bunch spacing (ns) 1 0.5

Beam current (A) 110 120

Kickers parametersCLIC DRs

1 GHz 2 GHz

Deflection angle (mrad) 1.5

Aperture (mm) 20

Effective length (m) 1.7

Field rise and fall times (ns) 560 1000

Pulse flat top (ns) 900 160

Flat top reproducibility ±

Injection stability ±

Extraction stability ±

Injection uniformity (%) ± 0.1 (over 3,5 mm radius)

Extraction uniformity (%) ± 0.01 (over 1 mm)

Repetition rate (Hz) 50

Vacuum (mbar)

Longitudinal beam impedance (Ω/n) 0.05

Transverse beam impedance (kΩ/m) 200

16th January 2014 TWIICE workshop 1/12

KICKER TECHNOLOGY AND PARAMETER CHOICES

PARAMETERS FOR THE STRIPLINES OF THE EXTRACTION KICKER OF THE

DAMPING RINGS

Characteristic impedance 50 Ω

Field inhomogeneity (%) [CLIC: 1mm radius] ± 0.01

Longitudinal beam coupling impedance (Ω per turn) < 0.05

Transverse beam coupling impedance (kΩ/m) < 200

Striplines have a significantly lower longitudinal beam coupling impedance, than a screened MKI magnet, at frequencies above 400 MHz .

The imaginary component of the longitudinal beam impedance is known to be significant, for the MKI magnet above 600 MHz.

The use of striplines rather than a screened ferrite loaded magnet is supported by experience at KEK/ATF, where metallized ceramic tubes were used to reduce the beam coupling impedance of ferrite loaded kickers: the thickness of the metallization was difficult to accurately control and, as a result, two kickers had very different pulsed magnetic characteristics.

16th January 2014 TWIICE workshop 2/12

STRIPLINE KICKER OPERATION

ODD MODE: The electric field lines have an odd

symmetry about the centre line, and a voltage null exists between the two electrodes.

There is a virtual ground plane midway between the electrodes (capacitance = 2C12)

The effective capacitance between an electrode and ground is .

EVEN MODE: The two electrodes are at the same

potential, so there is no charge stored in the capacitance between them.

The resulting capacitance of either electrode to ground is .

COUPLED LINES → TWO OPERATION MODES: ODD AND EVEN MODE

Each mode has its own electromagnetic field pattern and its own characteristic impedance:

16th January 2014 TWIICE workshop 3/12

CHARACTERISTIC IMPEDANCE CALCULATION: ANALYTICAL APPROXIMATION

The geometric cross-section of the striplines defines the characteristic impedance and the homogeneity of the electromagnetic field in the aperture.

FIRST-ORDER ANALYTICAL APPROXIMATION: plate-type electrode

: parallel plate capacitance, where edge effects and fringe fields are neglected.

: capacitance between a strip conductor and an infinite ground plane.

is the permittivity of free-space

is the surface area of one plate

16th January 2014 TWIICE workshop 4/12

CHARACTERISTIC IMPEDANCE CALCULATION: ANALYTICAL APPROXIMATION versus HFSS SIMULATION

ANALYTICAL EQUATION HFSS SIMULATION

It is confirmed that a small stripline beam pipe radius results in closer values of even and odd mode characteristic impedance.

The even mode characteristic impedance of striplines is always higher than the odd mode. For half-moon electrodes (simulation), the odd mode impedances are all equal. For a 50 Ω even

mode characteristic impedance, an odd mode characteristic impedance of 40.9 Ω is achieved with a stripline beam pipe radius of 20 mm.

