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Observation and Simulation of Space-Charge Effects in a Radio-Frequency Photoinjector Using a Transverse Multibeamlet Distribution

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Observation and Simulation of Space-Charge Effects in a Radio-Frequency Photoinjector Using a Transverse Multibeamlet Distribution PHYSICAL REVIEW SPECIAL TOPICS - ACCELERATORS AND BEAMS 12, 124201 (2009) Observation and simulation of space-charge effects in a radio-frequency photoinjector using a transverse multibeamlet distribution M. Rihaoui,1,2 P. Piot,1,3 J. G. Power,2 Z. Yusof,2 and W. Gai2 1Northern Illinois Center for Accelerator & Detector Development and Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, USA 2High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA 3Accelerator Physics Center, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA (Received 25 March 2009; published 29 December 2009) We report on an experimental study of space-charge effects in a radio-frequency (rf) photoinjector. A 5 MeV electron bunch, consisting of a number of beamlets separated transversely, was generated in an rf photocathode gun and propagated in the succeeding drift space. The collective interaction of these beamlets was studied for different experimental conditions. The experiment allowed the exploration of space-charge effects and its comparison with 3D particle-in-cell simulations. Our observations also suggest the possible use of a multibeam configuration to tailor the transverse distribution of an electron beam. DOI: 10.1103/PhysRevSTAB.12.124201 PACS numbers: 29.27.Àa, 41.85.Àp, 41.75.Fr The beamlets, arranged in a quincunx pattern, were gen- I. INTRODUCTION erated by intercepting the beam with a mask downstream Many accelerator applications call for the production of of a thermionic electron source. The experiment unveiled high charge (few nC), low transverse emittance (few m), important physics related to phase mixing in charged- high peak current (several kA) electron bunches. Such particle beam and associated effects such as emittance applications include accelerator-based light sources [1], dilution and halo formation. The latter experiment was high energy physics accelerators [2], novel acceleration recently repeated at the University of Maryland Electron techniques, e.g., based on dielectric wakefield accelerators Ring [12]. In both cases, the beamlets were created after [3]. The dynamics of such generally space-charge- the beam had reached 5–10 keV. dominated electron beams is intricate: it can develop trans- In this paper, we present the results of a similar experi- verse instabilities and is subject to phase space diluting ment performed in an rf photoinjector using a 5 MeV effects [4,5]. Typically, high-brightness electron bunches bunched electron beam. In contrast to the previous experi- are produced in a photoinjector: a high quantum efficiency ments, the electrons in our beam do not go through many photocathode, illuminated by a laser, is located on the back betatron oscillations (they do not even make one oscilla- plate of a radio-frequency (rf) resonant cavity. High charge tion) and the beamlets interact from their creation at the bunches are photoemitted and rapidly accelerated to rela- cathode. Masking the transverse distribution of the photo- tivistic energies [6] and the electron bunch properties are cathode drive laser allowed the generation of easily con- strongly affected by the photocathode drive laser trollable beamlets patterns. The evolution of the beamlets parameters. transverse density provides information on transverse Several theoretical and experimental studies have pre- space-charge effects which are validated against particle- viously addressed detrimental effects due to an inhomoge- in-cell (PIC) simulations. Furthermore, our experimental neous initial distribution in a rf photoinjector, e.g., observations point to a possible use of multibeamlet con- photocathode drive laser transverse distribution nonuni- figuration to transversely control a beam. This could lead formities. In most investigations, the beam is statistically to new ways of manipulating an electron beam using characterized using its root-mean-square (rms) properties. space-charge interaction between several beams in a pho- For instance, Ref. [7] experimentally explores the impact toinjector. Several schemes based on space-charge interac- of a transverse modulation on the rms transverse emittance. tion have been proposed as ways to control or shape Although this is a universal characterization relying on the charged-particle beams. For instance, Ref. [13] discusses concept of ‘‘equivalent beam’’ [8,9] important details of the design of a very fast kicker based on such a scheme. the beam evolution might be missed. The pioneering work Similarly, a technique using an electron lens to compensate of Reiser and co-workers underlined the importance of tune shift induced by beam-beam effects in circular col- studying the evolution of the beams transverse distribution liders was recently demonstrated in the Tevatron collider at in contrast to its rms properties only [10,11]. One of the Fermilab [14]. Finally, the generation of multiple-beamlet experiments conducted at University of Maryland inves- beams discussed in this paper has relevance to heavy ion tigated the propagation of a 5 keV DC beam, composed of fusion research [15] and intensity modulated radiation five beamlets, in a ‘‘long’’ periodically focusing beam line. therapy [16]. 