Plans for an initial test phase for the FERMI project

Nuclear Instruments and Methods in Physics Research A 393 (1997) 225-229

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RE2zH

ELSEVIER

Plans for an initial test phase for the FERMI project R.P. Walker”,*, D. Bulfone”, F. Cargnello”, M. Castellano”, F. Ceveninid, F. Cioccib, G. Dattolib, G. D’Auria”, F. Daclona, B. Diviacco”, A. Doriab, M. Ferianis”, G.P. Galleranob, L. Giannessib, M. Giannini”, E. Giovenaleb, G. Margaritondo”. ‘, A. Massarottia, N. Pangos”, P. Patteric, A. Renierib, A. Rindi”, R. Roseia32, C. Rossia, C. Rubbiaa,3, S. Tazzari”, F. Tazzioli”, L. Tosi”, R. Visintini”, F.Q. Wei”, A. Wrulich”, D. Zangrando” aSincrotrone Tries&. Padriciano 99, 34012 Meste, Ita! bENEA, Via E. Fermi 27, 00044 Frascati. Ital! “INFN, Via E. Fermi 40. 00044 Frascati. Italy dINFN and Gin. Naples, Pad 20. Mostra D’Olrremare. 80125 Naples, Ita!\,

Abstract

Recent activities related to the FERMI project are presented, concentrating on the re-designed beam transport optics including an assessment of second-order effects. Recent improvements to the linac are described and some details about the implementation of the FEL and its optical systems are also presented.

1. Introduction

The goal of the proposed FERMI project [1,2] is to construct a new FEL user facility in the infra-red and far infra-red to complement the existing high brightness VUV/Soft X-ray synchrotron radiation facility at Trieste (ELETTRA). Before embarking on the full project an initial test phase is planned, the main scope of which is to verify the feasibility of using the existing linac as an FEL driver while it continues its role as injector for ELETTRA. The initial goal is to demonstrate lasing at a wavelength of 16 urn, for which cavity mirrors and diagnostics are available. Thereafter the range will be extended to 5-20 urn, by installing new cavity mirrors with holecoupling. The cost of this phase will be kept to a minimum by making use of existing components as much as possible, which include beam transport dipoles and quadrupoles, the undulator and optical cavity, and some diagnostics, all from the suspended INFN/ENEA

* Corresponding author. Tel.: + 39 40 3758225; fax: + 37 40 126338; e-mail: [email protected]. ’ EPFL, Switzerland. ’ Department of Physics, Univ. Trieste, Italy. ’ CERN, Geneva, Switzerland. 0168-9002/97/$17.00 Copyright PI1 SO1 68-9002(97)00474-9

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“SURF” FEL experiment [3], as well as many spare parts from the ELETTRA facility. The layout of the first phase is shown in Fig. 1. The main features of the FERMl project have been described previously. In this report we include some new aspects, concentrating on the re-design of the beam transport optics including an assessment of second-order effects Recent improvements to the linac are described, and the engineering layout and plans for cavity alignment and photon beam diagnostics are also presented.

2. Beam transport calculations With respect to the original beam transport design [1,2] a better solution has recently been found with a reduced number of quadrupoles between the linac and undulator (9 instead of 12) which results in somewhat smaller beam sizes while also maintaining sufficient tlexibility for different optics. The same arrangement of 4 bending magnets has been retained that allows the dependence of transit time with energy (TV= dt/(dE/E)) to be varied, including an isochronous mode (a = 0). The transport line (Fig. 1) consists essentially of two 90 arcs with opposite bending directions, resulting in a total translation of the beam axis of 7.5 m. The design is complicated by the fact that it must contain a drift region of about 3.5 m in order to pass through the shielding wall. In

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addition, the design has to be compatible with the existing magnet apertures and strengths for a wide range of energies (20-75 MeV). Overall the line must be achromatic and provide the required matching conditions at the planar undulator: a vertical waist at the undulator entrance with characteristic beta function, fl = iLr,~/~‘2r& (i.e. 0.3-1.0 m); a horizontal waist at the undulator centre with /I = L/2, where L = 2.2 is the undulator length. Optimisation was carried out using the TRANSPORT program [4]. Fig. 2 shows the betatron and dispersion functions for the isochronous case at 30 MeV. At other energies, there is only a small variation, due to the changing matching condition at the undulator entrance. Only slight adjustment of quadrupole settings is required to change the degree of non-isochronicity while maintaining overall achromaticity. The betatron functions remain very similar to those of Fig. 2, the main difference being the dispersion function in the region between the two arcs, So far calculations have been carried out for a path length dependence of IX= _ 3.5 ps/% (obtained by setting the two arcs as two achromats) and + 3.5 ps/%, corresponding to the previously determined optimum settings for bunch compression [2], as well as r = 0.

