An EUV-FEL is one of the promising candidates for the future high power EUV light source of more than 1 kW. While the design study on the FEL light source has been progressed, a most important milestone should be a real demonstration of the high repetition rate ERL-based FEL light production. In FY2019, a real Mid-Infrared FEL (MIR-FEL) project based on the compact ERL in KEK started and the beam commissioning was started from the beginning of March 2020. At the conference, the present results obtained from the MIR-FEL and the expected remained-study-works on future EUV-FEL will be presented.
It is important to develop the high power EUV light source up to 1 kW to realize the 3nm node to reduce stochastic variation and achieve a higher through put. To this end, we have proposed an energy recovery linac (ERL)-based free electron laser (FEL) ,which will produce more than 10 kW EUV light to provide the light into several scanners. We studied the feasibilities to reduce the size of the accelerator system itself, and the part of them was presented at the previous symposium last year. And we also gave an idea to develop the POC of the ERL-FEL by using compact ERL (cERL) as a first stage of the EUV-FEL. In this paper, we present the upgrade plan of cERL for the POC.
The technologies to be clarified are both of to realize the SASE-FEL based on the ERL and to achieve short electron bunch around 100fs for EUV-FEL light generation. The former is that the energy of the electron beams after the FEL generation can be recovered with the ERL accelerator systems with the high repetition rate such as more than 100MHz. The POC will be completed at the wavelength of near infrared, because of the size limitation of the cERL. The latter will be also realized at the cERL by the bunch compression scheme using a combination of electron beams with a momentum chirp and magnet assemblies with a non-zero longitudinal dispersion.
The design values of the upgrade are as follows; the energy of the recirculation electron is 80 MeV, energy of the injection electron is 5 MeV, wavelength of the FEL is 1.35 micron, electron beam current is 10mA, bunch charge is 60 pC/bunch, repetition rate is 162.5 MHz. We have already studied the electron trajectory by using simulation, and checked the feasibilities. We will give detailed studies on the POC.
It is important to develop the high power EUV light source up to 1 kW to realize the 3nm node, which is expected to be in production at 2023-24. To this end, an energy recovery linac (ERL)-based free electron laser (FEL) must be a most promising candidate, so that our group has done some feasibility studies from the view point of accelerator technology. In order to realize the EUV-FEL high power light source, it is also important to recognize the demand of end users and related problems on the FEL light source. Last year, we attended many conferences and workshops to learn these items and also we organized one day workshop “EUV-FEL Workshop” at Tokyo. You can find the presentation materials in a website of http://pfwww.kek.jp/PEARL/EUV-FEL_Workshop/presentaions.html.
One of the most important requirements is to reduce the size of the EUV-FEL system. The total system size is about 200 m (L).x 20 m (W) at our current design of the EUV-FEL with 160m linac, where the acceleration energy and current are 800 MeV and 10 mA, respectively. However, we had comments from semiconductor industry that it is too long to install the light source in a usual LSI Fab, so that we have to find out solutions to reduce the length of the accelerator systems to ~100 m. To this end, there are following several challenges.
1) Increasing the field gradient of the superconducting RF (SRF) cavity to reduce the total length of the linac.
2) Higher Q to reduce the RF loss in higher field gradient SRF cavity.
3) Reduction of the acceleration energy by introducing shorter period undulator .
4) Double loop accelerator system, in which the electron passes through a same linac twice and accelerated up to twice energy or accelerating cavities are placed on both loop sides.
The R&D directions of the above challenges on accelerator technologies will be presented.
A circularly polarized X-ray (CPX) beamline has been constructed at 6.5 GeV Accumulation ring. The insertion device is an elliptical multipole wiggler. The CPX from this device is used by the following two branch lines. One is for magnetic Compton scattering and/or high resolution Compton scattering experiments. The monochromator of this branch line is a quasi- doubly bent crystal monochromator that comprises an array of twenty pieces of water-cooled singly-bent crystals. The obtained performance is as follows; the energy range is 40 - 70 keV, the flux is 6 X 1012 at 60 keV, the energy resolution (Delta) E/E equals 1.4 X 10-3, and the beam size at the sample position is 3 X 8 mm2. Another is for magnetic X-ray Absorption Near Edge Structure (XANES) and magnetic Bragg scattering experiments. The optics of this branch line is a combination of a saggital focusing double crystal monochromator and a bent plane mirror. In a moment, the monochromatized X- ray from 6 to 28 keV is available and the focused beam size is about 0.4 X 3 mm2. The heat load problems at the above monochromators and some experimental results by using this beamline are also presented.