Call for proposals

Time frame for the 14^th Call for Proposals:

Allocation period	Call opening	Call deadline	Reports on previous experiments due by	PRP Meetings	Feedback to proposers
Year 2026	April 2024	Tuesday 24 June 2025, at 16:00 CEST (Hamburg/Schenefeld time/Central European Summer Time)	Thursday 26 June 2025	15-16 October 2025	late 2025

14^th Call for Proposals for User Experiments

The 14^th call for proposals at the European X-Ray Free-Electron Laser Facility (European XFEL) is open until:

Tuesday 24 June 2025, at 16:00 CEST
(local Hamburg/Schenefeld time / Central European Summer Time)

Please refer to the information below and apply via the User Portal to the European XFEL (UPEX). Beamtime will be allocated in 2026 at these scientific instruments:

Femtosecond X-Ray Experiments (FXE) at Beamline SASE1
Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX) at Beamline SASE1
High Energy Density Science (HED) at Beamline SASE2
Materials Imaging and Dynamics (MID) at Beamline SASE2
Spectroscopy and Coherent Scattering (SCS) at Beamline SASE3
Small Quantum Systems (SQS) at Beamline SASE3

This is the only call for experiment proposals enabling access to the European XFEL in 2026. Due to important accelerator maintenance tasks, the European XFEL will not accommodate user experiments in the second part of 2025. X-ray photon delivery is expected to resume in early 2026, with user operation currently anticipated to start in April 2026. Beamtime in both semesters of 2026 will be allocated based on this single call, and feedback to applicants will be provided in late 2025. The next regular call for proposals is planned in 2026, for experiments in the first semester of 2027.

Before submitting a proposal, and for more information about user community projects, check with the relevant instrument group about feasibility conditions:

In the framework of this call,

In addition to regular proposals, the option of standard configurations for the HED and the MID instruments is available (see Section 1. below). The choice must be specified in the proposal form. At HED, priority access for the HIBEF User Consortium will reduce the available general user time by 30%. Questions regarding priority access should be directed to the HIBEF spokesperson, Prof. Thomas Cowan (HZDR) (cf. HIBEF webpages).

The SCS instrument accepts proposals to all experiment stations and detectors. Details can be found in the fact sheet on the SCS instrument webpages.
SPB/SFX welcomes regular experiment proposals that can take advantage of the full capabilities of the instrument.
- PCS beamtime: A rolling access call for sample screening and limited scope data collection will open at SPB/SFX in late 2025, for allocation on an ongoing basis in 2026. This call will incorporate and extend the previously available protein crystal and sample screening. The opening of this call and further details will be announced in due course.

Individual contact to the instrument groups is strongly encouraged to discuss specific requirements and technical feasibility of proposals in detail.

Proposals submitted in this call will be reviewed for their scientific quality by panels of international experts and checked for technical feasibility and safety conditions.

1. Standard instrument configurations in this call

An efficiency advantage can be established on some instruments with the development of “standard configurations”. The goal is to allow user experiments to be grouped together back-to-back without involving a major change in setup every time. A number of instruments will make standard configurations available in this call.

1.1 HED standard configurations

HED users are always welcome to submit regular (non-standard platform) proposals that fit the instrument capabilities, according to the details given on the our HED-HiBEF website. However, before submission, it is mandatory to discuss your proposal with the instrument staff, as not all combinations of our capabilities are technically feasible.

The following standard configurations will be available in the allocation period related to this call:

1.1.1 ReLaX standard configurations

Details: Conditions for the 14^th call for proposals are on the HED webpages.
The HED instrument group is available for questions and discussions about your projects.

1.1.2 DiPOLE 100-X standard configuration
Details: Conditions for the 14^th call for proposals are on the HED webpages.
The HED instrument group is available for questions and discussions about your projects.

1.1.3 Interleaved-mode proposals in DiPOLE XRD standard configuration

The DiPOLE XRD standard set-up at HED makes it possible to enable very efficient usage of beamtime, by interleaving even very different scientific projects using the same set-up. The DiPOLE shots delivered in intercalated repetitive manner allow the involved user groups to check results before going on with different samples and shot parameters.

Proposals submitted in this specific interleaved mode must comply with the standard DiPOLE XRD configuration and accept an interleaved mode of access with other experiments benefiting from the same set-up and properties over approximately a week of beamtime.
The allocated blocks of user beamtime will be preceded by a block of common beamtime and interleaved with short common blocks for standard reference measurements, as well as experimental set-up checks.
Typically, this mode would allow to schedule up to 10 interleaved proposals within a week of DiPOLE standard set-up. In previous DiPOLE XRD experiments, a four-hour shift allowed for probing 100 to 200 samples (one or two frames).

Laser- and diagnostics parameters (e.g. pulse shapes, VISAR settings)	Limited and defined by HED, in order to reduce setup changes
Beamtime block / shift duration	Four (4) hours Request number of blocks/shifts in the proposal form accordingly.
Laboratory space and the safety assessment	Organized by HED for whole allocation.
Name of submission cycle in UPEX	HED SASE2 - 202603 HED Interleaved-mode proposals in DiPOLE XRD standard configuration

Details: Conditions for the 14^th call for proposals are on the HED webpages.
The HED instrument group is available for questions and discussions about your projects.

1.1.4 Diamond Anvil Cell (DAC) standard configuration

Details: Conditions for the 14^th call for proposals are on the HED webpages.
The HED instrument group is available for questions and discussions about your projects.

