New Light Source Project
aiming for unique studies of microscopic motions in matter of all kinds

Facility Overview

The NLS Science Case demands high repetition rate, ultrashort, high brightness, high coherence X-rays and a suite of light sources tightly synchronised to these X-rays spanning the THz to vacuum UV range. To realise this goal a unique facility has been designed combining high repetition rate seeded soft X-ray FELs and advanced laser sources. The following provides a brief overview of the NLS facility, further details of which are presented in Part III of the NLS Conceptual Design Report

Facility Layout and Description

Overall layout of the NLS facility

NLS facility

Figure1 : Plan view and architectural layout of the NLS facility

Figure 1 shows the overall layout, main components and scale of the NLS facility.
Such a layout has been chosen in order to leave the “straight-ahead” direction free, to provide an easy means of extending the facility.
The baseline specification for the facility described in the above Sections will be met by three types of radiation source:

  • A suite of three FELs will cover the range from 50 eV to 1 keV in the fundamental, with overlapping tuning ranges as follows:

FEL-1: 50‑300 eV,      FEL-2: 250-850 eV,      FEL-3: 430-1000 eV.
Harmonics will extend the output to 5 keV.

  • Conventional laser sources, synchronized to the FEL sources, will cover the range from 60 meV (20 µm) to 50 eV.

  • Coherent THz/IR radiation from 20–500 µm will be generated by the electron beams after passing through each FEL, for optimal synchronization between the FEL pulse envelope and THz/IR field for pump-probe experiments.

To meet the required FEL tuning ranges with realistic electron beam parameters, given the chosen undulator design, and without demanding excessive undulator lengths, requires a minimum electron beam energy of 2.25 GeV. A common electron energy for all three FELs, together with variable gap undulators, assures the required independent operation and easy tunability of the three FELs. The FEL undulators are based on the well developed APPLE-II scheme in order to provide the required fully variable polarization with the highest possible degree of polarization.
The high repetition rate of equally spaced pulses, initially 1 kHz and increasing in subsequent phases up to 1 MHz, demands superconducting technology for the linear accelerator (linac), operating in continuous wave (CW).

Schematic layout of the NLS facility

Figure 2: Schematic layout of the NLS facility

Figure 2 shows a schematic layout of the facility. The baseline electron gun is a modified version of the successful DESY FLASH/XFEL gun, optimized for 1 kHz operation. The linac consists of a number of accelerating modules based on the well developed TESLA/XFEL design. These are however pulsed machines and although the cryomodule design provides a good starting point for meeting the NLS requirements, some re-engineering is needed to accommodate the higher dynamic head load, higher power couplers and HOM absorbers demanded by CW operation. The required engineering changes are considered in detail in Part III Section 3.6.6. Following a detailed analysis of associated capital and operational costs, as well as other relevant factors, the nominal accelerating gradient has been set at 15 MV/m (Part III Section 3.4), resulting in a requirement for 18 cryomodules (compared to 14 in the earlier design before the cost optimization had been carried out).


Three bunch compressors (BC1-3) are located at optimized locations (205 MeV, 460 MeV and 1.5 GeV) along the linac to compress the electron bunches while maintaining high beam quality. A 3rd harmonic cavity is included to optimize the beam dynamics by linearising the longitudinal phase space. A laser heater serves to introduce a controlled amount of energy spread in order to overcome the microbunching instability.

The linac is followed by a collimation section to remove unwanted beam halo before the beam enters the spreader region which directs successive electron bunches into different FEL lines by means of a set of kicker magnets. This arrangement was chosen for its flexibility. One of the lines parallel to the FELs is a diagnostic section which incorporates a transverse deflection cavity for full slice analysis of the electron beam. With this arrangement sophisticated beam diagnostics can be carried out on-line, by occasionally deflecting bunches into the diagnostics line. To provide the required temporal coherence of the FEL radiation, as well as the 20 fs pulse lengths, each FEL will be seeded with laser pulses obtained from High Harmonic Generation (HHG) in gases. Our current assessment, based on the rapid progress being made in this area, is that within the next ~5 years it will be possible to deliver HHG pulses with at least 400 kW peak power, with 1 kHz repetition rate, tunable over the range 50-100 eV. To obtain the required FEL output up to 1 keV, a one- or two-stage harmonic generation scheme is used, as shown schematically in Figure 3.

