SLAC Accelerator Development Program: X-Band Applications

Chris Adolphsen

OHEP Accelerator Development ReviewJanuary 24-26, 2011

X-band Development Objectives• Long Term: Provide Higher Energy Reach for a Linear Collider

– CLIC assumes ~100 MV/m acceleration in X-band structure powered with a drive beam

– Collaborating with them to use X-band klystrons to drive the structures as a first stage to reduce the technical risk and allow earlier construction

• Short Term: Other Applications – X-band ideal for compact (50-100 MV/m gradient), low bunch charge (e.g.

250 pC) linacs, in particular light sources similar to LCLS– Allows developments from the US High Gradient program to be

implemented on a larger scale

• Immediate: Expand Vendor Base– Improve X-band klystron performance and have industry build them (see

previous talk).SLAC Accelerator Development Program

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World’s Highest Gradient (80 MV/m)Accelerator in Routine Operation

CLIC Collaboration (CERN/SLAC/KEK)• High Gradient Characterization Prototype CLIC Linac Structures

– T18 series – first optimized structure with ramped gradient design– T24 series – next iteration structure optimized for higher efficiency

• Special Structures/Tests to Explore Breakdown Limits– Pulse heating study with the TD18 structure– Dual Mode Cavity– C10 structures

• High Power Tests of Power Extraction Structures (PETS)– Nominally driven with 100 A beams, which are not available – Instead test rf-driven versions

• Structure Simulations (see Arno Candel’s talk later today)– Dark current propagation within structure– Dipole mode propagation between the PETS and main accelerator

structures

SLAC Accelerator Development Program Page 3

CLIC T18-Disk Structure

Field Profile Along the Structure

Cells 18+input+output

Filling Time: ns 36

Active Length: cm 17.5

a/λ (%) 15.5 ~ 10.1

vg/c (%) 2.6 - 1.0

Phase Advance Per Cell 2p/3

Power Needed <Ea> = 100 MV/m 55.5 MW

Es/Ea 2

Require breakdown rate < 4e-7 /pulse/m with 230 ns pulses

The gradient including beam loading will be 10-20 MV/m smaller

Structure Installed in NLCTA for Testing

TTFCouplers

Marx Modulator

X-Band2*300 MW

L-BandLasers

E-163 DLA

S-Band

Echo-7RF TestingXTA

L-band Big Pipe

X-band2-Pack

500 MW

Also have two X-band rf stations in the Klystron Test Lab (ASTA) for structure/component testing

0 50 100 150 200 250 3000

20

40

60

80

100

120

BKD Rate (hr-1)Average gradient (MV/m)

50 ns 100 ns 150 ns 200 ns

Processed structure by progressively lengthening the pulse at constant gradient (110 MV/m)

Second T18 Structure Tested at SLAC

T18 by KEK/SLAC at SLAC #1

T18 by KEK/SLACat KEK

T18 by CERNat SLAC

TD18 by KEK/SLACat SLAC

TD18 by KEK/SLACat KEK

Unloaded Gradient [MV/m]

T24 by KEK/SLACat SLAC

1400

3900

550

Total testing time [hr]

1100

3200

200

T18 by KEK/SLACat SLAC #2

280

60 70 80 90 100 110

HOM Damped

HOM Damped

Gradients Achieved at a CLIC-Acceptable Breakdown Rate

Pulsed Heating Effect in TD18It appears that for pulsed temperature

increases above ~ 50 degC, the breakdown rate becomes strongly dependent on this heating

SLAC Accelerator Development Program Page 8

* Phys. Rev. ST Accel. Beams 14, 010401 (2011)

Cutaway view of 1/8 of the TD18 cell showing its surface magnetic field *

Dual Mode Cavity for Heating/Field Study

Built rf cavity resonant in two modes, which when driven independently, allow the rf magnetic field to be increased on the region of highest electric field without affecting the latter

So far have powered the TEM3 mode, achieving 200 MV/m surface fields

SLAC Accelerator Development Program Page 9

* Phys. Rev. ST Accel. Beams 14, 010401 (2011)

Top) electric field on the center conductor with 3.3 MW driving the TEM3 mode

Bottom) center conductor magnetic field with and without 18.3 MW driving the TE011 mode - on the same scale with a peak of 1 MA/m

Structure Performance vs Group Velocity

• At ASTA, have measured two 10 cell, constant impedance TW structures (C10), with group velocities of 1.35 % and 0.7 % of c.

