1Real-Time Transverse Emittance and Phase-Space Monitor
K. Jordan, J. Song, P. Piot, R. Legg, D. Kehne, R. Li, E. Feldl, G.A. Krafft,J.-C. Denard, G. R. Neil, C. L. Bohn
Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA
Abstract
Jefferson Lab is developing a 10 MeV injector to provide
an electron beam for a high-power free-electron laser (FEL).
To characterize the transverse phase space of the spacecharged-dominated beam produced by this injector, we designed an interceptive multislit emittance diagnostic. It incorporates an algorithm for phase-space reconstruction andsubsequent calculation of the Twiss parameters and emittance for both transverse directions at an update rate exceeding 1 Hz, a speed that will facilitate the transverse-phasespace matching between the injector and the FEL’s accelerator that is critical for proper operation. This paper describes
issues pertaining to the diagnostic’s design. It also discusses
the acquisition system, as well as the software algorithm andits implementation in the FEL control system. First results
obtained from testing this diagnostic in Jefferson Lab’s Injector Test Stand are also included.
1 Introduction
Jefferson Lab is building a tunable high-power free-electron
laser operating in the mid-infrared to study technologies required for high-average-power operation [1]. This FEL incorporates a 350 keV photocathode gun capable of generating a charge-per-bunch beam of typically 135 pC. After
Achromatic bending system
Solenoid

Cryounit

Solenoid
Buncher
Photocathode

Telescope
Multislit mask 2

Multislit mask 1

ε x=8.8mm-mrad, σ x=1.9 mm, σ ’x=0.3 mrad
ε y=6.2 mm-mrad, σ y=1.4 mm, σ ’y=0.2 mrad
ε x=6.6 mm-mrad, σ x=2.3 mm, σ ’x=0.55 mrad
ε y=6.0 mm-mrad, σ y=2.3 mm, σ ’y=0.47 mrad

Figure. 1. Schematic of the FEL injector. Normalizedemittance, beam size and beam divergence at 135 pC bunchcharge are given at the cryounit exit and the injection pointaccording to PARMELA.
acceleration with two CEBAF five-cell superconducting cavities the beam is at 10 MeV and injected via an achromatic
bending system into the driver linac that accelerates the electrons up to about 42 MeV prior to the wiggler interaction.
The transverse emittances at the wiggler location are required to be small (less than 13 mm-mrad) to operate thelaser with a high gain and high extraction efficiency at fullpower. The specified transverse beam parameters at the wiggler constrain the emittance in the injector to be likewise
small (Fig. 1). Therefore, in order to verify the emittancefrom the injector for operations, two multislit transverseemittance diagnostics are going to be installed (Fig. 1) inthe injector. A first monitor, located at the cryounit exit, will
provide the necessary information to set the four matching
quadrupoles of the downstream telescope. The second device allows beam parameters at the injection point in orderto check the injector and linac are properly matched.
2 Multislit technique
Techniques of phase-space sampling for emittance measurement are widely used for space-charge-dominated beams.
The idea is to select one or several sample beamlets, at specific positions, by means of an intercepting mask in whichsmall apertures have been machined. The beamlets retain
the transverse temperature of the original beam at such low
charge that space charge forces do not contribute significantly to the transverse momentum. The beamlets drift
through a length long enough to allow the transverse momentum to impart a measurable contribution to the transverse position. The phase-space parameters can then becomputed using the second-order moments ( x2 , x02 ,xx0 ) of the beamlet profiles [2]. There are several typesof emittance monitors reported in the literature whose differences are essentially in the type of selecting apertures (pepper pot, slit,...) and the profile monitor used (wire-scanner,fluorescent screen, ...). Among the different methods wechoose to intercept the beam with a multislit mask [3,4] since
it provides the fast and precise measurement we need during the commissionning and operation of our injector. The
multislit assembly consists of a series of slits which collimate the space-charge-dominated incoming beam into theemittance-dominated beamlets that drift up to a profile monitor, an optical transition radiation (OTR) viewer in our case.
The principal drawback of this diagnostic is its narrow emittance acceptance compared with other methods [5]. Therefore we optimized the slit design for a specific charge perbunch that we have chosen to be the full operating charge:
135 pC.

