Wan, W

Zholents, A A

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BUNCH COMPRESSOR AND DE-COMPRESSORIN THE FEL FOR SATELLITE POWER BEAMINGW. Wan, FNAL, Batavia, IL and A. Zholents, LBNL, Berkeley, CAAbstractA FEL of average power 200 kW is being designed at theLBNL for satellite power beaming[1, 2]. It utilizes the ra-diation of100 MeV electrons with200 A peak current.In order to obtain the desired peak current, the 5mm longelectron bunches delivered by a linear accelerator are com-pressed to 1mm. Furthermore, it is important for the FELoperations that the compressed bunches have a uniform lon-gitudinal density distribution over the entire bunch length.After the FEL, the electron beam is returned to the linearaccelerator for deceleration. Since the electron beam ac-quires approximately 6% energy spread during radiation inthe FEL, bunch de-compressor is used between the FEL andthe linac to expand the electron bunches back to its origi-nal length and to reduce the energy spread. In this paper wepresent design and analysis of the bunch compressor and thebunch de-compressor that perform needed functions.1 INTRODUCTIONThis work is part of a broader study aiming at the design ofa FEL for satellite power beaming [1]. At present a FEL ofaverage power 200 kW at the wavelength of 840 nm is beingdesigned at the LBNL [2, 3]. The FEL utilizes a100 MeVelectron beam with the average beam power of 10 MWprovided by a RF photoinjector and a linear accelerator.The design assumes a low beam peak current in the injec-tor and the linear accelerator and a high beam peak currentin the FEL. A compression of electron bunches is neededbetween the linear accelerator and the FEL. The goal of thecompression is not only a short bunch in the FEL, but also abunch that has a uniform longitudinal electron density pro-file over most of the bunch length. The former helps to im-prove the efficiency of the energy transfer from electronsto light; the latter helps to minimize the energy spread ofelectrons after the radiation. This is important because theFEL design assumes that the electrons are decelerated inthe linear accelerator to below 12 MeV before they are sentto the dump, which is employed for energy recuperation inthe linac and the elimination of induced radioactivity in thedump. Therefore, too large energy spread of the electronswill hamper the deceleration process.In the process of the radiation in the FEL the electronslose about 2% of their energy and acquire roughly an en-ergy spread of 6%. Bunch de-compressor uses this energyspread to disperse the compressed electron bunch back toits original length. It also creates a correlation between en-ergy of electrons and their position along the bunch, whichWork supported by DOE under Contracts DE-AC03-76SF00098 andDE-FG03-95ER40926.Figure 1: The longitudinaldistributionof electron peak cur-rent at the end of the RF photoinjector. Point to point fluc-tuations on the plot are due to limited statistics in the simu-lation.is used to reduce energy spread of electrons at the end of thedeceleration.2 BUNCH COMPRESSOR ANDDE-COMPRESSORThe design requirement for a bunch compressor is a reduc-tion of the electron bunch length from approximately 5 mmat the end of the linear accelerator to 1 mm at the beginningof the FEL. This will raise the electron peak current in theFEL up to more than 200 A.At this point we assume that an electron bunch leavingthe RF photoinjector has a normalized longitudinal emit-tance of 6 cm and a uniform longitudinal distribution, asshown in Fig. 1 (This distribution is considered in a designof the RF photocathode gun). We begin bunch compressionby creating energy shift of the electrons correlated with theelectron position along the bunch. This is accomplished byaccelerating the electron bunches in the linac at a -13 phaseoff the peak accelerating field. Ideally, we want to producea linear dependence of the electron energy shifts on the lon-gitudinal coordinates of the electrons relative to the bunchcenter. But, it is impossible because the acceleration takesplace in a sinusoidal RF field and, together with the linearterm, it inevitably produces nonlinear terms. However, onecan use the actual dependency of the electron energy shifton the longitudinal coordinates and derive a required path-length difference as a function of electron energy. Usually,the first three terms are sufficient for an accurate descrip-0-7803-5573-3/99/$10.00@1999 IEEE. 2465Proceedings of the 1999 Particle Accelerator Conference, New York, 1999 9.35  m Figure 2: The layout of the bunch compressor-0.6 -0.4 -0.2 0.2 0.4 0.650100150200250300350400-0.6 -0.4 -0.2 0.2 0.4 0.650100150200250300350400Figure 3: The longitudinal distributionof electron peak cur-rent after bunch compression: a) without errors, b) with er-rors. Point to point fluctuations on the plot are due to limitedstatistics in simulations.tion. In our case they turn out to be:L = −0:514EE− 3:88EE2− 177EE3(1)where all coefficients on the right hand side of the equationhave a dimension expressed in meters. Now, this functioncan be programmed into the performance of the bunch com-pressor to produce the uniform longitudinal distribution atthe exit. This is to force all electrons to radiate in the iden-tical conditions in the FEL and, therefore, to minimize theenergy spread produced in the process of the radiation.In this design we incorporated the bunch compressor intoa magnetic arc performing a 180 beam turn from the endof the linear accelerator to the FEL. Its layout is shown inFig. 