R = 20 mm

Higher odd mode characteristic impedance

Good field homogeneity

HFSS SIMULATION

16th January 2014 TWIICE workshop 5/12

BEAM COUPLING IMPEDANCE: ANALYTICAL EQUATIONS

𝑍∥=2𝒁𝒆𝒗𝒆𝒏(𝝓𝟎

2𝜋 )2

[2sin2(𝝎𝑳𝑐 )−𝑖 𝑠𝑖𝑛( 2𝝎𝑳

𝑐 )]𝑍⊥=[ 𝑍 ∥

𝝎 ]𝑝𝑎𝑖𝑟 [ 𝑐𝑹2 ][ 4𝝓𝟎 ]

2[sin2(𝝓𝟎

2 ) ]

FOR UNTAPERED STRIPLINE BEAM POSITION MONITORS is the even mode characteristic

impedance is the coverage angle of a single

electrode is the angular frequency is the striplines length is the stripline beam pipe radius

At low frequencies:

Longitudinal impedance per harmonic:

where is the DR radius.

Transverse impedance:

High frequency beam coupling impedance is an open question to be studied.

16th January 2014 TWIICE workshop 6/12

BEAM COUPLING IMPEDANCE: CST SIMULATIONS

Geometric parameters: = 1,8 rad = 1,7 m = 20 mm

16th January 2014 TWIICE workshop 7/12

Thanks to S. Smith

FIRST RESULTS IN TAPERING STUDIES

Advantages of tapering the transition between the electrode and the adjacent beam pipe:

Minimimize the non uniformity of transverse deflection as a function of the transverse position; Minimize the contribution of the kicker to the machine impedance; Minimize the reflection coefficient at high frequency

16th January 2014 TWIICE workshop 8/12

(𝑍∥)𝑡𝑎𝑝𝑒𝑟𝑒𝑑=2𝑍 𝑒𝑣𝑒𝑛( 𝜙0

2𝜋 )2 sin

2(𝜔𝑙𝑐 )

(𝜔𝑙𝑐 )

2 [2sin2(𝜔𝐿𝑐 )−𝑖𝑠𝑖𝑛( 2𝜔𝐿

𝑐 )](𝑍⊥ )𝑡𝑎𝑝𝑒𝑟𝑒𝑑=[ (𝑍 ∥)𝑡𝑎𝑝𝑒𝑟𝑒𝑑

𝜔 ]𝑝𝑎𝑖𝑟 [ 𝑐𝑅2 ] [ 4𝜙0 ]

2 [sin2(𝜙0

2 ) ]

MANUFACTURING PROCESS: Trinos Vacuum Projects

16th January 2014 TWIICE workshop 9/12

FIRST LABORATORY TESTS AT CERN

S11 PARAMETER

The frequency between peaks is ≈95 MHz, which corresponds to the two-way delay of the electrodes.

The electrode supports (Macor rings) increase the magnitude of the reflection parameter starting from 300 MHz, of every third peak.

The separation between these maxima corresponds to the distance, there and back, between the equally-spaced Macor rings.

16th January 2014 TWIICE workshop 10/12

FUTURE BEAM TESTS

1) Striplines not used as a extraction device and without inductive adder Longitudinal and transverse beam coupling impedance

2) Striplines not used as extraction device and with inductive adder Field inhomogeneity Pulse shape and repeatability Long term reliability of the system

3) Once long-term reliability is demonstrated: possible use as extraction device

Verification of the stripline dimensions.

Vacuum compatibility.

High voltage performance.

Longitudinal and transverse beam coupling impedance measurements..

FUTURE TESTS WITHOUT BEAM

16th January 2014 TWIICE workshop 11/12

SUMMARY

The stripline design provides the performance specified for the extraction kicker of the CLIC DRs: excellent field homogeneity, good power transmission and reasonable broadband beam coupling impedance.

The stripline design has been carried out by using analytical approximations and simulations to optimize the stripline geometry.

Beam coupling impedance has been studied both analitically and with CST simulations, only for a low frequencies. The behavior of the kicker at high frequencies is an open question to be studied.

A prototype of the extraction stripline kicker for the CLIC DR has been manufactured by Trinos Vacuum Projects (Valencia, Spain).

Laboratory tests have commenced at CERN labs. Results for the reflection parameter S11 match quite well with the results predicted by the simulations. Other laboratory tests will be done in the next months.

The stripline and the inductive adder will be finally tested separately and jointly with beam.

16th January 2014 TWIICE workshop 12/12