3.9 GHz Deflecting Mode CavityTimothy W. Koeth

October 3 & 4, 2005

History of 3.9 GHz DMC

Cell Shape

Cavity Simulations

The “Other Modes” concern and modeling

R/Qs

Wake Field Simulations

Design: OM couplers

Infrastructure

Deflecting Mode Cavity HistoryThe Deflecting Mode Cavity (DMC) was initially planned for use in the CKM fixed target experiment’s beam line, but now it finds it’s primary use as a diagnostic tool. The operating frequency, 3.9 GHz, was chosen to be a multiple of the A0 facility’s 1.3 GHz system. This will permit the use of the cavity as a beam-slicing device that can measure the longitudinal profile of a short bunch.

13 CELL CAVITY

OFF

13 CELL CAVITY

ON

B-F

ield

head is deflected up

tail is deflected down

Cell shape CONSIDERATION(R/Q)’, G1, and Bmax-surf were studied for 9 shapes

G1 does not strongly depend on iris radius:

A-series: 250Ω

B-series 230Ω

C-series 228Ω

Cell shape CONSIDERATIONFrequency separation between the π-mode and the 12π/13-mode needed to have a relative separation greater than 3E-4

End cells have a slighly different shape to match the beam tube to the structure.

C15 Profile was chosen.

Beam Line SimulationsFull 3-D E-M field made for 13 cell polarized model was generated in HFSS

Uniformity map of the the integrated transverse kick along the the 13 cell cavity

The peak magnetic field along the cavity.

(illustrating field flatness)

The electric field along axis is zero

Iris

Beam Line SimulationsThe 3-D field map was used with the ASTRA code to understand the cavity’s time resolution

Resolution: 10 to 20 fsec

2.0mm 730fsec

The simulation consisted of a bunch composed of two Gaussian peaks of different intensity and generation time. This enabled us to compare the time to spacial correlation preservance.

BLUE: Vertical Position

GREEN: Time domain

CAV. OFF

CAV. ON

Viewing screen Vertical Projection

Intesity [arb]

Intesity [arb]

[Meters] [T

ime]

[Meters]

[Meters]

[Meters]

[Meters]

[Meters]

Lower, Same, and Higher Order Modes

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7000.00

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10000.00

0 20 40 60 80 100 120 140 160 180

Phase [degrees]

TM010 TM020 TM011 TMwierd TM021 Light Cone . . .

m = 0 modes m = 1 modes

m = 2 modes not shown, but modeled.

Requirement: to be maintain the DMC in “standby” in the beam line at 4.5 K.

Does the cavity have any “problematic” modes sitting near the light cone ?

Using MAFIA, dispersion curves for modes up to cut-off were checked

0.00

1000.00

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0 20 40 60 80 100 120 140 160 180

Phase [Degrees]

Determination of R/QR (m)/Q in MAFIA for CKM 13 cell

1.0E-04

1.0E+00

1.0E+04

1.0E+08

1.0E+12

0 2000 4000 6000 8000 10000 12000 14000f (MHz)

R(m

)/

Q (

W/m

(2m

))

m=0 modes m=1 modes m=2 modes

R m

Q 1

r2m

2k m r

2

r2m

dz ˜ E zm r,z e i z / c

2

4U m

Possible Modes of Concern:

We show the R(m)/Q values for all the modes found in the 13 cell MAFIA simulation, in units of Ω for m = 0, Ω/m2 for m = 1, and Ω/m4 for m = 2.

TM010:

-9π/13 R/Q:187

-10 π/13 R/Q:354

-11 π/13 R/Q:129

TM110:

- π (2nd Polr.) R/Q:2.3x106 !

LOM, SOM, HOM Couplers

SAME ORDER MODE COUPLER:

90o rotated Qext:105 Qext-op:108/9

INPUT COUPLER

Qext: 107

Input Coupler

• Qext: 107

• Power Requirement~400 Watts

• Coaxial feed through

HOM CouplerHOM coupler a modification of DESY design

HOM COUPLER

Qext at 3.9GHz 109/10

Qext else: 104/5

Trapped HOMs ?

A study of possible trapped HOM above cut-off, and were compared to R/Q determined by MAFIA.

Freq: 5322 R/Q 243Ω/m2

An example of a potential trapped mode

Knowledge comes from measurement !

Possible solution is to increase energy in the end cells by compressing end cell more than the body cell.

