& *H Nuclear Instruments and Methods in Physics Research A 375 ( 1996) ABS 56-ABS 58 NUCLEAR . __

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INSTRUMENTS __ & METHODS

IN PHYSICS RESEARCH

ELSEVIER Section A

Optical aspects for lasing in the visible and UV with FELICITA I*

T. Schmidt*, A. Geisler, D. Niille, M. Ridder, S. Tammen

University of Dortmund, Institute for Acceleratorphysics and Synthrotronradiation, 4421 Dortmund. German?;

In this contribution the mirrors in the optical resonator for the FEL at the storage ring facility DELTA, University

of Dortmund, are discussed. The wavelength regime covering the full visible and UV requires dielectric multi- layer mirrors. As the reflectivity of the available materials decreases with the wavelength it determines together with

the gain parameters of FELICITA I the shortest possible wavelength for this device.

Optimum performance of an FEL requires optimum

electron beam parameters, especially concerning peak current, emittance and energy spread. DELTA is designed

as a low emittance storage ring. But it fulfills not only the requirements of a third generation synchrotron source, but also provides long straight sections for insertion devices and is optimized for high peak currents [I]. Thus it has all features of a dedicated FEL driver.

The first experiment FELICITA I is based on an

electromagnetic undulator. The special feature of this device is that it can be tuned from a pure undulator into an

optical klystron mode. Therefore the complete characteris- tics of this device can be altered without any modification

of the hardware, especially concerning gain and power levels. With the installed power supplies it is possible to

tune the K-value of the undulator sections up to values of K = 3.06 with the corresponding wavelength of 741 nm for

500 MeV electrons. The original design value was K = 2.1 resulting in 418 nm for the fundamental. With this device it is possible to achieve lasing in the deep UV either with energies around 700 MeV or using the maximum K-value to operate at the third harmonic of the spontaneous emission at 500 MeV As the gain scales inversely with the third power of the electron energy it is worth operating at

the lowest energies possible. Rather pessimistic calcula- tions give gain values around 3% for the FEL-mode with a pure undulator field configuration and about lo-20% for the optical klystron-mode in the visible and UV

The maximum gain for FELICITA I is achieved with a mirror radius of the optical cavity of 7.48 m at which the

*This work is supported by the Bundesministerium fur

Forschung und Technologie under contract 05 3PEAAI 0.

* Corresponding author. Tel. +49 231 775 5374, fax +49 231

755 5383, e-mail thomas@marvin.physik.uni-dortmund.de.

best overlap of laser pulse and electron over the undulator length is given. At this value the optical cavity is very sensitive to a misalignment (Fig. 1). Therefore the first set of mirrors has a radius of curvature of 8 m, which gives a significant improvement of the stability of the cavity by a

rather convenient gain reduction. See Table 1 for variation of gain and stability versus curvature radius.

FELICITA I will cover the visible and UV part of the

electromagnetic spectrum. At shorter wavelengths the

parameters of the substrates and the reflecting multilayers

become more and more important. The planity of the substrates influences the mode structure of the laser pulse whereas the microroughness induces scatter. The typical planity or disturbance from the reference radius is A/ 10, measured with He-Ne lasers. Therefore it is only A/3 or even worse at operation wavelengths of FELICITA I. The 8 m fused silica substrates from several manufacturers with

planities of h/IO and A/20 and different specifications for the microroughness are in house. Additional sapphire

substrates of 10 m radius are available, which are worse to polish due to their higher hardness. For each substrate the microroughness will be measured with an interferometric

system at ESPCI in France in order to compare the

polishing quality. In the case of FELICITA I the integrated power radiated

40 Tilt of both mIrrora : dd = I wmd

1

Fig. 1. Tilt of optical axis in Prad by a tilt of both mirrors of

1 prad as a function of mirror radius.

