MeV-UED Run 6 Scientific Capabilities

SLAC MeV-UED Run-6 will focus on advancing gas- and liquid-phase ultrafast chemical science, building on the successes of previous gas-phase user offering (Run-2 and Run-4) and the recent liquid-phase early science experiments (August 2024). This run will offer both gas-phase and liquid-phase sample deliver for general access user experiments. We will utilize gas-liquid sample chamber while integrating the latest improvements at the MeV-UED facility. Run-6 will build on the established sample delivery methods, including the flow-cell and slit-jet for gas-phase studies and the liquid-jet for liquid-phase experiments. It will also take advantage of the successful implementation of high-repetition-rate operation (1080 Hz). Meanwhile, R&D efforts will focus on enhancing single-shot single-electron detection capabilities, implementing time-of-arrival jitter corrections with newly deployed electron detectors, and further refining sample delivery systems for both gas and liquid phases.

Below table is a list of key capabilities for Run-6 experiments

• Sample Delivery
  • Current sample delivery capabilities
  • Sample Delivery R&D
• Optical Laser Properties
  • New capabilities of the DUV generation
• Detectors
  • P43 phosphor screen & Andor iXon Ultra 888 EMCCD
  • ePix10k detector - direct electron detection with single shot capability up to 360 Hz 
• Data acquisition mode
  • Single shot detection, ePix based
  • Integration mode, Andor based

MeV-UED Electron Beam

The SLAC MeV-UED has commissioned a new solid-state modulator and new klystron source for the source photocathode. Parameters from the new system are given in the table below.

Sample Delivery

In the gas-phase experiment during Run-6, two standard sample delivery methods are available: (1) cw Flow Cell, ideal for systems with moderate vapor pressure, including gases and volatile liquids. (2) cw Slit-Jet, designed for delivering low-vapor-pressure powdered or crystalline samples. Historically, our best results with the flow cell have been achieved using samples with vapor pressures above 0.5 Torr at room temperature. Before Run-4, samples requiring higher temperatures to increase their vapor pressure could only be used in combination with a Parker valve, which operated at 50% of its maximum repetition rate (180 Hz). Since Run-4, we have implemented an in-vacuum cw slit-jet and catcher sample delivery system, successfully conducting experiments across a broad temperature range, from 70°C to below 300°C. Unlike the flow cell, the slit-jet introduces excess heat, which may induce sample reactions or decomposition. Therefore, to ensure proper sample delivery, a pre-experiment test is required to confirm that the sample can reach sufficient vapor pressure at the target temperature without undergoing thermal degradation.

In the liquid-phase experiment, we utilize a standard sample delivery system based on a liquid-jet setup. The liquid sample is introduced through a converging sheet-jet nozzle into the interaction region under vacuum. After passing through the interaction point, the liquid is collected by a catcher and recirculated back to the sample reservoirs. The jet is maintained at a thickness of 250 nm, with a sample consumption rate of approximately 2.7 mL/min. To ensure stable operation and successful sample recirculation, the vapor pressure of the pure solvent or solution is limited below 25 Torr, preventing excessive vapor load in the vacuum system. This limit helps maintain vacuum conditions below the tripping threshold. Early science experiments have demonstrated successful operation with solvents such as water and 1-propanol. For solutions, volatile solutes are preferred, as salts tend to crystallize when the jet enters vacuum due to rapid cooling effects, which can obstruct smooth operation.

Sample Delivery R&D

MeV-UED remains committed to advancing its sample delivery capabilities. In the gas phase, we will continue to enhance the performance of both the flow-cell and slit-jet delivery systems. For the flow-cell, we plan to introduce mild heating to the external sample reservoir and transport lines, maintaining temperatures below 60°C. This will help increase vapor pressure, particularly for liquid samples with relatively low volatility under ambient conditions. For the slit-jet, several key improvements are planned. We aim to explore heating temperatures ranging from 300°C to 500°C for samples requiring extensive thermal input. Additionally, a larger and more efficient catcher design will be implemented to optimize sample collection. To enhance stability and efficiency, a more precise temperature regulation system will be introduced, ensuring consistent heating while minimizing manual intervention. Furthermore, gas-assisted cooling systems will be employed to prevent overheating and enable controlled cooling for venting requirements. In liquid-phase sample delivery, ongoing R&D efforts are focused on expanding the range of compatible solvents to include those with higher vapor pressures. Key objectives include improving the efficiency of the catcher and recirculation system and extending the upper vapor pressure limit beyond 50 Torr for target solvents used in experiments. Further information or requesting for custom development for the user's sample on a best effort basis, please contact the local UED team.

Optical Laser Properties

Our MeV-UED laser team continues to expand laser capabilities, with a key advancement being the successful deep UV wavelength generation demonstrated during the Run-4 R&D efforts. By leveraging wave mixing between the 2^nd harmonic generation (400 nm) and the visible output (480–600 nm) from the OPA + NirUVis mixer, we achieved the generation of ultrashort pulses in the 220–240 nm range with pulse energies in the tens of μJ. We plan to introduce this new laser capability in the Run-6 experiments, starting with in-house experiments.

Detectors

In Run-6, we will continue to offer our conventional electron detector, which combines a P43 phosphor with an Andor EMCCD, while also introducing the ePix10K single-shot electron detector. The ePix10K is designed to enhance electron detection efficiency, providing single-electron sensitivity and an extended Q-range for improved data acquisition. With its specialized design for high-energy electron and X-ray detection, the ePix detector is expected to enable pump-laser (with a high limit of a few electron volts) background-free detection. Although it has demonstrated superior electron detection efficiency, the ePix system significantly increases the complexity of data acquisition. As this is an early-stage implementation, experiments using ePix carry inherent risks. However, by maintaining the availability of the Andor EMCCD, we ensure a seamless transition between detectors without interrupting the experiment, eliminating the need for a bunker entry to make the switch.

Single-shot based data acquisition

MeV-UED is entering a new phase of data acquisition with the implementation of the single-shot (up to 360 Hz) ePix10K detector. Following several successful in-house and user experiments in Run-5 utilizing this direct electron detector, we are exploring the integration of a single-shot data acquisition framework alongside diagnostics for laser pulses and electron bunches.

Single-shot data acquisition offers several advantages, particularly for gas phase experiments. In gas-phase studies, electron scattering is inherently weaker due to the lower sample concentration. With the ePix10k’s single-electron detection capabilities and hit identification, signal detection is significantly improved, especially for larger-angle scattered electrons (high Q), where fewer electrons contribute to the diffraction pattern. Additionally, unlike conventional phosphor-EMCCD detection, the ePix10k direct detector provides aberration-free imaging, preserving diffraction patterns with higher fidelity, particularly in the large-Q regions. Furthermore, single-shot data acquisition with ePix10K enables precise synchronization of pump-laser and probe-electron bunch diagnostics. A key advantage is the ability to track shot-to-shot variations, such as time-of-arrival jitter in electron bunches, enhancing the accuracy and reliability of ultrafast measurements. By continuously monitoring these diagnostics on a per-shot basis, scattering signals can be further processed with proper corrections and filtering, ultimately improving the signal-to-noise ratio and the accuracy of pump-probe measurements.

As we integrate the direct detector, we remain committed to supporting our conventional phosphor + Andor EMCCD electron detection system, which has demonstrated high reliability and consistency throughout the history of SLAC MeV-UED. The current detector configuration enables rapid switching between the two systems without interrupting electron beam delivery, i.e., no need for bunker entry. Detector selection and data acquisition mode are determined based on user group requests.