h i

h i h i3 Design consideration
3.1 Analytic conditions
Due to mechanical constraints, a drift distance (L) was chosen to be 620 mm. The choice of the geometric parameters(slit width w and spacing d) are then imposed by three conditions.
(i) The beamlets must be emittance-dominated since
space-charge expansion would cause appreciable broadeningof the angular spread over the drift. We calculate the spacecharge-over-emittance ratio defined from the transverse KV equation to be about R 11.75 using the beam parameters predicted with PARMELA. After the multislit mask thevalue of the ratio, R0 , for one beamlet is scaled accordingly
to: R0 = R=12 (w= )2 ,  being the rms beam size. Setting w to 100 m yields a ratio R0 =2.8E-3. This guarantees
the space-charge force contribution to transverse momentumis insignificant.
(ii) The beamlet profiles must not overlap on the screen,
which implies the drift length (L) and the inter-slits distance(d) are related by: 4 0 L < d.  0 is the rms beam divergence.
(iii) Finally, the resolution in position and in divergenceshould be comparable, i.e. =d = L 0 =, where  is theresolution of the detector.
The two latter conditions yield a slits spacing of about
1:1 mm. The values determined for w and d have been used
as a starting point for accurate numerical simulation of theslits with PARMELA.

'

3.2 Optimization with numerical simulations
We developed a MATLAB-based program to retrieve thephase-space parameters and distribution from a simulated
OTR image. The image is generated from the distribution
predicted by PARMELA, retracing each electron from before the slits up to the OTR screen. We optimized the design, after many iterative runs, changing the slit width andspacing, to try to minimize the error in the retrieved parameters over the required emittance range (normalized rms emittance between 4 and 20 mm-mrad). The values we finallyfound reasonable are: w=75 m and d=1:5 mm. According
to our simulation results for different emittances (by scaling
the nominal one) we are confident in achieving a relative error on emittance and Twiss parameters of the order of 10%.
Table I compares the nominal emittance with the one computed from the beamlet’s OTR image.
Table I

Typical retrieved value for the normalized emittanceobtained simulating the multislit mask using a phase-space
distribution predicted by PARMELA

quickly dissipates the heat toward the exterior of the vacuum chamber. In order to reduce the deposited power below
the damage threshold of 3 W we will use a low duty factor
beam mode for emittance measurement, which is possible
since the physics of our beam is only dominated by singlebunch effects (bunch spacing is 8:02 m). The multislit mask
thickness is a compromise between noise and angular acceptance: if the mask is not thick enough, electrons that gothrough copper can contribute to the OTR pattern. On theother hand, increasing the thickness would imply more stringent tolerance on the slit alignment with respect to the beamaxis. After estimating multiple scattering, which is the dominant process for 10 MeV electrons, we set the thickness to5 mm, which yields an angular acceptance of approximately
10 mrad. For this angle 10% of the electrons are lost because of edge scattering. The final design of the multislit
assembly has two sets of thirteen slits that allow x and ytransverse-emittance measurement. When the slits are removed from the beam path, a high-frequency shield insures
beam-pipe continuity to minimize the wakefield impedance.
4 Control and acquisition system
The beamlet’s OTR pattern is monitored with a chargecoupled-device camera whose analog video signal is digitized and processed using a DATACUBE VME-based frame
grabber operating with its own VME input-output controller(IOC). In addition to digitizing, the CPU of the video boardperforms elementary image-processing functions such as
background subtraction, projection along an axis, etc. The
output of the DATACUBE, in our case a projection that
contains the beamlet profiles, is transferred to the IOC on
which we have implemented VxWorks routines. After identifying each beamlet profile and the slit it comes from, thecode computes the emittance and Twiss parameters. The results can then be accessed from any X-station via the EPICS
channel-access protocol. We developed X-window based
screens that display emittance, Twiss parameters and possibly phase-space isocontours. The achieved speeds are, respectively, about 1 and 2 sec for updating parameters andplot refresh, a speed that allows observing the phase spaceparameters in real time while tuning the injector. Storing
raw data and projections is also possible at each stage of theprocess for more detailed off-line analysis, e.g. using (timeand CPU consuming) powerful image processing tools.
5 First results