1098-4402=09=12(12)=124201(11) 124201-1 Ó 2009 The American Physical Society M. RIHAOUI et al. Phys. Rev. ST Accel. Beams 12, 124201 (2009) II. EXPERIMENTAL SETUP The experiment was performed at the Argonne Wakefield Accelerator (AWA) [17]; see Fig. 1. The accel- erator incorporates a photoemission source, henceforth referred to as the rf gun, consisting of a 1 þ 1=2 cell rf f : cavity operating at ¼ 1 3 GHz in the TM010; mode. An ultraviolet (uv) laser beam impinges a magnesium photo- cathode located on the back plate of the rf gun half cell. The thereby photoemitted electron bunch exits from the rf gun with a maximum kinetic energy of approximately 5 MeV and is allowed to drift for a few meters with no external field save for the solenoid magnets. Two 100 m FIG. 2. (Color) Picture (a) of the aluminum mask used to shape thick Ce:YAG screens [18] are used to measure the beam’s the laser beam into a series of transversely separated beamlets transverse distribution. An integrated current monitor [19] and corresponding schematic of the mask geometry (b). In the provides the charge per bunch. The uv laser system con- experiments reported in this paper only five or six holes were not obstructed [shown as thicker line circles in (b)] and the mask was sists of a titanium sapphire laser amplified with a regen- oriented to yield the pattern shown in (b) on the virtual photo- erative amplifier and two linear amplifiers [20]. The cathode [see also Fig. 4 (left image) for the corresponding uv infrared laser is frequency tripled to ¼ 248 nm and is laser transverse density]. amplified in a single-stage krypton fluoride excimer am- plifier. The laser system is located 20 m from the photo- cathode and the laser beam is transported via an optical 1:1 tive CCD camera, located outside of the vacuum chamber, imaging system. and is a one-to-one optical image of the laser on the The rf gun is surrounded by three independently pow- photocathode. ered solenoidal magnetic lenses and referred to as L1, L2, The transverse distribution on the photocathode was and L3; see Fig. 1. During the experiment reported in this controlled with a mask, located in the object plane of the paper only L3 was used. The transverse distribution of the imaging system, with a series of identical holes that could uv laser on the photocathode can be measured from a be selectively blocked (see Fig. 2). This object plane is duplicate image of the laser beam on a ‘‘virtual photo- located in the laser room thereby allowing laser shaping cathode.’’ The virtual photocathode consists of a uv sensi- without interrupting the accelerator operation. The mask’s center-to-center hole separation is 2 mm and the hole diameter is 1 mm. The uv laser beam temporal profile was measured with a streak camera and can be approxi- mated by a Gaussian distribution with rms duration of t ’ 2ps. II. EXPERIMENTAL RESULTS AND COMPUTER SIMULATIONS The evolution of the beamlet pattern was explored for two cases of total charge: Q ¼ 20 and 1000 pC, respec- tively referred to as ‘‘low’’ and ‘‘high’’ charge cases henceforth. A. Space-charge effects and solenoid focusing nonlinearities In this section we begin by estimating the role of space charge in the two regimes explored in the experiment. At a given axial location z with respect to the photocathode, a measure of the importance of space-charge effects in driv- ing the beam dynamics is given by the Debye length FIG. 1. (Color) Overview of the AWA beam line. Here, only the 1=2 elements pertaining to our experiment are shown. The legend "?ðzÞ I0 DðzÞ¼ ðzÞ ðzÞ : (1) represents solenoidal magnetic lenses (L), optical mirror (M), 2 IðzÞ virtual cathode (VC), integrated current monitor (ICT), and transverse profile monitor (YAG1 and 2). The distance along This expression is obtained by using the properties of a the beam line is also shown. beam bunch corresponding to its ‘‘rms equivalent uniform 124201-2 OBSERVATION AND SIMULATION OF ... Phys. Rev. ST Accel. Beams 12, 124201 (2009) pffiffiffi I z Q= z I 1 a) ellipsoid’’ [21]; ð Þ ½ 5 tð Þ, 0 ¼ 17 kA denotes d) B 0.5 " z −1 the Alfve`n current, ?ð Þ is the transverse normalized rms 10 0 emittance, ðzÞ is the beam’s Lorentz factor, and ðzÞ −0.2 0 0.2 À2 1=2 z−z (m) ½1 À ðzÞ . Comparing the Debye length to the mean sol interparticle distance Ãp and the beam transverse size ? ) 50 provides an estimate of the importance of space-charge −1 b) 0 χ (m effects. If D single-particle dynamics dominates 1 ? B −50 whereas Ãp D <? indicates space-charge effect are −0.2 0 0.2 z−z (m) ð< Þ sol substantial.
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