ACCELERATOR-RECUPERATOR FOR HIGH POWER FREE ELECTRONLASER
N.A.Vinokurov1, Budker INP, Novosibirsk, Russia

7Abstract

46A 100-MeV 8-turns accelerator-recuperator intended todrive a high-power infrared free electron laser (FEL) is
under construction in Novosibirsk now. The first stage ofthe machine includes one-turn accelerator-recuperator
that contains full-scale RF-system. It was commissioned
successfully in June 2002.

1 INTRODUCTION
The efficiency of conversion of the beam power to theradiation power is rather small in an FEL, being typicallynot more than a few percent. For high power applications,therefore, it is necessary to recover the beam power after
the FEL interaction. The main reason for the energy
recovery, except of simple energy saving, is the dramatic
reduction of the radiation hazard at the beam dump.
One of the possible methods of the beam energy
recovery is to return the beam to the radiofrequency (RF)accelerating structure, which was used to accelerate it. It
was proposed first by M. Tigner [1] for collider, and afterthat applied to FEL [2,3]. If the length of path from theaccelerator through the FEL to the accelerator is chosen
properly, the deceleration of particles will occur insteadof acceleration, and therefore the energy will return to theaccelerating RF field (in other words, the used beam will“pump” RF oscillations in the accelerating structure
together with the RF generator). Such a mode ofaccelerator operation was demonstrated at the Stanford
HEPL [4]. The first high power free electron laser using
such accelerator-recuperator (or energy recovery linac)
was successfully commissioned recently [5]. An obvious
development of such an approach is the use of multipass
recirculator [6,7] instead of simple linac. By increasing of
the number of passes, cost and power consumption can bereduced. However, the threshold currents for instabilities
also decrease, so the "optimal" number of passes exists[8]. The general scheme of such FEL is shown in Fig.1.
The high power infrared FEL for the Siberian center ofphotochemical research, which is under construction now,is the implementation of this approach.

1E-mail: vinokurov@inp.nsk.su

3

3

62
15Figure 1: The scheme of the FEL with theaccelerator-recuperator. 1-injector; 2-RF accelerating
structure 3-180-degree bends; 4-FEL magnetic system;
5-beam dump; 6-mirrors; 7-output light beam.

2 ACCELERATOR
The accelerator-recuperator layout is shown in Fig. 2.
The 2 MeV electron beam from the injector passes 8times through the accelerating structure, getting the 98
MeV energy, and comes to the FEL, installed in the laststraight section. After the loss of about 1% of its power
the beam passes 8 times more through the accelerating
structure, returning the power, and comes to the beamdump at the injection energy. Some parameters of theaccelerator are listed in the Table 1:
Table 1: Main accelerator parameters
RF wavelength, m1.66
Number of RF cavities
16Amplitude of accelerating voltage at 0.8
one cavity, MV
Number of orbits
8Injection energy, MeV
2Final electron energy, MeV
98
Bunch repetition frequency, MHz
2 - 22.5
Average current, mA
8 – 100
Final electron energy dispersion, %0.2
Final electron bunch length, ns0.02 - 0.1
Final peak electron current, A
100 – 20

Figure 2. Scheme of the microtron-recuperator (1 - electron gun; 2 - bending magnets; 3 - RF resonators; 4,5 – injectionand extraction magnets; 6 - focusing quadrupoles; 7 - straight sections with the quadrupole lenses; 8 - FEL magneticsystem; 9 - beam dump).

The 300 keV electron gun of the injector
produces the 1 ns electron bunches with charge
2nQ and repetition frequency up to 22.5 MHz. It
has the DC power supply (rectifier) and termioniccathode with the greed. After passing themodulating RF cavity, the electron bunch iscompressed in a drift section down to 200 ps andaccelerated up to 2 MeV in the next two RFcavities. The measured emittance of the 2 MeV
beam is 4π 10-6 m·rad and the measured energy
spread is less than 15 keV. After that electrons are
injected into the common straight section of theaccelerator-recuperator, using two pairs of theidentical bending magnets with opposite magnetic
field signs (injection chicane). At the entrance tothe main accelerating system the bunch length is
100 ps. The project of the 300 keV photoinjector
was developed [9] to replace the thermionic gun infuture.
The accelerating structure consists of 16 RF
cavities. Each cavity has mechanical tunings for thefundamental and high order modes. The effective
accelerating voltage is 0.8 MV at the thermal
power consumption about 70 kW. So, the total RF
power is more than 1 MW. The details of the RF
system design and tests were described in paper[10].
The orbit geometry was chosen to meet thefollowing conditions:the lengths of all orbits (except of the eighth one)are equal to integer number of the RF wavelength;
the distances between straight sections are equal;
each 180 - degree bend is achromatic.
First condition is necessary for synchronousacceleration. The eighth orbit is longer, than theseventh one by 1.45 of the RF wavelength to obtaindeceleration at the next eight passes through the RFstructure. The second condition makes the design
more compact. The third condition eliminates
coupling of horizontal transverse and longitudinalmotions and makes magnetic lattice more flexible.
The splitting magnets are round. The quadrupoles
into the 180-degree bends make each of thesebends achromatic. The quadrupoles at the longstraight sections are optimized to focus properly
both accelerating and decelerating beams.
The lengths of the straight sections were chosensuch, that, when the electron bunches are injected
at every eighth period of the RF voltage (e.g. with afrequency of 22.5 MHz), the bunches under
acceleration and deceleration are not overlapping
each other on the common track (in theaccelerating cavities), but fill all available buckets
phases homogeneously. In this case the interactionof the electron bunches, having various energies,
decreases significantly.
Calculations of the longitudinal and transverse
beam dynamics show that the microtron recuperator is capable to operate with an average
current above 0.1 A. The final bunching takes placeon the last track, and that allows to achieve a highpeak current (about 100 A) without significant
emittance degradation. The expected power of theFEL is up to 100 kW.

