MeV-UED Run 5 Scientific Capabilities

LCLS MeV-UED Run 5 will provide support for materials science research and materials under extreme conditions communities. LCLS MeV-UED has led to a new paradigm in ultrafast electron scattering by employing higher electron beam energy, resulting in stronger diffraction and significantly reduced space-charge effects, enabling atomic spatial resolution alongside subpicosecond temporal resolution. MeV electrons experience less multiple-scattering and exhibit a virtually flat Ewald-sphere. MeV-UED thus facilitates quantitatively-understood observables to validate physical modeling. MeV-UED has enabled broad scientific opportunities in ultrafast structural dynamics and chemical dynamics, including the first observation of the transition of heterogeneous to homogeneous melting, mapping the energy flow in superconductors, atomic movie capturing light-induced structural distortions in perovskite solar cells, and the discovery of a novel method of switching exotic properties on and off in topological material (SLAC-MeV-UED Pub). 

In the upcoming Run 5, LCLS MeV-UED will offer two specialized chambers dedicated to solid-state experiments. The first chamber, designated as the Cryo and Quantum Materials Chamber, is optimized to facilitate experiments conducted at or below room temperature and features a tunable optical pump. The second chamber, known as the Single-Shot and High Temperature Chamber, offers a range of configurations to accommodate versatile experimental requirements. These configurations include single-shot experiments, high-temperature experiments (from RT to 800 K), and experiments involving electrical excitation or biasing. Detailed specifications of the available options are outlined below. For additional information and to discuss experimental feasibility, please reach out to the UED instrument team. We highly encourage potential users to contact us with any special requirements they may have.

Cryo and Quantum Materials Chamber

Short description:

This chamber is configured for measurements of structural dynamics from RT to low temperature and it is the primary chamber for most solid state UED experiments. The chamber configuration is kept relatively static to ensure the best performance of the cryo system. The chamber is located closer to the UED gun than the 2nd chamber enabling slightly higher time resolution, and slightly lower electron beam emittance than in the 2nd chamber. However, the proximity to the UED gun requires a cleaner vacuum environment. 

Single Shot & High Temperature Chamber

Short description:

This chamber allows maximum user versatility. It can be reconfigured into any one of several different modes depending on user requirements. Three operational modes are offered in this chamber during UED run 5: solid state single-shot UED, high-temperature (300 K - 800 K) UED, and electrical pump/bias UED. Experiments using novel user-provided sample environments will only be considered for this chamber (please discuss feasibility with UED staff prior to submission). 

Single Shot & High Temperature Chamber Operational Modes

Single Shot Mode — Single Shot & High Temperature Chamber

Description: This mode operation allows irreversible solid-state experiments to be performed. A large array of identical samples are used and after each measurement a fresh sample is moved into the beam. Major areas of science addressed using this technique are ultrafast melting dynamics, irreversible phase transitions, warm dense matter and the potential for dynamic compression. Science Example Instrument Review Paper 

Parameter	Value
Samples	Accessible sample area 25 x 45 mm  Example of a fork sample holder designed for a 27 x 50 mm2 Si chip with an array of  freestanding membrane windows. The samples slightly overfilled the accessible area. *Please contact UED staff about custom mounts Max throughput: 2000-3000 samples per chip with 12 hours typical measurement time
Sample Temperature	RT (Discuss with staff about experiments above and below RT)
Electron Beam Parameters (typical)	2 - 4 MeV 20 - 100 fC (105 - 106 electrons / pulse) ~200 um diameter spot at sample <1 Hz (sample alignment dependent) < 200 fs FWHM pulse duration (charge dependent)
Optical Pump	See laser parameters table below for pulse energies

High Temperature Mode — Single Shot & High Temperature Chamber

Description: In a newly offered mode, a new custom sample holder with integrated heaters allows the precise control of the sample temperature from RT to 800 K. This facilitates exploring solid-solid phases transitions, quantum order, and structural dynamics at elevated temperatures.

Electrical Pump/Bias Mode — Single Shot & High Temperature Chamber

Description: Electrical pulses synchronized to the UED pulses are provided to allow electrical excitation and/or dynamic biasing of the sample or device. This mode is provided to allow UED to address structural dynamics of in-situ micro-electronic systems under operating conditions. It is assumed that users will provide the sample ‘device’ on a thin film window suitable for UED experiments. 

Optical Laser Properties

Detectors

Two detectors are offered for run 5. 

Sample Considerations

All UED experiments are conducted in transmission geometry. It is important that samples are sufficiently thin to allow the MeV beam to transmit to the detector. The following is given as general guidance for the thickness of UED samples and their supports. Users will be required to ship samples to SLAC before any allocated beamtime to allow time for installation and screening. Screening of samples may be required before an experiment. Please contact your assigned UED Point of Contact (POC) for details.

Samples should be thin, of the orders of 10s of nm, with only an essential support. Commercial TEM grids, either mesh grids or  Si₃N₄ chips, are ideal sample supports. The electron beam size at the sample is typically ~100 um diameter. Samples that are smaller than 10 x 10 um² may prove difficult to measure due to low scattering signal. The sample thickness and support requirements are UED technique dependent. The following table summarizes these different requirements.  

*Please note that laser pump penetration may be an important additional consideration in many experiments.