• Breakdowns appear to be mostly on first regular cell – in process of redesigning the coupler

SLAC Accelerator Development Program Page 10

50 100 150 20010 -7

10 -5

0.001

0.1

Gradient [M V /m ]

Brea

kdw

nPr

obab

ility

[1/p

ulse

/m]

260 ns

130 ns

RF

Power Extraction Structures (PETS) • Large aperture (23 mm, vg = 46%c) structures that weakly

couple to a 100 A drive beam to extract rf power• Second rf-driven version tested at ASTA, which included

HOM absorbers, met breakdown rate specs at 135 MW

SLAC Accelerator Development Program Page 11

56 MV/m Max at 135 MW

High Power (Multi-MW) X-Band Applications

• Short bunch FELs– Energy Linearizer: in use at LCLS, planned for BNL, PSI, Fermi/Trieste and

SPARX/Fascati– Deflecting Cavity for Bunch Length Measurements

• 100’s of MeV to Many GeV Linacs– LLNL 250 MeV linac for gamma-ray production (MEGa-ray)– LANL 6-20 GeV linac for an XFEL source to probe dense matter (MaRIE)– Trieste 1 GeV linac extension– Alternative to SC for the proposed 2.25 GeV NLS linac– 2.6 GeV linac for a soft X-ray FEL facility at KVI, U. Groningen, NLD– SLAC study of a 6 GeV Linac for a Compact XFEL (CXFEL) source– X-band Gun (with LLNL) and Test Beamline

SLAC Accelerator Development Program Page 12

X-Band Energy ‘Linearizer’ at LCLSA

fter B

C1 sz =

227 mm

Non-linearity limits compression… …and spike drives CSR

sz =200 mm

sz = 840 mm

Energy-Time Correlation

X-Band Structure: 0.6 m long, 20 MV

LLNL 250 MeV X-band Linac forCompton Gamma Ray Production

LANL MaRIE Project: 50 keV XFEL

20 GeV, 70 MV/m X-band Linac(space limited)

C-band structures at 26 MV/m10 m

C-band- Klystron 5.7 GHz, 50 MW, 2.5 μs, 100 Hz

120 MW 0.5 μs

40 MW 2.5 μs

30 MW30 MW

3 dB

3 dB3 dB

30 MW30 MW

SwissFEL Main Linac Building Block

SLED RF pulse compressor

Hans Braun: “X-band was not considered because no commercial klystrons available”

Recently issued bids to have two vendors each build a 50 MW XL4 klystron

Linac-1250 MeV

Linac-22.5 GeV

Linac-36 GeV

BC1 BC2

UndulatorL = 40 mS

RFGun

X

undulator

LCLS-like injectorL ~ 50 m

250 pC, gex,y 0.4 mm

X-band Linac Driven Compact X-ray FEL

X X

X-band main linac+BC2G ~ 70 MV/m, L ~ 150 m

Use LCLS injector beam distribution and H60 structure (a/l=0.18) after BC1

LiTrack simulates longitudinal dynamics with wake and obtains 3 kA “uniform” distribution

Similar results for T53 structure (a/l=0.13) with 200 pC charge

Units CXFEL NLCBeam Energy GeV 0.25-6 2-250Bunch Charge nC 0.25 1.2RF Pulse Width* ns 150 400Linac Pulse Rate Hz 120 120Bunch Length μm 56, 7 110Linac Gradient MV/m 70 65

Operation Parameters

* Allows ~ 50-70 ns multibunch operation

CXFEL wakefield effects are comparable at the upstream end of the linac as the lower bunch charge and shorter bunch length offset the lower energy, however the bunch emittance is 25 times larger

Layout of CXFEL Linac RF Unit

50 MW XL4

100 MW 1.5 us

400 kV

480 MW150 ns

12 m

Nine T53 Structures (a/l = 13%) or Six H60 Structures (a/l = 18%)

Power and Field Levels Already Demonstrated !!

5.59 Cell X-Band Gun

200 MV/m at Cathode

X-Band Gun Development (with LLNL)

Emittance ~ 0.5 micron for a 250 pC Bunch, Longitudinal Emittance Less Than ½ of that at LCLS

Comparison of 4D emittance along the gun computed with ImpactT (‘instant’ space charge) and PIC 3D (‘delayed’ space charge plus wakes with true geometry) at two bunch charges and three laser offsets

Optimization Using Stacked LasersAt NLCTA, will be able to run

short laser pulses and stack two pulses.

For 250 pC bunches, emittance = 0.3 mm (95% particles) with single Gaussian (500 fs FWHM) vs 0.25 mm (95% particles) with stack of two Gaussians (300 fs FWHM each).

s = 0.76 ps

New Gun/Structure Beamline NLCTA

Adding a new beamline segment to NLCTA to characterize rf photocathode guns and to test high gradient structures

Major portion of the FY11 funds directed at this project

Cut-away view of an X-band Deflecting Cavity for bunch slice diagnostics

Summary• Our X-band program builds on the 15 year effort at SLAC

to develop this technology for a linear collider• Have extensive resources including several high power

X-band stations and experts in rf and accelerator design• There has been a revival of interest in X-band in recent

years with the– Adoption of the technology by CLIC– Use of X-band linearizers in existing FELs– Desire for compact linacs for future light sources

• Our program is geared to meeting this growing demand, while in the long term, seeking a cost effective solution for a multi-TeV collider

SLAC Accelerator Development Program Page 24