In view of the requirement to operate at relatively low energy where the energy spread of the beam is expected to increase, calculations have been extended to second order, using TRANSPORT. Inspection of the secondorder transport matrix from the linac exit to the undulator entrance ( Ti.j,k) clearly shows that momentum dependent terms (k = 6) are the most important. The main effects are: _ a non-linear dependence of horizontal position with momentum, i.e. non-linear dispersion ( T1.6.6). This term is also mainly responsible for the shift of the centte-of-mass of the beam in the horizontal plane. Due to symmetry, there is no shift of the centre in the vertical plane. _ momentum-dependent terms in the vertical motion (T3.~6~ T5.3.6, and T4.4.6 in particular) causing a distortion of the phase-space ellipse. ~ a non-linear path length dependence on momentum (T5.6.6).

Calculatrons show that the T1,6,6 and T 5.6.hterms vary little either with the different optics settings, or energy, and are both sufficiently small ( -0.36 mm/%’ and - 1.1 PS/%~. respectively) that beam energy fluctuations should not create significant horizontal or longitudinal (in the isochronous case) motion at the entrance to the

Horizontal & vertical betatron amplitudes [m]

Horizontal & vertical dispersion functions [m]

Fig. 2. FERMI

transport line optical functions

for 30 MeV, isochronous

operation;

solid-horizontal

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dotted-vertical plane.

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undulator. The increase in bunch length even in the worst case at 20 MeV is negligible. The effect of the non-linear path length dependence on the proposed method of bunch compression by means of linac dephasing has also been investigated, using a revised version of the simulation described in Ref. [2]. Some increase in pulse length and consequently a reduction in the peak current is observed, however the simulations still show that peak currents of around 100 A should still be achievable for Xl-75 MeV beam energies, compared to the nominal value of 38 A for an uncompressed bunch. In order to examine the change in the phase space distributions we firstly examined the beam ellipse calculated by TRANSPORT, which is based on the second moments of the beam intensity distribution (assuming a Gaussian input beam). The largest effect observed was at 20 MeV for the double-achromat optics, and is shown in Fig. 3. It can be seen that there is a significant difference between the first-order (inner solid line) and second-order (outer dashed line) results. The interpretation of the second-order beam ellipse is not however straightforward, as pointed out by the TRANSPORT program authors [4]. In order to gain a better understanding of the non-linear effects we have therefore carried out tracking calculations using a Monte-Carlo approach. A large number of electrons were transformed from the initial six-dimensional Gaussian distribution at the linac exit to the undulator entrance, using the first- and second-order transfer matrices provided by TRANSPORT. Fig. 4 shows the resulting phase space contours, from which it is evident that the beam distribution has much larger tails compared to a Gaussian distribution. tending to increase the second-moment. This can also be seen from Fig. 3 which includes two iso-intensity contours calculated from the tracking program, both of which would coincide with the one-sigma ellipse in the case of a Gaussian distribution: the contour at 60.7% relative intensity (inner dash-dotted line), and the contour containing 39.3% of the electrons (dotted line). It is clear that there is a significantly smaller increase in the phase space area for the core of the beam than indicated by the second-moment ellipse. The distortion is much smaller in the isochronous case, and negligible in both cases in the horizontal plane. At 50 MeV, where the momentum spread is much reduced, no significant effect remains. The overall conclusion therefore is that secondorder beam transport effects are not expected to pose a significant problem for FEL operation even at the lowest energies presently envisaged. In any case, non-isochronous transport line optics are only likely to be employed at higher energies to provide bunch compression, for which second-order effects are negligible.

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Fig. 3. First- and second-order beam ellipses at the undulator entrance in the 20 MeV double-achromat optics (see text).

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Fig. 4. Vertical phase space distribution for the same case as Fig. 3 calculated with a Monte-Carlo tracking program; contours from 10% to 90% of peak intensity at 10% intervals.

