LCLS-II will be the first XFEL to be based on continuous-wave superconducting accelerator technology (CW-SCRF). The linac energy of 4 GeV will drive two tunable-gap undulators (SXU, HXU), LCLS-II will generate soft X-ray pulses from 0.25 to 5 keV (2.5 Å) at repetition rates up to 1 MHz. The energy upgrade (LCLS-II-HE) will take advantage of infrastructure already being installed as part of the ongoing LCLS-II construction project.

LCLS-II is based on new accelerator cryomodules to be installed in the first 750 m of the 3 km SLAC linac tunnel (L1, L2, L3). By adding additional cryomodules in the final 250 m of the refurbished tunnel (L4), LCLS-II-HE will double the electron beam energy to 8 GeV and thus increase the spectral reach of the hard X-ray undulator (HXU) to more than 12 keV at repetition rates up to 1 MHz. Anticipated improvements in electron beam emittance will extend the energy reach of LCLS-II-HE to 20 keV at high repetition rates. At the same time, the existing Cu-linac and new tunable-gap hard X-ray undulator (HXU) will provide photon energies up to 25 keV, and multi-mJ pulses from the soft X-ray undulator (SXU) at 120 Hz. Both accelerators (Cu-linac and CW-SCRF) can operate simultaneously, driving either SXU, or HXU, or both simultaneously (e.g. by time-multiplexing pulses from a single linac).

LCLS-II-HE will:

• Deliver two to three orders of magnitude increase in average spectral brightness beyond any proposed or envisioned diffraction-limited storage ring (DLSR).
• Provide temporal coherence for high-resolution spectroscopy near the Fourier transform limit with more than 300-fold increase in average spectral flux (ph/s/meV) beyond any proposed or envisioned DLSR.
• Generate ultrafast hard X-ray pulses in a uniform (or programmable) time structure at a repetition rate of up to 1 MHz – a qualitative advance beyond the burst-mode nature of the European-XFEL, and a 100,000-fold improvement in temporal resolution compared to storage ring sources.

The X-ray performance of LCLS-II-HE in photons/pulse and average brightness are shown below:

Calculated photons per pulse for high-repetition-rate operation from LCLS-II soft X-ray (SXU) and hard X-ray undulator (HXU) at 4 GeV, and proposed LCLS-II-HE (8 GeV). Projected high-energy performance is bounded by the blue and green lines, dependent on electron beam emittance (with the resultant highest photon energy being between 12.8 and 20 keV). Note that ph/pulse is constant with repetition rate up to ~300 kHz, and scales inversely with repetition rate above ~300 kHz.

Average spectral brightness of current, planned, and potential future X-ray science facilities including diffraction-limited storage rings (DLSRs), and the EuXFEL. LCLS-II provides ~10,000-fold increase in average brightness in the soft/tender X-ray range and LCLS-II-HE provides ~1,000-fold increase (in the fundamental) to >12.8 keV, with a potential reach to ~20 keV (assuming improved e-beam emittance, indicated by the dashed line). All XFEL curves assume SASE operation. Self-seeding will increase the average brightness of XFELs by an additional factor of 20 to 50.

DLSR contributions from M. Borland (APS) and C. Steier (ALS)

Pulse structure from the LCLS warm Cu-linac at 120 Hz, burst-mode structure from the pulsed SCRF linac of the EuXFEL at 5 MHz/10Hz, and the uniform (programmable) bunch structure from the CW-SCRF linac of LCLS-II and LCLS-II-HE.

Further details are available in the report:
Linac Coherent Light Source II High Energy Upgrade (LCLS-II-HE) Conceptual Design Report (SLAC-R-1098 )