The Frascati C-band structure and the high-power test of the cavity at KEK

B. Spataro (LNF-INFN, Frascati, Italy)

on behalf of the SPARC/X C-Band group

SLAC, XB11 17/05/2011

Design: D. Alesini and V. Spizzo (Univ. of Rome “La Sapienza”)

Mechanical design and Realization: D. Alesini, V. Lollo, R. Di Raddo, P. Chimenti

Test at KEK:D. Alesini, L. Palumbo, R. Boni, A. Gallo, S. Quaglia, F. Marcellini, A. Battisti, G. Di Pirro, R. Clementi

Toshiyasu Higo, Shuji Matsumoto, K. Kakihara (KEK)Silvia Verdu Andres (CERN)

Klystron, Modulator, Installation, logistics, financial support: R. Boni, G. Di Pirro, L. Palumbo

Beam dynamics: M. Ferrario, V. Fusco, C. Vaccarezza

SLED design: B. Spataro, F. Marcellini

THANKS TO ALL CONTRIBUTORS

OUTLINE

1) Design Overview

2) Realization

3) High power RF test at KEK

SPARC

RF GUN:-2.856 GHz-1.6 cell

LINAC:-S BAND-3 sections (3 m long)

UNDULATORS

-Diagnostics (RF deflector)-quadrupoles matching-seeding

Solenoids

SPARC PHOTO-INJECTOR

E ≈ 170 MeVGUN

KlystronN°1

≈ 13 MV/m

ACC. SECTION ACC. SECTION

20 ÷ 22 MV/m ≈ 130 MeV

ACC. SECTION

KlystronN°2

PULSECOMPRESSOR

ATT

3 dBsplitter

SPARC ENERGY UPGRADE WITH C BAND SYSTEM

C-bandStation

C-bandENERGY

COMPRESSOR

C-band acc. structures> 35 MV/m

5712 MHz – 50 MW

1.4 m 1.4 m

≈ 50 MW(Half pulse)

≈ 50 MW

≈ 100 MW/0.20μs2.5 μs

E > 240 MeVThe new C-band system consists mainly of:

2 accelerating sections (1.4 m long)

1 C-band klystron (50 MW), by Toshiba Ltd (JP)

1 Pulsed HV modulator supplied by ScandiNova (S)

WR187 waveguide system

400 W solid state driver supplied by MitecTelecom (CDN)

SLED

The SPARC energy will be upgraded from 170 to >240 MeV by replacing a low gradient S-band traveling wave section with two C-band units. We decided to implement this system at SPARC to explore the C Band acceleration (RF components, construction at LNF TW sections, SLED, syncronization) in hybrid scheme with S Band:

-Higher gradients -Very promising from the beam dynamics point of view

Single cell design: optimized to work with a SLED RF pulse

Splitter: to have a symmetric feeding system.

Coupler design: based on the “new generation” of couplers proposed at SLAC for high gradient X Band structures (waveguide couplers)

C BAND structure

THE C BAND STRUCTURE AND THE PROTOTYPE

Previous the construction of the two TW sections, a prototype with a reduced number of cells, has been realized. The goals of this prototype were:

-test all design and construction procedure -test the structure at high power at KEK (Japan) in the framework of a

collaboration between LNF-KEK.

(and many dicussions with V. Dolgashev during his visit in Italy)

SINGLE CELL DESIGN

Elliptical shape

The structure has been designed as TW constant impedance (cells with equal radius) in order to:

-simplify the fabrication

-reduce the peak surface field when the structure is feed by a SLED pulse

-reduce the unbalance between the accelerating field at the entrance and at the end of the structure, due to the combination of power dissipation along the structure and SLED pulse profile.

The single cell has been designed:

-exploring the different TW cell parameters as a functiion of the iris aperture (a), thickness (t) and ellipticity (r1/r2).

-combining the parameters with the real RF SLED pulse and calculating the maximum surface field, accelerating field,....