1.2 MID – Small-angle MHz XPCS standard configuration

In this call, regular experiment proposals to MID (refer to the conditions mentioned on the instrument webpages) are also welcome. Please discuss with the MID instrument scientists prior to submission.
In addition, this standard configuration is available:

AGIPD MHz area detector, 1 Mpx, 200 µm pixel size
Photon energy: 7–12 keV, up to 2 mJ/pulse, up to 200 pulses/train, 10 trains/sec
Min. correlation function lag time 440 ns, max. lag time 88 µs
q-range (8 m sample–detector distance): ~7e-3 – 0.1 Ang^-1 (small angle scattering)
Beam size on sample: 1–10 µm with local optics, >10 µm with tunnel optics
Standard mounts for sample in capillaries and scanning
Mounting of user-supplied sample environments possible (contact: mid-info@xfel.eu before proposal submission)
Further details: MID instrument group

2. Cross-instrument proposals

If, on the advice of our scientists, specific parts of your project should be conducted at different European XFEL instruments, the selection of two instruments is allowed in the proposal form, in order to submit a cross-instrument proposal. Please discuss with the respective instrument groups and make sure that the proposal and the experiment description addresses experiment sessions on both instruments exhaustively.
The cross-instrument proposal will receive individual IDs on submission to both instruments and will be reviewed by the relevant Proposal Review Panels. This option should not be used to submit the same experiment to two different instruments in order to increase the chances for beamtime allocation. Highly targeted experiment proposals for a specific instrument have better chances of success.

3. X-ray beam conditions for the allocation period

The conditions expected for this allocation period are listed below. Case-by-case verification of specific feasibility conditions with the instrument groups is required.

The provision of attosecond pulses at SASE2 and SASE3 is still under development and we therefore consider this a special mode. For specific parameters (photon energies, polarization states, purity, expected pulse durations), you need to inquire with the respective instruments.

	Photon energy range	Expected pulse energy**
SASE1	5–9.3 keV	2 mJ
	>9.3–14 keV	1 mJ
	>14–24 keV	0.5 mJ
SASE2	5.8–9.3 keV	2 mJ
	>9.3–12 keV	1 mJ
	>12–24 keV	0.5 mJ
SASE3	0.4–1.5 keV	5 mJ
	>1.5–2.5 keV	2 mJ
	>2.5 keV	0.5 mJ

Bunch distribution: 350 X-ray pulses per instrument assume equal distribution at 2.25 MHz operation with higher or smaller numbers for higher/smaller intra-train frequency. Max. 2250 electron bunches within 500 µs are available for distribution to the instruments (4.5 MHz). Exact bunch distribution is based on needs of particular experiments and the capabilities of the photon delivery systems.

** Pulse energy depends on bunch charge, electron energy, and photon energy. The values in this table refer to average pulse energies in an optimized parameter setting. Depending on how much the actual parameters differ from these optimum values, the attainable pulse energies can be lower. It is very important to contact the instruments in order to evaluate the feasibility of your requirements

Explanations:
1. The above parameters correspond to the standard SASE operation mode.
2. The following non-standard modes are available but may require more tuning and could be less reliable:
      a. Hard X-ray self-seeding (SASE2; 7–14 keV)
      b. Hard X-ray two-colour with variable delay (SASE2; 6–10 keV; 0–0.5 ps)
      c. Soft X-ray two-colour with variable delay (SASE3; 400–3000 eV; 0–1 ps)
      d. Short bunches (<10 fs FWHM); requires coordinated scheduling as other instruments and available number of bunches might be affected; time-diagnostics is only partially available
      e. Variable polarization (circular left/right; variable linear) at SASE3
      f. Full trains at instruments with << 10 Hz rep. rates (~2250 pulses)
3. The following special modes are available but will likely require more tuning and may be less reliable. Interested PIs are strongly encouraged to contact the Leading Scientist of the respective instrument prior to proposal submission.
      a. Attosecond pulse delivery at SASE3 (e.g. ≥200 as, few 100 µJ @ 1 keV)

Experiments requesting these non-standard and special modes should address the development of new techniques and fields and are expected to involve large communities and facility staff. If included in the final schedule, continuous (24 h) beam delivery would be planned in this case. Since there is a vast range of detailed specifications for these special modes, proposers are requested to contact the corresponding instrument staff in order to clarify requirements.

4. Experiment reports about previous beamtime

Experiment reports are mandatory and must be submitted 3 to 6 months after the end of each experiment. However, Main Proposers and/or Principal Investigators who have received beamtime in the past and who submit new proposals or continuation proposals must provide the experiment reports related to previous proposal(s) via the User Portal to the European XFEL (UPEX) by Thursday, 26 June 2025. Missing experiment reports may result in withdrawal of new proposals from the review process. Please refer to the complete information on this web page:

https://www.xfel.eu/users/user_guide/experiment_report

5. NEPHEWS Twinning Programme

European XFEL is part of the NEPHEWS consortium and partner in the twinning programme within this collaboration.

The main goal of this programme is the transfer of knowledge necessary to function in the environment of research infrastructures between a new user and a more experienced research group. In due time, the facility may contact spokespersons of accepted proposals in this call to enquire about their readiness to host a new XFEL facility user, with suitable background and scientific interests, as an additional temporary member of their team.

More information:

https://beamtime.eu/programmes-support/twinning/

6. Information for users and funding options

Practical information for users (e.g. about travel funding) can be found here:

https://www.xfel.eu/users

General enquiries should be directed to the European XFEL User Office

Email: useroffice@xfel.eu

Phone: +49 (0)40 8998-6937 or -6767

Before submitting a proposal, always discuss with the relevant instrument group about specific feasibility conditions