3 FIRST STAGE
The first stage of the machine [11] is asingle-orbit accelerator-recuperator, that contains
full-scale RF-system, but reduced number of orbits(Fig. 3).
Main parameters of the first stage ofaccelerator-recuperator are listed in Table 2.
Table 2: The first stage accelerator parameters
RF wavelength, m1.66
Number of RF cavities
16Amplitude of accelerating voltage at 0.8
one cavity, MV
Injection energy, MeV
2Final electron energy, MeV
14Bunch repetition frequency, MHz
2 - 22.5
Average current, mA
4 – 50
Final electron energy dispersion, %0.2
Final electron bunch length, ns0.02 - 0.1
Final peak electron current, A
50 – 10Figure 3. Scheme of the first stage of the high power free electron laser.

The FEL is installed in the long straight section of thesingle orbit of the accelerator-recuperator. It consists of
two undulators, a magnetic buncher, two mirrors of
optical resonator, and an outcoupling system. Bothundulators are identical. They are electromagnetic planarones, of length 4 m, period is 120 mm, gap is 80 mm, anddeflection parameter K is up to 1.2. One can use one orboth undulators with or without the magnetic buncher.
Both mirrors are identical, spherical, made of gold plated
copper, and water-cooled. The outcoupling system
contains four adjustable planar 45 copper mirrors(scrapers). These mirrors scrape radiation inside theoptical resonator and redirect small part of it to user. Thisscheme preserves the main mode of optical resonator welland reduces amplification of higher modes effectively.
The buncher is simply a three-pole electromagnetic
wiggler. It is necessary to optimize the relative phasing ofundulators.
The expected radiation parameters are shown Table 3.
Table 3: Expected radiation parameters
Wavelength, mm
0.1...0.2
Pulse length, ns
0.02...0.1
Peak power, MW
1...7
Average power, kW
0.6...7
The reliable operation of the 2 MeV injector at averagecurrent 50 mA was achieved last year. The measured
beam parameters are suitable for the FEL operation. The
commissioningofthefirststageofaccelerator-recuperator was done successfully in June
2002. The commissioning of FEL is expected by the endof 2002.

4 FOURTH GENERATION X-RAYSOURCE
The idea of a diffraction-limited fourth generation
X-ray source, based on the use of long undulators andhigh quality (i.e. low emittances, low energy spread andsignificant current) electron beams in MultiturnAccelerator-Recuperator Source (MARS), was proposed
recently [12,13]. This scheme combines the main
advantages of storage ring (low radiation hazard andrelatively low RF power consumption) and linac (lownormalized emittance and energy spread can be conserved
during the acceleration process). Time of acceleration in
the accelerator-recuperator is small in comparison withthe radiation dumping time in a storage ring (103 – 104times), therefore diffusion processes (quantum fluctuationof synchrotron radiation in arcs and intrabeam scattering)
can not “spoil” emittance and energy spread. The energy
recovery is necessary for such a projects, as atseveral-GeV energies the beam power is significant andthe reduction of the radiation hazard is one of the criticalissues of the project.

Our eight-turn accelerator recuperator for FEL canserve as a low-energy prototype for MARS.

5 CONCLUSION
Accelerators–recuperators are the new step inaccelerator technology. They became possible due to asignificant progress in the CW RF accelerating systems.
Their possible applications are very wide 2 and theprospects are very promising.

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2In addition to radiation source applications, mentioned in thispaper, the nuclear physics experiments using internal target
technique at accelerators-recuperators were proposed recently[14].