slac accelerator development program: x -band rf power sources

SLAC Accelerator Development Program:

X-Band RF Power Sources

Michael V. Fazio

OHEP Accelerator Development ReviewJanuary 24-26, 2011

Outline

• Goals and relevance to SLAC/DOE missions• Historical perspective on SLAC high power X-

band source development• Issues limiting X-band source performance• Future plans• Collaborations and commercialization• Summary

SLAC Accelerator Development Program

Project Goals Support SLAC and DOE Mission Serving the National Interest

• Overall Project Goals– Understand the physical mechanisms limiting the performance of high

power X-band (11-12 GHz) klystrons – Use the knowledge to improve power, pulse width, and reliability as

accelerator power sources. – Increase availability of commercial X-band technology for use in advanced

accelerator applications• Relevance to SLAC/DOE Mission

– Nurtures the 45+ years of SLAC’s unique high power RF experience in support of advanced accelerator technology for DOEs HEP research program

– Advances high power RF source S&T in support of new mission applications

• Compact accelerators• Advanced x-ray and gamma-ray photon sources• Specialized components (linearizers, RF deflectors)


SLAC Has Two Decades of Experience with High Power X-Band Technology

• 1990’s through 2004 R&D effort to develop an X-band klystron for the proposed Next Linear Collider

• Culminated in the development of the solenoid focused XL series of tubes capable of 50 MW in a 1.5 µs pulse and 90 MW at 100 ns pulse width.

• A periodic permanent magnet focused klystron, the XP series, was also designed and tested, and demonstrated 75 MW at 1.6 µs at 120 Hz.


More than 40 klystrons have been built and tested including 18 XL4s and 5 XL5s


SLAC X-Band Klystron Development

Design Specifications

Solenoid Focused PPM Focused (6 Tubes) Solenoid

Tube NameXC

(8 Tubes)XL1-XL4(21 Tubes) XL-PPM 75 XP-1 75 XP-3 75 XP3-4

XL5(5 Tubes)

Frequency (GHz) 11.424 11.424 11.424 11.424 11.424 11.424 12

Peak Pout (MW) 100 50 50 75 75 75 50

RF Pulse Length 1us 1.5us 1.5-2.4us 2.8us 3.2us 1.6us 1.5us

Beam Voltage (kV) 440 440 465 490 490 490 440

Beam Current (A) 520 350 190 257 257 257 350

mP 1.8 1.2 0.6 0.75 0.75 0.75 1.2


XL-4 Klystron Has Been the Workhorse with Some Operating a Few 10,000s of Hours Mostly Below 35 MW

• X-band RF source tube for NLCTA, ASTA , LCLS, and RF breakdown experiments.

• XL4/5 is last iteration of a line of 100 and 50 MW solenoid focused tubes.

• To date 18 tubes have been produced with several tubes having over 10k hrs of filament time.

• Performance specifications- 11.424 GHz- 50 MW peak, 9 kW average - 1.5µs @ 60Hz- Solenoid focusing

We do not know long term reliability at 50 MW

5 XL5s Built for PSI, Sincrotrone Trieste, & CERN


• XL4 variation for higher frequency 11.992 – 11.994 GHz• Same gun, magnet, drift tube, collector and in general

same cavity and waveguide mechanical layout• Performance Specifications

- 50 MW, 1.5us, 100 Hz• A few changes from XL4 to reduce operating gradients

and increase reliability- Cavities re-tuned, output structure improved- Improved output coupler, bends etc.- New window design- Rounded edges

• After processing and operation at ≥50MW at 60Hz for 7 hours, detected no rf breakdown in the tube during a 24 hour heat run.

• XL5 work is directly funded by customers as “Work for Others” and is still ongoing

Observe Breakdown and RF Pulse Shortening as Power and Pulse Length Increase

• Breakdown events damage the output structure• Lifetime limitation issues are not understood• At 75 MW, iris surface field ~ 70 MV/m

– Lower than in 3% vg accelerating structures– but higher than sustainable (~ 50 MV/m) in waveguide with

comparable vg (~ 20%) as the klystron TW output structures.• Need to understand the difference between RF-induced

breakdown and electron-beam induced breakdown


0 200 400 600 800 1000 1200 1400 1600 1800 20000

2

4

6

8

10

12

Time: ns

Kly

stro

n ou

tput

: arb

.u.

0 200 400 600 800 1000 1200 1400 1600 1800 20000

0.2

0.4

0.6

0.8

1

Time: ns

TWT

outp

ut: a

rb.u

.