Elliptical shape(V. Dolgashev)

STRUCTURE FINAL PARAMETERS ( with no coupler)

PARAMETER prototype Final structure

Frequency (fRF) 5.712 [GHz]

Phase advance per cell 2/3

Number of accelerating cells (N) 22 71

Structure length included couplers (L) 0.54 [m] 1.4 [m]

group velocity (vg/c): 0.0283

Field attenuation () 0.206 [1/m]

series impedance (Z) 34.1 [M/m2]

Filling time 50 [ns] 150 [ns]

Es peak/Eacc 2.17

Hs peak @ Eacc=35 MV/m 87.2 [kA/m]

Pulsed heating @ Eacc=35 MV/m <1 oC

Average Accelerating field with SLED input pulse and after one filling time

60.2 [MV/m] 51.6 [MV/m]

Peak surface field with SLED input pulse at the beginning of the pulse (max and @35 MV/m)

140 (96) [MV/m]

Energy gain (max and @ 35 MV/m) 23.2 [MeV] 64.1 (42) [MeV]

Output power 0.85Pin 0.60Pin

Average dissipated power @ 10 Hz with SLED pulse length equal to one filling time

7.6 [W] 59.6 [W]

The dimensions of the single cell have been optimized to simultaneously obtain:

-lowest peak surface electric field on the irises with the SLED input pulse;

-an average accelerating field of, at least, 35 MV/m with the available power from the klystron;

-the largest iris aperture for better pumping, reduced wakefields contribution and reduced filling time of the structure (related to the breakdown rate probability).

COUPLER DESIGN The design of the coupler has been divided in two parts:

-the design of the waveguide coupler;

-the design of the splitter.

Advantages: -integrated symmetric feeding

-low H field pulsed heating

-Better pumping

Disadvantage: -multipolar field components (quadrupole)The analysis of the multipolar field components have been done (see technical note (D. Alesini et al.).

waveguide coupler

splitter

STRUCTURE REALIZATION

REALIZATION OF CELL AND COUPLERS @ COMEB S.r.l.

Output coupler:Electro discharge machiningRa<1m tolerances 20m

Input couplercumputer controlled

milling machineRa<0.2m tolerances 10m

Cells Turning machine “Shaumblain”

Cell Ra<0.05m tolerances 2m

Cooling pipes

Tuning by deformation

BRAZING @ LNFThe structure has been brazed @ LNF in several steps:-Cells-Couplers-Final brazingIntermediate RF measurements have been done during realization

Main problem during the realization: -alignment of coupler and cells in the final brazing process

Prototype

SLED

HIGH POWER TESTS AT KEK: TEST AREA (1/2)

PF=klystron output power

STRUCTURE CHARACTERIZATION AT HIGH POWER (2/2)

m

bpp

Lft

breakdownsnoBDR

struct

shotsno

rep.

.

See S. Verdu AndresTalk for comparison with X-Band BDR

TE037 storage cavity

TE028 storage cavity

Frequency 5712 MHzQo = 135000 (unloaded)

β = 6ΔF ~ 13 MHz (min. frequency separation )

Relevant sizes : L ~ 24 cm ( cavity axial length) L/D = 1 ( with D the cavity diameter)

For the SLED design we investigated cavities with ellipsoid – like shape in order to increase the quality factor Qo and remove the modes degeneration.P. Fernandes, R. Parodi, C. Salvo, B. Spataro: "The Design of the TE Storage Cavities for a RF Pulse Compressor System". NIM A 288 (1990) pp. 549-554.

CONCLUSIONS FOR THE C BAND

1) Design: SLED oriented, large irises, low filling time (D. Alesini, et al., SPARC note RF-11-002, 2011)

2) Realization: LNF+ local companies

3) Results of high power RF test at KEK: 50 MV/m with BDR<1e-6

As responsible of the Frascati Laboratory for the technological transfer activity, we have given to the local private company (COMEB S.r.l. involved in our projects ) a strong economic impact, new know – how, new permanent recruitment, new orders from other institutions ( domestic and international) and so on, in the order of the following areas:

1) X - Band : thank you very much again to S. Tantawi and V. Dolgashev for them help !!! ;2) RF deflectors designed and realized for the CERN,PSI,ELETTRA ;3) First PETS prototype fabricated for the CERN ;3) New Frascati RF gun ( fabrication in progress);4) Design and realization of the Frascati C band section ;5) Training for young scientist as : L. Faillace (Radiabeam), A. Falone( PSI), C. Vicario (PSI) and many other ones !!!

