TWO-CHANNEL RECTANGULAR DIELECTRIC WAKE FIELD ACCELERATOR STRUCTURE

EXPERIMENT*

G. Sotnikov,1,2 T.C. Marshall,1,3 S. V. Shchelkunov,4 A. Didenko,1,5 J.L. Hirshfield1,4

1Omega-P, Inc., 199 Whitney Ave., New Haven, CT 06511 USA

2NSC Kharkov Institute of Physics and Technology, Kharkov, Ukraine3Columbia University, New York, NY USA

4Beam Physics Laboratory, Yale University, 272 Whitney Avenue, New Haven, CT 06511 USA5Fermi National Accelerator Laboratory, Batavia, IL 60510 USA

Outline of talk:• General aim is to understand subtleties of a two-channel DWFA.• Why two channels?• Tradeoff between transformer ratio and accelerating gradient.• Power radiated into modes of the structure.• Deflecting forces.• Outline of experiment planned at ANL with W.Gai, J.Power, and

M.Conde. • A new concept for a symmetric two-channel DWFA.

*Sponsored by US DoE, Office of High Energy Physics

CROSS SECTIONAL VIEW OF 2-CHANNEL DWFA STRUCTURE

Wide channel is for high-current low-energy drive beam, while narrow channel is for low-current high-energy test beam.

Advantages:• Separate channels can simplify separation of beam transport issues.• Energy flow is continuous from drive beam to test beam. No coupling structures.• Transformer ratio >>2 is possible, but at the expense of accelerating gradient.• Axially-uniform structure is relatively simple to fabricate.

Disadvantages:• Dielectrics can impede thermal transport, leading to cooling issues.• Transverse asymmetry can lead to excessive deflecting forces & emittance

growth.

THEORETICALLY-PREDICTED ACCELERATING & DRAG FORCES

(for the witness bunch located at z = 0.845 cm behind the drive bunch center)design mode LSM31

acceleratingchannel

2 6 mm2

drive channel 12 x 6 mm2

slab 1 thickness

1.24 mm

slab 2 thickness

2.29 mm

slab 3 thickness

1.05 mm

slab 4.76

drive bunch parameters

Gaussian length

z = 2 mm

energy 14 MeV

charge 50 nC

number of bunches

1

transformer ratio

~12.6

THEORETICALLY-PREDICTED TRANSVERSE FORCES IN BOTH CHANNELS

Transverse deflection is not insignificant, but witness bunch remains in its channel over its entire 12 cm length. Transverse wake forces can be greatly reduced for a

three-channel structure. (Hear the following paper by T.C. Marshall, et al.)

Mode powers, test beam deflection, energy gain

14 MeV, 50 nC drive bunch (6x2x4 mm3)

Cross section of 2-channel DWFA experimental structure

Channel shown below is cut into a copper block using EDM, and the three cordierite slabs are to be slid into place.

Engineering design for 2-channel DWFA experiment

At left, view along beam axis is shown. At right, view from above is shown.

A TWO-CHANNEL DWFA WITH GOOD SYMMETRY

(requiring a stable high-charge annular drive bunch)

AXIAL FORCES ON DRIVE AND TEST BUNCHES IN COAX DWFA

4-mm long, 50 nC annular drive bunch head is at z = 100 mm;test bunch is at z = 86 mm; transformer ratio 11/1.1 = 10

TRANSVERSE FORCES ON DRIVE AND TEST BUNCHES IN COAX DWFA

4-mm long, 50 nC annular drive bunch head is at z = 100 mm;test bunch is at z = 86 mm, r = 0.8 mm; transformer ratio 10

Note that transverse force on test bunch is stabilizing!

SUMMARY

Properties of 2-channel rectangular DWFA have been simulated and calculated for experiments based on ANL bunch parameters.

Double-pulse RF photocathode is to create a 50 nC drive bunch and a smaller time-delayed test bunch.

Energy gain and radiation spectrum will be measured.

This is the first experiment designed to elucidate the performance of a two-channel, two-beam DWFA.

Evolution into a working accelerator requires invention of a multi-channel structure with higher symmetry than the rectangular structure.

Preliminary simulations for coax DWFA show good symmetry with a high transformer ratio and intrinsic transverse stability of the test bunch.

The rectangular two-channel structure may be a good candidate for a fast kicker.