0 500 1000 1500 2000 2500 3000 3500 4000-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

Time: ns

Mod

ulat

or H

V: a

rb.u

.

In XL4 9 events occurred during 17 hrs running at 50 MW with 1.44 us pulse width.


75 MW XP1

50 MW XL-PPM

Tube Autopsies Show Output Waveguide Coupler Iris Damage from RF Pulse Heating

XL4-12

Waveguide coupling iris

SEM Photos of a 75 MW PPM X-band Klystron Output Section Showing Surface Damage

Developed Approach to Test New Output Structures without Building a Whole Klystron

• Reduces cost and turnaround time for testing new designs

• Should prove useful for comparing different structures

• Caveat: The power enters the cavity differently than when coupled from a bunched electron beam. Power flow and field pattern will not be the same.


Approach uses an RF Driven Output Structure

Pulse Heating Caused by Magnetic Field Driven Surface Currents Are Also an Issue


Thick Lip0.08inch

(2.032mm)

Thin Lip0.024inch

(0.6096mm)

• Pulse Heating Estimation for 75 MW, 400 ns Pulses:– ΔT ~ 112 ºC for thin lip, Bmax=0.82Tesla– ΔT ~ 31ºC for thick lip, Bmax=0.43Tesla– Emax = 86 MV/m for thin lip– Emax = 83 MV/m for thick lip

Test Structures

Larger Iris Radius Improves Breakdown Rate by ~ Order of Magnitude


40 60 80 100 120 14010

-2

10-1

100

101

Power (MW)

Bre

akdo

wn

rate

(1/h

r)

Thin Lip VersionFat Lip Version

1180 ns

430 ns

160 ns

496 ns

240 nsXL4 Klystronat ~ 1000 ns

We Observe that High Temp. H2 Brazing (>1000 C) and Bakeout Improves Surface and RF Performance


Before Etch After Chemical Etch After Heat Cycle

Surface Topography of Copper Cavity Noses (Magnification: 1000x)750 C Heat Treatment

• Heat treatment is a key to improving performance but not quantified• Need to investigate

Future Plans• Lifetime testing the (thick-lipped) XL4• Develop new ideas for more robust output structures• Continue testing rf-powered output sections to see if they have similar

breakdown characteristics as beam-powered sections• In FY12 planning to build an XL4 with a demountable output structure

for rapid test turnaround to enable a variety of structures to be tested under realistic beam conditions

• Enables us to confirm results from the RF-driven output section tests under realistic beam conditions


MAGIC“simulation of 75MW-XP3 illustrates typical beam behavior

Collaborating with numerous institutions to further the development of X-band high power sources

• CERN, Sincotrone Trieste, PSI, Brookhaven, and Lawrence Livermore• Livermore - compact 250 MeV Compton gamma-ray source funded by the Dept.

of Homeland Security. MEGa-ray, uses a SLAC built X-band linac system with an RF photoinjector.

• Los Alamos National Laboratory - pre-conceptual design of a 50 keV x-ray FEL light source for the Matter and Radiation Interaction in Extremes (MaRIE) Project, based on X-band RF and linac technology developed at SLAC.

• CERN - SLAC is developing a staged approach to a multi-TeV linear collider where the first stage relies on an X-band klystron based linac. Could reduce the risk associated with the Two Beam Accelerator technology that CERN is developing resulting in an earlier turn-on of a linear collider.

• LCLS utilizes an X-band structure powered by an XL klystron as a longitudinal phase space linearizer as part of its routine operation. Similar Linearizers using XL klystrons are being constructed for XFEL projects at Trieste, FERMI, and PSI and for the Brookhaven ATF.


Commercialization Effort Underway

• Commercial availability of the klystron is perceived as the major limitation to the use of X-band technology

• Request for Proposals was advertised in 2010 for commercial procurement of an XL type klystron.

• Three Responses were received and are in the evaluation process.

• Will procure 1 or 2 depending on cost.


Summary• Demand for high power X-band sources is clear and

growing for both science and national security applications• On the path to establishing confidence in the reliability of

X-band sources for applications requiring 24/7 operation• Strong leveraging and working with other Labs• Transferring the technology to other labs and industry• Program is cost effective and leverages other work to

advance the technology base and the unique high power RF competency at SLAC

• Demonstrating increasing impact on the field• Program is critical to SLAC and important for the broad

DOE mission


slac accelerator development program: x -band rf power sources

Documents

accelerator power sources

peak power

mw rating

xp series

ns pulse width

project goals support

high temperature

s of hours