Some words for the X-Band activities status

The work of the X-Band fabricated with the EBW is made possible by the efforts :

SALAF Group, INFN - LNF

V. Dolgaschev, S. Tantawi , A.D. Yeremian, SLAC

Y. Yigashi, KEK

L. Palumbo, University of Roma 1

5th Collaboration Meeting on X-band Accelerator Structure Design and Test Program May 16h -18th, 2011- National Accelerator Laboratory Menlo Park, SLAC

Triple choke standing wave structure

Studies on the mechanical drawings in order to separate vacuum and RF-joint to test molybdenum and hard alloy structures are in progress, too.

(A.D. Yeremian, V.A. Dolgashev, S.G. Tantawi, SLAC)http://accelconf.web.cern.ch/accelconf/IPAC10/papers/thpea065.pdf

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00Distance [mm]

0.00E+000

5.00E+007

1.00E+008

1.50E+008

2.00E+008

2.50E+008

3.00E+008

3.50E+008

Co

mp

lexM

ag

_E

Ansoft LLC HFSSDesign1XY Plot 4

m1

m2

m3

Name X Y

m1 27.7200 152518884.2512

m2 40.8100 304028128.6167

m3 54.6700 151671480.9496

Curve Info

ComplexMag_ESetup1 : LastAdaptiveFreq='11.42219GHz' Phase='0deg'

Preliminary Solid Model

11.410 11.415 11.420 11.425 11.430 11.435Freq [GHz]

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

dB

(S(I

np

utP

ort

Cp

lr:1

,Inp

utP

ort

Cp

lr:1

))

Ansoft LLC HFSSDesign1XY Plot 1

m1

Curve Info

dB(S(InputPortCplr:1,InputPortCplr:1))Setup1 : Sw eep1

Name X Y

m1 11.4222 -26.3351

mode e.m. distribution

mode fieldprofile on axis

mode coefficientreflection

Use of the Electron Beam Welding technique

EBW was used for a welding test of an X-band cavity sample.

Sample pre-bonding @ 300°C

0.04 mm

0.6 mm

cavity

Tool to keep together the 2 half-cavities during pre-bonding

The pre-bonding is used in order to prevent :

1) microgaps left by welding (on the cell surface)[vacuum leakage tests gave about 10-10 mbar litre/sec]2) accidental pocket air inclusions3) EBW damages in the internal surface of the structure

3 mm

EB welded sample

Prototype ready to beused for the EB technique

Cross section of the prototype

…… use of the Electron Beam Welding technique

The welding meets the requirementsof the applicable specification SI 01.003 revANo cracks have been found in the fusion zoneand in the heat affected zone.There are only small porosities at the root-side of the weld joint which are, however, within the limits of the specs.

Moreover dimensional mechanical tests before and after welding gave negligible difference.

Macrographic inspection of EB welded joints, made on a X-band test specimen

pre-bonding zone

EBW zone

The joints in the pre-bonding region demonstrated to work well andadditional tests are in progress, too.

Macrographic inspection of EB welded joints, made on a X-band test specimenCarrried out by HTC (High Technology Center April 14 – 2011)O

Test Specimen (after welding process)

Specimen Identification

The joints in the pre-bonding region demonstrated to work well. Vacuum leakage tests after the Welding process gave about 5 10-10 mbar litre/sec

Cavity (inside)

Electron Beam Welding region.Again no cracks have been found in the fusion zone and in the heat affected zone

Triple choke section mechanical drawing

Gasket in order to separate vacuum and RF-joint to test molybdenum and hard alloy structures

Triple choke section solid model

Copper real triple choke cavity afterElectron Beam Welding process

Copper Triple choke fabrication

Assembling of all cells for making the pre-bonding process

Single cavity

Devices after the pre-bonding process ready for making the Electron Beam Welding process

Prototype

Real model

Bead-pull measurements of triple choke cavity made of Cu and E.B.W. process

Frascati low power test data

Mode Frequency [GHz]

Qo β1 β2

11.442 8368 0.0822 0.039

2/3 11.381 8289 0.1908 0.059

/3 11.342 7962 0.0521 0.076

An about 40 MHz frequency increase of the mode has been obtained after the welding process. Investigations are in progress !!!

Fields profile on axis measurements

Q0=8368

mode

Q0=8289

2 /3mode

Q0=7962

3mode

Triple choke cavity

The baking of the triple choke cavity structure is in progress, too.

Thanks a lot for your attention !!!!