Laboratory Directed Research and Development

Proposal Title

Lead Scientist: / Chris adolphsen
Phone: / 3560
Email: /
Date: / 3/17/2010
Department/Division/
Directorate: / Linac Design/ARD/AD
Other Scientists: / Cecile Limborg, Feng Zhou and Arnie Vlieks
Proposal Term / From: 10/2010
Through: 09/2011

Published By:

SLAC National Accelerator Laboratory

2575 Sand Hill Road

Menlo Park, CA94025

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SLAC National Accelerator laboratory

Abstract

This proposal is to evaluatean X-band (11.4 GHz) rf photocathode gun for use as a low emittance electron sourcefor future accelerator applications, in particular, for free electron lasers (FELs). The approach entails both modeling the beam dynamics of X-band injectors and measuring the beam parameters of a SLAC X-band gun in anew test area at theNext Linear Collider Test Area (NLCTA).

Summary of Proposal

Description of Project

The performance of free electron lasersFELs depends strongly on the transverse size (emittance) of the electron bunches which generate electromagnetic radiation as they traverse the undulators.in which the light is generated. This size is generally limited by the properties of the bunches as they emerge from their source, typicallywhich is generallyan rf gun consisting of a copper rf accelerator cavity andin which a laser is shined on the backplane to liberate electrons from its backplane. The electron source for the LCLS facility at SLAC is an rf gun running atof the same frequency (2.9 GHz, S-band) as the rf used to accelerate the bunches in the SLAC lLinac. This project will explore whether a higher frequency (11.4 GHz, X-band) gun, in which theacceleratingor gradient can be twice as large, can produce lower emittance betterbunches that can be used tofor generateingelectromagnetic radiation light in free electron lasers.

This project is an ideal match to the technical capabilities and science directives at SLAC. It merges the 11.4 GHz X-band rf technology developed over the last two decades at the lab with its mission to develop forefront accelerators and light sources for photon science research.

Expected Results

From beam dynamics simulations done so far, the bunch quality from an X-band based electron source isas goodsimilar or better than the present S-band gun used atthat at LCLS. In particular, the bunch transverse size is comparable but because the rf frequency is four time larger, the bunch length is about four times shorter. With additional optimization the transverse emittance is expected to be even smaller. The shorter bunch length, which puts less demand on the downstream bunch compression stages of an FEL, and perhaps can be exploited in an emittance exchange lattice to further reduce the transverse emittance.

Proposal Narrative

Purpose/Goals

The main purpose of this proposal is to evaluate the transverse and longitudinalemittances of bunches from anX-band rf gun to see if itoffers an advantages over S-band guns, such as the one used at LCLS. An X-band gun at SLAC has been shown to operate reliably with a 200 MV/m field at the cathode,which is almost twice as large assignificantly higher than the115 MV/m gradient in the LCLS gun. The higher gradient should result in a smalleroughly balance the space charge related transverse emittance growth for the same bunch charge.,but Also with a four-times shorter bunch length due to the four times higher frequency, which would make the subsequent bunch compression will be easier. SAlso, such a gun can be also be used with an X-band linac to produce a compact FELlight source that would require only one rf frequency for beam generation and acceleration.

The two figures below show examples of simulatedbunch size and bunch emittance evolution through a 200 MV/m, 5.5 cell X-band gun with solenoidal focusing, followed by two, 53 cm X-band structures operating at 70 MV/m. The left plot is for a 33 pC charge with 0.07 um thermal emittance and the right plot is for a 333 pC bunch charge with0.22 um thermal emittance. These thermal emittance values are larger than theoretical estimates but somewhat smallerthan those measured at LCLS. The resulting normalized projected emittances are0.12 µm for 33 pC bunchesand 0.51 µm for 333 pC bunches, and probably can be reduced with further optimization.For comparison, emittances of0.20µmfor 20 pC bunchesand 0.7 µmfor 250 pC bunches have beenmeasured at LCLS.

Approach/Methods

To evaluate the performanceofX-band guns, we would continue to improve the gun and accelerator structure modeling to include allrelevant effects on the beam. For example, the SLAC ACD group has a 3D PIC code that wouldallow us to fully gauge the effect of wakefields in the gun. We wouldalso continue to collaborate with the MegaRay group at LLNL, which plans to use an X-band gunas an injector for a 250 MeV accelerator. The combined beam simulation efforts thus far have shown that alignment, field and timing errorsshould be manageable.

In parallel, a gun test stand would be setup in the NLCTA aAlcove areaand an X-band gun would be evaluated there. This area is currently unused but will be upgraded in FY10 to allow high gradient testing of X-band structures. The following equipment would be refurbished and re-used for this project:

1.aA partially assembled X-band gun thatis availablethat isis a “‘spare”’ for one that Arnie Vlieks designed and built for an earlier Compton experiment at SLAC. A new, low quadrupole component (“‘racetrack”’) coupler would be built for itand brazed to the existing gun cell stack.

2.In addition, a solenoid also from the Compton test. is available,Itwhich may need some modifications,

3. and quadrupole magnets with steering correctors and BPMs will become available after the NLCTA chicane is reconfigured this year.

4.aA number of X-band accelerators from the NLC program are available and one or two would be used to increase the bunch energyto about 60 MeV so accurate emittance measurements can be made. THowever, these structures would need to be retrofitted with racetrack couplers.

5.Also, we would take advantage of the 266 nm wavelength laser systems at NLCTA that was used for the E163 experiments.,We willand install a vacuum line to transport the picossecond-long light pulses to the gun cathode to produce the electron bunches. Finally,

6.parts from the now dismantled Eight-Pack rf distribution system at NLCTA would be used to build an rf splitter system with remote phase and amplitude controlto power the gun..

Besides using/modifying/extending existing components and systems, new beamline diagnostics (charge and profile monitors)and steering magnets would need to be acquired.TAlso, the proposal would also fund the efforts to coordinate the assembly and installation work, operate the test stand and to take and analyze the required data.

Specific Location of Work

As noted above, the gun measurements would be made in theNLCTA aAlcove areawhichthat will be configured to do high gradient structure testing. The Station 1 X-band rf will be plumbed into this area with circulator waveguide to provide up to 300 MW of X-band power in 250 ns long pulses at 60 Hz. The area will also come equipped with a spectrometer magnet, profile monitor and faraday cup that will be used to measure structure dark current, but can also be used to characterize accelerated bunches.Below are illustrations of the X-band gun and how the aAlcove area would lookupstream of the spectrometer to test the gun as proposed.

Anticipated Outcomes/Results

Based on simulations done thus far, we expect the bunch emittance from the X-bandgun to be comparable to or better than the S-band gun that at LCLS for the same charge. With the shorter bunch length, using such a gun would be an advantage for accelerator applications such FEL and Compton light sources, especially if emittance exchange can be used to further reduce the transverse phase space.

Budget

This project would be completed in FY11 and would require 0.5 FTE of labor for simulations and 0.5 FTE of labor forthe experimental work. The total labor cost with indirects would be about 220 k$. Specific M&S costs (with indirects) include: gun racetrack coupler and assembly (30 k$), solenoid modifications and bake-able gate valve (20 k$), diagnostics, quads and steering magnets (50 k$), new racetrack couplers for an accelerator structure (50 k$), laser plumbing including optics (75k$), and a waveguide splitter to power the gun (40 k$). These items total 265 k$,so with labor, the FY11 budget would be 485 k$.

Approvals

Department/Division Administrator

Department Chair/Division Manager

Associate Laboratory Director

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