Recent Euclid Wakefield Experiments @ AWA

C. Jing, S. Antipov, A. Kanareykin, P. Schoessow, Euclid Techlabs, LLCM. Conde, W. Gai, W. Liu, J. Power, Z. Yusof, HEP, ANL

HG Workshop, Feb. 2011

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I. Experiment on Transformer Ratio Enhancement Using a Ramped Bunch Train

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Wakefield Transformer Ratio

Transformer ratio limited: R≤2 @ a longitudinally symmetric drive bunch, but it can be enhanced greater than 2 using asymmetric bunch.

Transformer ratio R =Max energy gain of the witness bunch

Max energy loss of the drive bunch

Q

W-min

W+max

q

Collinear Wakefield Acceleration

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Scheme I---Single Triangular Bunch

Scheme II---Ramped Bunch Train

Reference: Schutt et. al., Nor Ambred, Armenia, (1989)

Reference: Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985)

c (z)

W+

W-z

To Enhance the TR

zd d

W -

W+

d

(z)

RBT: d=(1+1/2)λ, acceleration for the second bunch, Q1=3Q0, W+=(3-1)W0

+=2W0+, W0

- =(3-2)W0- =W0

-, R=2R0

Rn= nR0~2*n for the large number of bunches

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The previous experiment by joint effort from Euclid Techlabs and AWA (2006)*

Measured Enhancement

factor of R2/R1=1.31

Inferred R2=2.3

* Funded by DoE SBIR Phase II

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The latest experiment by joint effort from Euclid Techlabs and AWA (2010)*

What’s the same comparing to the previous experiment? Same Ramped Bunch Train Technique. Same DLA Structure

What’s new comparing to the previous experiment? Laser stacking technique to elongate the bunch

length of AWA beam. Improved data taking conditions (upgraded LLRF, remotely

controlled delay line for the witness bunch, independent controlled shutter for each bunch, etc.)

*Results will appear in PRSTAB soon.

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time (ps)

Inte

nsi

ty (

arb

. u

nit.

)

20 10 0 10 20time (ps)

Inte

nsi

ty (

arb

. u

nit.

)

20 10 0 10 20

7 mm

-BBO-BBO

14 mm

2 crystal set

Laser stacking for RBT Experiment

FWHM~24 ps

FWHM~8 ps FWHM~24 ps

Streak camera measurement

Bunchlength=2.7mm from Parmela simulation

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Measurement

Direct measurement of the wakefield transformer ratio for a single bunch:

)(/5.2

)(/1.41

STDnCKeV

meannCKeVW dNormalized selfwake

)(/7.8

)(/7.79

STDnCKeV

meannCKeVW dNormalized wake behind

R1=1.94

Tune spacing and charge ratio to achieve: 11

21

d

dd

W

W

Direct measurement of the wakefield transformer ratio Enhancement after the 2nd bunch:

)(21

)(6.14421

STDKeV

meanKeVW dd

Wake behind the 1st bunch

)(1.15

)(4.821

STDKeV

meanKeVW d

R2/R1=1.75Wake behind the 2nd bunch

R2=3.4

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II. Experiment on the 1st Tunable DLA Structure*

* Funded by DoE SBIR Phase II

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Geometric and accelerating parameters value

Radius: b0, b1, b2 4.79mm, 6.99mm, 7.49 mm,

Effective Length 101.6mm

Dielectric constant: dielectric, ferroelectric 6.8, 310 (at room Temp.)

Loss tangent: dielectric, ferroelectric 2*10-4, 2*10-3

Freq. of two dominant wakefield modes 7.8GHz, 14.1GHz (at room Temp.)

Q of two dominant wakefield modes 385, 1250

Peak wakefield by 50nC drive bunch (z=2.3mm) 16MeV/m

Tunable DLA Structure

By introducing an extra nonlinear ferroelectric layer which has dielectric constant sensitive to temperature and DC, the frequency of a DLA structure can be tuned on the fly by controlling the temperature or DC voltage.

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Ferroelectric layer

Ceramic layer

Bench Test

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Wakefield Experiment

The experiment demonstrated that by varying the temperature of the structure over a 50C temperature range, the energy of a witness bunch at a fixed delay with respect to the drive beam could be changed by an amount corresponding more than half of the nominal structure wavelength.

26.2cm

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Bench Test---DC Voltage

Although introduction of a high DC voltage to a tunable DLA structure in the vacuum environment appears to be challenging at this moment, this approach is still attractive because of its extremely short response time compared to the temperature control. One can conclude the best solution for the future tunable DLA structures would be a combination of ”coarse” but slow temperature tuning by 100s of MHz and rapid fine tuning with high voltage dc biasing applied.

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Summary

•Wakefield transformer ratio of 3.4 has been achieved in the recent experiment at AWA facility with help of the elongated bunch length.

•A novel low loss BSTM ferroelectric material has been used in dielectric based accelerators as a method of frequency tuning. Wakefield acceleration experiment show an excellent tuning capability through control of either temperature.