PARMELA (mm-mrad)

RETRIEV ED (mm-mrad)

8.094
3.947
24.282

8.077
4.014
23.016

3.3 Some mechanical considerations
In order to avoid using any cooling system, we have opted tomake the slits out of copper and have a thermal bridge thatSince the 10 MeV energy region of the injector was not yet
available, we chose to conduct the preliminary tests on theJefferson Lab Injector Test Stand at 250 keV. The configuration is similar to that described elsewhere [5], consisting ofa photocathode gun, a solenoid, and a diagnostic beamline.
The charge-per-bunch was set to approximately 5 pC so thatthe achieved emittance is within the multislit assembly acceptance. Also, because of the low energy, we had to use afluorescent viewer instead of an OTR viewer.

On Figure 2 we present an example beamlet profile. Depending on the solenoid setup, we were able to illuminate
up to 8 slits, a very good number for emittance measurement with a 10% level accuracy. Unfortunately, we werenot able to perform our measurement with such high number of peaks because of the reduced available area on the
fluorescent viewer. Several cross-checks have been made
20000

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0 ..........................

Intensity (a.u.)

18000

02000

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with the single-slit-wire emittance diagnostic used in Reference [5] and both methods agreed within 10-15% for theemittance and the -function. However, there is still substantial disagreement concerning the -parameter, probably
due to a not precise enough scale calibration of our fluorescent screen. This problem should be solved on the final
version of the diagnostic where we plan to have precise calibration lines on the OTR screen.
2.0pulse 10 sec
pulse 50 sec

1.8(mm-mr)

SOLENOID FIELD 275 G

1.4
1.2
1.00.825010The authors would like to thank S. Benson, for useful
discussions, and H. Liu and B. Yunn for their help withPARMELA. We are indebted to M. Shinn for her help.
This work was funded by the U.S. DOE contract DE-AC0584ER-401 and the Office of Naval Research.
References

Figure. 2. Typical beamlet profile obtained on a fluorescentscreen at 250 keV with a charge-per-bunch of 5 pC.

0.6 -110Acknowledgements

10000 12000 14000 16000

x ( m)

1.6

We also measured the emittance versus the charge perbunch, for two macropulse width, as depicted in Figure 3,keeping the solenoid field value constant. The discrepancies between the two sets of data at low charge are probably
due to low charge ghost pulses that are created as a consequence of laser light leaks between two consecutive micropulses. Therefore the wider the macropulse, the larger
the ghost pulse contribution to emittance is.
Further studies and quantitative comparison of the multislit mask, with the single-slit-wire technique in the Injector
Test Stand and with the one-quadrupole-OTR method in theFEL injector, will be forthcoming.

2510

12Charge (pC)

Figure. 3.
Unnormalized emittance versus charge fortwo different macropulse widths. Error bars of 15% havebeen considered after comparison with the single-slit-wiremethod.

[1] C. L. Bohn, “Recirculating Accelerator Driver for aHigh-Power Free-Electron-Laser: A Design Overview”,
Proceeding of the Particle Accelerator Conference 97.
[2] P. Lapostolle, “Quelques effets essentiels de la charged’espace dans les faisceaux continus”, Rep. CERN/DI70-36 (1970).
[3] S. C. Hartman, “The UCLA High-Brightness RF PhotoInjector”, PhD thesis, University of California, Los Angeles (unpublished) and J. Rosenzweig and G. Travish, “Design Considerations for the UCLA PBPL SlitBased Phase Space Measurement Systems”, PBPL Internal (Feb 1994).
[4] M. Crescenti, U. Raich, “A Single Pulse Beam Emittance Measurement for the CERN Heavy Ion Linac”, U.
Raich, private communication.
[5] D. Engwall et al., “A High-Voltage-DC GaAs Photoemission Gun: Transverse Emittance and Momentum
Spread Measurement”, Proceeding of the Particle Accelerator Conference 97.