LOM Coupler

LOM COUPLER

SOM COUPLERQext: 109/10 @ 3.9GHz

Qext: 104/5 @ 3.9GHz

LOM at opposite end of input coupler to reduce direct capacitive losses from the input coupler

Current design has pertruding tip only 11mm from cavity axis

F=2702.3 Q=697

F=2945

HFSS model was build from I-DEAS surface mesh file.

Both resonances close to design frequency of 2830 MHz. Notch frequency is 4100 MHz. All resonances in final tuning range.

Frequency [MHz]

LOM Coupler Modeling in HFSS

SOM CouplerSOM coupler will be mechanically adjusted to find the node

of the TM110 mode

LOM COUPLER

SOM COUPLER

SOM COUPLERS

Qext: 104/5 @ 3.9GHz

Polarization Splitting

Δf ~ 50kHz

Polarization SplittingPolarization separation with a copper model

Typical compression is 3mm, which is about 10MHz separation

Wake Field SimulationsBunch Spacing 1μSec

Bunch Charge 12nC

Bunch Length 3pSec

Ebeam 40 MeV

Operating V┴ 3 MV/cavity

Qext 1x106

Rsurf at 3.9 GHz 100nΩ

-3

-2

-1

0

1

2

3

-3 -2 -1 0 1 2 3

Typical Input Parameters:

Position & angle

of offset beam:

0.0

0.1

0.2

0.3

0 20 40 60

Bunch

DM

V0.0

0.1

0.2

0.3

q m

rad

i.e. The 17th bunch, entering 0.5 mm away from the center looses ~200keV and is kicked 32 μrad

A WORK IN PROGRESS !

Vertical Dewar Test Results

RSURF vs TC/T

10.0

100.0

1000.0

10000.0

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

W

Preliminary 3-cell Result R BCS (Nominal, deltaT = 0)

Q vs PDEFL

1.0E+08

1.0E+09

1.0E+10

0 1 2 3 4 5 6 7 8

PT kick (MV/m)Run 1 Run 2 Design Goal

Preliminary

Cavity design parameters 13 cellsBMAX = 80 mT EMAX = 18.6 MV/mL EFF = 0.5 mP = 5 M V/m

Vertical Dewar Test Results

Date Rres(nΩ) Peak Field (MV/m)

Early 2003 250 5.1

Feb, 2003 120 3.6

Feb, 2004 1300 5.4

Dec, 2004 1000 5.4

April, 2005 200 3.3

June, 2005 10,000 N/A

Date Rres(nΩ) Peak Field (MV/m)

June, 2005 190 N/A

March, 2005 220 3.3

Brief Testing History:

Cavity: “Short” Cavity: “Soft”

Cavity: “Thick”

Date Rres(nΩ) Peak Field (MV/m)

August, 2004 60 7.5 !

Possible cause for poor surface resistance:

-Contaminated water system(s)

-Hydrogen poisoning from long acid etch

-Cavity orientation wrt to environmental flux lines ?

-Hydrogen in welded region at about 1ppm ?

- Losses on tip of Input Coupler

Horizontal CryovesselFNAL already has a prototype horiztonal cryovessel that will host two 3.9 GHz cavities. With minor modification, we will retrofit for use in the PhotoInjector Beam line.

Cryogenics Feed

Thermometry, LOM, SOM, HOM ports

RF input Coupler(s)

Capable of hosting two 3.9 GHz cavities.

DMC HLRF

We have a 3kW CW 3.9 GHz Klystron for CKM operation

Power requirement is only 400W for 13 cell

Using DESY’s SimCon Series for LLRF control.

Developement and testing with Superconducting cavities underway at DESY and Fermilab.

Successful demonstration of Piezoelectric suppression of microphonics.

DMC LLRF

Deflecting Mode Cavity Status:

Fall 2005: Fine tuning L/S/HOM design

December 2005: Cavity assemblies in construction

Summer 2006: Finish cavity production

Fall 2006 : Assembled cavity in cryovessel

Winter/Spring 2007: Commission with beam

Acknowledgements

Helen Edwards, Leo Bellantoni, Timergali Khabiboulline, Don Mitchell, Mike Foley, Allan Rowe, Bill Soyars, Peggy Crayton

HiGHLY RECOMMENDED READING: FERMILAB-TM-2144 May 2001

RF Design of a Transverse Mode Cavity for Kaon Separation