MeV-UED Run 7 Scientific Capabilities

SLAC MeV-UED Run 7 will provide support for materials science research and materials under extreme conditions communities. SLAC 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 "real" 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). 

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.

Contents

• Cryo and Quantum Materials Chamber
• Single Shot & High Temperature Chamber
• Single Shot & High Temperature Chamber Operational Modes
  • Single Shot Mode — Single Shot & High Temperature Chamber
  • High Temperature Mode — Single Shot & High Temperature Chamber
  • Electrical Pump/Bias Mode — Single Shot & High Temperature Chamber
• Optical Laser Properties
• Detectors
• Sample Considerations

Cryo and Quantum Materials Chamber

This chamber is configured for measurements of structural dynamics from RT to cryogenic 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

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 7: 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). 

Parameter	Value
Location	Downstream chamber (~2.25 m from the electron gun)
Measurement	Transmission ultrafast electron diffraction
Operational Modes	Solid state single-shot UED
	High temperature UED
	Electrical excitation or biasing
Sample Temperature	RT (see modes)
Samples	Requirements vary depending on mode (see modes tables for details)
Vacuum Level	< 10-6 Torr * *Samples and user components need to be compatible with HV ** Chamber pump down to operation ~1 hour
Electron Beam Parameters	2 - 4 MeV 1 - 100 fC (104 - 106 electrons / pulse) Single shot - 540 Hz (1080 Hz*) ~ 200 fs FWHM pulse duration (charge dependent) 100 um diameter at sample (typical) *reduced e-beam stability energy
Sample Motions	Five axis motion stage  Sample Vertical (Y) +/- 12.5 mm Sample Horizontal (X) +/- 25 mm Sample along beam (Z) +/- 15 mm* Yaw (rotation about Y) -50 deg – +50 deg* Pitch (rotation about X) +/- 50 deg* *depends on constraints and mode (no rotation about beam axis)
Laser Pump wavelengths	800 nm, 400 nm, 266 nm OPA UV - 2 um (See detailed laser parameters below)

Single Shot & High Temperature Chamber Operational Modes

Single Shot Mode — Single Shot & High Temperature Chamber

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

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.

Parameter	Value
Samples	Up to nine 3 mm TEM style grids  (Si3N4 or Mo are suggested as preferred grid materials)
Sample Temperature	RT -- 800 K +/- 0.1 K stability over 1 hour +/- 1 K uniformity
Additional Constraints	Rotations about the pitch and yaw axes are limited to ~22 degrees due to shadowing by the sample holder.

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

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.

Parameter	Value
Samples	Please contact Alex Reid to discuss sample integration
Electrical Pulses	Values represent pulse durations and amplitudes in an 50 Ohm impedance matched device.  Rise time Duration Max Voltage 50 Ohm Load ~150 ps >1.5 ns - several ns 4 V ~1.5 ns  >4 ns  5 V Pulses generated with SRS DG645 with optional SRD1 attachment (DC biasing is also feasible) (Contact staff for information about integrating user supplied electronics)
Electron Beam Parameters	2 - 4 MeV 1 - 100 fC (104 - 106 electrons / pulse) Single shot, 120 Hz, 180 Hz, 360 Hz, 540 Hz (1080 Hz*) ~ 200 fs FWHM pulse duration (charge dependent) 100 µm diameter spot at sample (for probing smaller devices it is suggested that the active area is placed a suitably sized aperture, e.g.  20 x 20 µm2 Si3N4 window)  *reduced stability

Optical Laser Properties

Parameter	Value
Repetition rate	Single shot, 120 Hz, 180 Hz,  360 Hz (1080 Hz*)
Typical optical pulse energy	> 8 mJ @ 800 nm > 0.8 mJ @ 400 nm > 0.08 mJ @ 266 nm ~ 16  µJ @ 200 nm For pulse energies at specific wavelengths (e.g 240 nm - 12  µm tunable) or other details please contact Patrick Kramer.
Nominal pulse duration	75 fs (FWHM)* * Depends on the wavelength **Tunable pulse durations up to 10s of ps at 800 nm with potential applications in ps laser heating and dynamic compression studies. .
Optical delay	0 - 1.5 ns* (physical delay stage) *Please contact UED staff for longer delay time ** Depends on the wavelength
Optical spot size	Profiles available:  Gaussian 200 - 1500 µm (FWHM) Flat top (300-400 µm)* *800 nm and 400 nm only, please contact Patrick Kramer.

Detectors

Two detectors are offered for run 7. A magnetic telescope is available for increased q-resolution for measurements in the Cryo/QM chamber. This lens system has been commissioned for use with the Andor detector, but use with the ePIX may also be possible. Please contact UED staff for more information.

P43 phosphor screen &  Andor iXon Ultra 888 EMCCD
Phosphor thickness	50-100 um
Point-spread-function	85 µm (rms)
Pixel size	13 x 13 µm2
Sensor size	1024 x 1024
Data format	.tiff
Pixel well depth	80k e-

 

ePIX detector -  direct electron detection
Pixel size	100 x 100  um2
Frame rate	540 Hz
Sensitive area	5 x 5  cm2
Gain	3 fixed plus 2 with auto ranging and programmable switch point
S/N in High Gain per MeV electron	530

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.  

Post-sample apertures down to 15 micron diameter are available for the selection of small sample regions for measurement. The apertures are mounted on translation stages downstream of the sample and can be used to select a small region of a larger sample, or a single sample flake while masking nearby flakes. Please contact UED staff for more information.

Technique	Comments	Thickness Range*
Diffuse Scattering on single crystal samples examining phonon dynamics	Optimized thickness is vital as these experiments are cross section limited. Samples should be freestanding or on ultra thin substrates as diffuse scattering is easily swamped by electron scattering for any support layer.	Low Z materials: <= 100 nm High Z materials: <= 15 nm   Support < 20 nm Si3N4
Debye Waller and Structure factor analysis	It is important to stay at thicknesses below significant multiple scattering.    Thin amorphous supports are ideal if required.	Low Z materials: < 200 nm High Z materials: < 30 nm   Support <= 50 nm Si3N4
Lattice deformation and superlattice order	Some multiple scattering can be tolerated.    Thin amorphous supports are ideal if required.	Low Z materials: < 300 nm High Z materials: < 50 nm   Support <= 50 nm Si3N4

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