0168-9002/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved

SSDI 0168-9002(95)01320-2

T. Schmidt et al. I Nucl. Instr. and Meth. in Phvs. Res. A 375 (1996) ABS _56-ABS 58 ABS 51

Table 1

Relative gain reduction with respect to the optimum mirror

curvature of 7.46 m and the allowed angular misalignment

R [ml Gain reduction [%I 80 [prad]

8 9.3 4.1

8.5 18.2 6.3

9 24.9 8.2

9.5 30.2 10.0

10 34.4 11.4

to the downstream mirror will be about 1 W in FEL mode

at 500 MeV and up to I5 W in OK mode at 700 MeV While fused silica has the better heat expansion coefficient (fused silica 2.33 X IO-‘/K, sapphire 7.7 X 10ehlK) sapphire has a better thermal conductivity (27 W/K to 1.3 W/K) and is therefore the better choice by high heat loads. As the heat expansion disturbs the planity, fused silica could also

be a good choice also at short wavelengths if either lasing on the third harmonic is provided or to use active cooling,

for instance using a nitrogen cooled copper wire. The temperature of the mirror holder has to be controlled

anyway, as the InchwormTM’ motors and the piezo step- pers used for mirror movements are limited to 80 and

120°C respectively. The coatings will be made by ion beam sputter tech-

niques in order to get the hardest possible thin films for the rough environment in a synchrotron beamline. For the alternating h/4 sheets of high and low index material a high difference in the refractive index combined with low

absorption coefficients is necessary. Scanning these values for various materials from 200 nm to the visible, reflectivi-

ty calculations were done. At 420 nm values of 99.9% can be achieved with Ta,O, whereas the maximum reflectivity

at 200nm decreases down to 98.5% with Al,O,. This requires a gain of at least 3% at 200 nm (see Figs. 2 and

3). Down to 220 nm YzO, is a good choice because no absorption occurs. Below this wavelength the absorption increases rapidly.

To align the optical cavity and to control the position of the mirrors online, green and red He-Ne lasers are used [2]. The cavity mirrors therefore needed additional reflec- tivities for these lasers, provided by undercoatings of A14 thickness for 546 and 633 nm. The red light for the stabilisation system is sent from the back of each mirror

through the substrate and is reflected at the undercoating. So the reference point is the dielectric coating itself and thermal expansion of the substrates have no influence. Fig. 4 shows the reflectivity through the substrate.

In this paper the optical technology used for FELICITA I was discussed. It was shown that all techniques to operate this FEL in the wavelength regime covering the visible and

’ Burleigh Instruments, Inc.

f B z

Fig. 3. Reflectivities for 420 nm. 20 multilayers for each material

including absorption. More layers do not improve the reflectivity.

The low index material is always SiOz. Ta?O, as high index will

be the best choice.

Fig. 3. Reflectivity for different numbers of multilayers from

SiO,, AlzO, with a central wavelength of 200 nm. Lasing in this

wavelength region requires at least a gain of about 4%.

Fig. 4. Reflectivity through a 5 mm thick substrate at the under-

coating: a central wavelength of 640 nm gives a reflectivity of

about 40% for the green adjusting laser.

EXTENDED SYNOPSES

ABS 58 T. Schmidt et al. I Nucl. Instr. and Meth. in Phys. Res. A 375 (1996) ABS 56-ABS 58

UV down to at least 200 nm are available. As the operation of the DELTA main ring starts in fall 95 no reliable data for gain calculations exist. Though pessimistic estimates

show that the gain of FELICITA I in both operation modes should be sufficient to cover the above mentioned wave-

lengths. The undulator is ready for operation and the assembly of FELICITA I is nearly finished. Therefore the

commissioning of the FEL will start simultaneously with

DELTA [3,4].

References

[I] G. Blokesch, M. Negrazus and K. Wille, Nucl. Instr. and

Meth. A 338 (1994) 151.

[2] D. N611e et al., Nucl. Instr. and Meth. A 358 ( 1995) ABS 14.

[3] D. N61le et al., these Proceedings (17th Int. Free Electron

Laser Conf., New York, NY. USA, 1995) Nucl. Instr. and

Meth. A 375 (1996) ABS 48.

[4] A. Geisler et al.. ibid., p. 445.