2. The arc’s lattice consists from two identical cells andresembles a second-order achromat. Each cell has 2 bend-ing magnets, 8 quadrupoles and 4 sextupoles. The strengthsof all quadrupoles and sextupoles are chosen to provide asmooth beam focusing in the arc and time-of-flight latticeterms defined by Eq. 1. Care was given to the choice of lin-ear optics to minimize the strengths of the sextupoles. As aresult, the magnetic field of the sextupoles at the pole tip iswell below 1 kG (assuming 5 cm radius of the beam pipe).To test the lattice we did particle ray tracing using COSYINFINITY [4]. We assumed 8 mm-mrad for normalizedvertical and horizontal beam emittance. The result of sim-ulation without lattice errors is shown in Figure 3. Hereone can see the longitudinal distributionof the electron peakcurrent after bunch compression. One can see that the finaldistribution has a flat top extended almost over the entireStatic errorsSetting errors (BB ) = 1 10−3 a)Tilt errors (mrad) ( ) = 0:2Misalignment errorsin x, (m) (x) = 150in y, (m) (y) = 30in z, (mm) (z) = 1Multipole errorsdipoles ( b3b1 ) = 1 10−4 at r = 3cm b)quadrupoles ( b3b2) = 5 10−4 at r = 5cm b)a) b1 , b2 and b3 are the dipole, quadrupoleand sextupole components of the magnetic fieldb) the same for quadrupoles and sextupolesTable 1: Specification of the errorsFigure 4: The longitudinaldistributionof electron peak cur-rent after bunch compression with no errors and sextupolesand octupoles turned off.bunch length even with all errors being included in simu-lation. Then we applied all kinds of lattice errors like mis-alignment errors, tilt errors, setting errors listed in Table 1and use two families of quadrupoles to offset the linear per-turbations to the lattice due to the setting errors. We assumethat similar knobs will be used with an actual beam for aninitial tuning of the lattice. Then we did particle ray tracingagain. A typical result of a simulation for one seed of er-rors is shown in Fig. 3. Obviously, errors have little effecton the performance of the bunch compressor. To demon-strate the importance of the sextupole corrections we showthe result of similar particle ray tracing for a case when allsextupoles are set to zero in Fig. 4. It is clear in this casethat sextupoles play a crucial role in producing the desiredbeam profile. In fact, with sextupoles off, the term T566,following the TRANSPORT notation, is about 135 m, re-sponsible for 3.4 mm bunch lengthening of a beam with en-ergy spread of0.5%. Since the present solution with sex-tupoles turned on works well for our purpose, no extra ef-fort was devoted to optimize the first-order layout to furtherminimize the second-order aberrations.2466Proceedings of the 1999 Particle Accelerator Conference, New York, 1999z,mmPeak current,AElectronenergy,M eV(E)Figure 5: The longitudinal distributionof electron peak cur-rent and energy distribution of the spent beam at the end ofthe linac.2.1 BUNCH DE-COMPRESSORThe function of the bunch de-compressor is the reverseof the bunch compressor. It takes the compressed elec-tron bunch with large energy spread acquired in the FELand uses this energy spread to disperse electrons. For thispurpose we use the second 180 arc to return electrons tothe linear accelerator. Physically this arc is similar to thefirst arc, but quadrupoles and sextupoles are set to differ-ent strengths. The de-compressor creates the correlationdesired between electron position in the bunch and its en-ergy. This correlation is used in the linac for a partial com-pensation of the beam energy spread. This is accomplishedby decelerating the electron bunches in the linac at a 13phase off the peak accelerating field. A result of the de-compression of the electron bunch and deceleration in thelinac is shown in Fig. 5. Here we present a distribution ofthe electron peak current and a distribution of the electronenergy in the bunch of spent electrons at the end of the linac.One can see that at the end of the deceleration the totalenergy spread of the electrons is roughly 2.5 MeV, while atthe beginningof the deceleration it is about 5 MeV as shownin Figure 5.3 CONCLUSIONThe study presented in this report shows that the carefullydesigned bunch compressor and de-compressor meet the re-quirement of the FEL for power beaming and thus enhanceits efficiency.4 REFERENCES[1] H. E. Bennett, et. al., NIM A341, (1994)124-131.[2] K.-J. Kim, et. al., “FEL Options for Power Beaming”, NIMA407, (1998)380.[3] A. Zholents, et al., “FEL Design for Power Beaming”, to bepublished in SPIE Proc. 3614.[4] M. Berz, TechnicalReport MSUCL-977, Michigan State Uni-versity, (1995).2467Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Bunch compressor and de-compressor in the FEL for satellite power beaming

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