3. Linac performance in the FEL mode The first 100 MeV part of the ELETTRA 1 GeV Sband linac consists of a triode gun, followed by a 500 MHz chopper, 500 MHz and 3 GHz pre-bunchers, a 4 MeV buncher and two 3.1 m travelling wave accelerating sections. The first tests of the FEL mode of operation were carried out in early 1995 [S]. At that time, switching to the FEL mode involved replacing the standard gun modulator used for injection with a separate modulator. The length of time required, and the risk of component damage, meant that only a brief series of measurements were made. Since then a new modulator has been constructed [6] that unifies multibunch, single

R.P. Walker et al. JNucl. Instr. and Meth. in Ph.vs. Res. A 393 (1997) 225-229

bunch and FEL operating modes on a single unit. Switching between modes is now much easier and takes less than 30 min to insert the pulse forming network and high-voltage transformer for the long macropulse formation. The first measurements with the new gun modulator were carried out in May 1996, which showed performance (emittance and energy spread) were close to the expected values [7]. To reduce the switching time even further, it is intended in the near future to make the long pulse mode permanent, which requires a re-matching of the lower and high-energy parts of the linac. Then, switching between modes will only require changing the gun trigger signal, enabling further tests of the FEL mode to continue in-between use of the linac to inject into ELETTRA.

4. Layout Fig. 1 shows the overall layout of the initial phase of the FERMI project. The large hall adjacent to the linac tunnel exists and needs only minor civil works to install the shielding wall separating the control and diagnostic area to the left, from the FEL area itself on the right. A similar interlock system to that employed in the ELETTRA linac and transfer line will control access to the FEL area. The FEL radiation beam will be able to be brought out into the diagnostic area for characterisation and possibly pilot experiments. Diagnostic elements in the transfer line include a horizontal collimator immediately following the first quadrupole where the dispersion is close to maximum (0.9 m) for energy spread selection. The beam position and profile will be determined using a total of 7 fluorescent screens, including one immediately before and another immediately after the undulator, and one after the final bend. Non-destructive stripline pick-ups will be installed at a later stage. Current measurement will be performed using two torroids. The linac vacuum system is protected against the possibility of a vacuum failure in the FEL line by a delay line, conveniently located in the long drift section through the shielding wall and a fast valve. A further 3 valves section the line into 3 regions, each pumped by a 60 l/s sputter ion pump (two in the first section). Other equipment include six small air-cored magnets for beam steering in both planes, and a beam stopper, as part of the personnel safety and access system. Apart from the delay line and steering magnets, all other components are either identical, or very similar, to components that are presently installed in the ELETTRA transfer line or storage ring.

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5. Optical alignment and diagnostics A simple scheme for initial optical alignment and diagnostics will be adopted, to minimize cost and complexity. At the beginning all optical components will be located in the FEL Hall. Alignment will be carried out using a HeNe laser injected through the upstream cavity mirror. A system of lenses will be used to match the beam profile to that of the trapped mode in the cavity, taking into account the defocusing effect of the planar-concave dielectric mirror, in order to provide a maximum number of reflections for mirror alignment purposes. FEL radiation will be extracted at the downstream mirror and focused on the monochromator and detector system. A system of two mirrors will allows the beam height to be adjusted to match the requirements of various diagnostic elements. One mirror will have remotely controlled angular adjustment in order to make measurements of the angular distribution of the radiation. The following procedure for alignment and commissioning will be adopted: align the electron beam to the fluorescent screens; align the HeNe beam to the screens; align the monochromator and detector to the HeNe beam; perform measurements of spontaneous emission in the absence of the downstream mirror; install the downstream mirror and align the cavity mirrors to the HeNe beam; adjust cavity length. etc. to obtain lasing and perform initial measurements. For simplicity, no attempt will be made to measure single-pass gain by means of an injected laser. Later. copper mirrors with hole coupling will be installed to extend the range of operation. At a later stage the FEL radiation beam will be transported into the control and diagnostic area for further studies.

References [l] [2] [3] [4] [5] [6] [7]

FERMI Conceptual Design Report. Sincrotrone Trieste. April 1995. R.P. Walker et al., Nucl. Instr. Meth. Phys. Res. A375 (1996) 252. M. Castellano et al., Proc. 3rd. European Particle Accelerator Conf., 1992 p. 611. K.L. Brown et al., CERN 80-04, March 1980. DC. Carey, SLAC-R-95-462, May 1995. G. D’Auria et al.. Proc. 1995 Particle Accelerator Conf.. Dallas, Texas, IEEE Catalog No. 95CH35843, p. 222. G. D’Auria et al., Proc. 18th Int. Linac Conf.. Aug. 1996. CERN 96-07, Nov. 1996, p. 854. G. D’Auria et al.. Proc. 5th. European Particle Accelerator Conf.. Institute of Physics Publishing I 1996). p. 825.

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