Schematic

Figure 3: Schematic of the harmonic cascade FEL scheme.


After exiting from each FEL and before being dumped, the electron beam passes through an undulator magnet to generate coherent undulator radiation in the
20-500 µm range. Broad-band radiation will also be generated using a bending
magnet source.


Eight experimental stations are currently planned. Each FEL will have one with directly focussed beam and one with a grating monochromator to improve spectral resolution and/or filter out unwanted spectral components. In addition a time-preserving grating monochromator is foreseen on FEL-1, and a crystal monochromator on FEL-3 for accessing the harmonics in the range 2-5 keV. The photon beam transport region has been designed to avoid the optical components being damaged by the high peak power of the FEL radiation

Performance Summary

Full start-to-end calculations have been performed to confirm the performance, using three linked computer codes: electrons are tracked from the gun through the first accelerating module (ASTRA), then through the linac, collimator and spreader (Elegant) and finally through the FEL (Genesis). The expected output from the FELs is presented in Table 1. The calculations for pulse energy and number of photons per pulse assume a 20 fs FWHM photon pulse. All figures are for the APPLE-II undulators in horizontal polarization. The power levels in circular polarization mode will be somewhat higher.


Table 1: Calculated output performance of the NLS FELs.


FEL

Photon energy (eV)

Output power (GW)

Energy per pulse (µJ)

Photons per pulse

Peak Brightness
(photons/s
/mm2/mrad2
/0.1%bw)

FEL-1

50

7.1

142

1.8 1013

1.9 1030

300

5.3

106

2.2 1012

5.0 1031

FEL-2

250

5.3

106

2.7 1012

3.5 1031

850

2.9

59

4.3 1011

2.2 1032

FEL-3

500

4.2

84

1.1 1012

1.1 1032

1000

2.7

54

3.4 1011

2.8 1032

 

Upgrade Paths

An important aspect of the design of NLS will be the possibility to extend its performance in future stages. The Science Case calls for the following options to be available for possible future development:

  • Higher repetition rate, eventually up to 1 MHz.

  • Shorter FEL pulses, ranging from sub-fs at 1 keV to a few-fs at 100 eV.

  • Additional FELs and experimental stations.

  • Higher photon energies, at least 1.5 keV in the fundamental, and potentially in excess of 2 keV. 

Higher repetition rates will require a different gun and several different types are under active consideration not only by NLS but by several laboratories world-wide. A corresponding upgrade of the photocathode, seed and experimental lasers will also be required. This is likely however given the timescales involved and the rapid progress being made in this area.


Various schemes have been put forward in the literature for generating sub-fs to fs FEL pulses. Start-to-end calculations are reported in the Conceptual Design Report
for three of the most promising schemes, demonstrating the feasibility and compatibility with the basic NLS design.      


The proposed layout of the facility (Figure 1) and the choice of spreader scheme (Figure 2) lend themselves well to the future extension of the facility. The electron beam transport line can relatively easily be extended along the direction of the linac axis into a second spreader region which then feeds a second FEL hall and experimental hall parallel to the first. Providing a second set of beamlines at higher photon energy can be achieved in a similar way by adding extra linac accelerating sections before the second spreader. These could be of the same type as employed in the main linac, but if it is not essential for the higher energy beamlines to operate at high repetition rate, there are in principle alternative possibilities. The second linac could for example be a high gradient normal conducting system in order to reach the highest energy in the most efficient manner, at reduced repetition rate. The possibility of using plasma wakefield acceleration also deserves consideration.