Project Heliacal: A Kilometer-Scale Laser-Plasma Wakefield Accelerator for 10 TeV Collisions

Project Heliacal proposes a linear electron-positron collider based on laser-plasma wakefield acceleration (LWFA). Unlike the LHC or future circular colliders, we skip the hundreds of kilometers of superconducting magnets. Instead, a series of high-power laser pulses (petawatt-class) are fired into a low-density plasma, creating a wakefield that accelerates electrons to ultra-relativistic energies over centimeters. The plan chains hundreds of such stages, each adding ~10 GeV, to reach 10 TeV in a total length of ~800 meters. The positron arm uses a separate, identical accelerator. The beams are focused to nanometer spot sizes at the interaction point using plasma lenses. This design rejects the mainstream reliance on brute-force scaling of magnetic fields and tunnel length, instead leveraging the immense electric fields achievable in plasmas (TV/m vs MV/m in RF cavities). The entire facility would fit on a university campus, costing roughly 1% of a traditional collider. The project includes a novel laser synchronization system using frequency combs, and a plasma source based on a lithium heat pipe oven for uniform density profiles. The goal is to demonstrate staged acceleration to 1 TeV within five years, then push to 10 TeV.

Particle physics has hit a wall: the energy frontier is stalled because circular colliders become exorbitantly expensive and physically limited by synchrotron radiation. The LHC's successor (FCC, CEPC) would cost tens of billions and require decades of construction. Meanwhile, laser-plasma accelerators have shown gradients 1000x higher than RF cavities in single-stage experiments, but no one has attempted a multi-stage collider. The mainstream accelerator community is wedded to mature but stagnant technology; they dismiss plasma concepts as too unstable or low-repetition-rate. This project scratches that itch: a practical, staged design that could bring 10 TeV collisions within reach of a single lab, re-energizing the field and opening new discovery potential for dark matter, extra dimensions, and more.

• Demonstrate 10 GeV energy gain in a single laser-plasma stage with >1% energy spread and >1e9 electrons per bunch
• Develop a plasma mirror-based coupling to inject successive laser pulses into each stage without destroying upstream optics
• Achieve stable, repeatable operation at 10 Hz repetition rate (not single-shot)
• Show controlled injection of low-emittance electron bunches using density gradient injection
• Build a 10-stage prototype accelerating electrons from 100 MeV to 100 GeV over 10 meters
• Design and simulate a full 1000-stage, 10 TeV collider with beam optics and background suppression
• Produce a cost estimate and siting plan for a 10 TeV collider on a 1 km linear footprint

• Not building a collider that exceeds the energy or luminosity of existing machines until the prototype validates the concept
• Not solving all beam-stability issues at the outset; acceptable to have lower luminosity than circular colliders initially
• Not developing commercial laser systems; we will use existing petawatt lasers (e.g., BELLA, Vulcan) with modifications
• Not addressing positron generation from scratch; we assume a conventional positron source injected into the plasma accelerator
• Not designing a detector for the collider; we focus on the accelerator only

Petawatt-class Ti:sapphire or diode-pumped laser systems (e.g., from BELLA or ELI) with pulse durations <50 fs and energy >30 J. Plasma sources: lithium heat pipe oven for cm-scale uniform density (10^17-10^18 cm^-3) with <1% variation. Diagnostics: magnetic spectrometer (dipole magnet + scintillator screen) for energy spectrum, streak camera for bunch length, interferometry for plasma density. Staging optics: plasma mirrors (thin fused silica) to separate stages and couple laser pulses. Control system: FPGA-based timing with femtosecond synchronization using optical frequency combs. Vacuum system: differential pumping between stages to maintain low pressure. Simulation tools: particle-in-cell codes (e.g., OSIRIS, PIConGPU).

The accelerator consists of a linear array of ~1000 identical stages, each 0.8 m long. Each stage contains a gas cell (lithium vapor) wherein a petawatt laser pulse drives a plasma wake. Electron bunches are injected at the front of each stage from a conventional RF gun (or from the previous stage's output) and are accelerated by the wakefield. Laser pulses are injected perpendicular to the electron beam via a plasma mirror at the entrance of each stage, which directs the laser into the plasma while transmitting the electron beam. After each stage, the laser pulse is discarded (or redirected for energy recovery). The stages are spaced by a drift section containing a focusing plasma lens (capillary discharge) to keep the beam focused and align it to the next stage's entrance. The entire assembly is housed in a vacuum tunnel ~1 km long, with laser bays every 100 m for amplifying the laser pulses (using chirped-pulse amplification). The interaction point at the end of the linac has a final focus using a plasma lens to achieve nanometer spot sizes. The positron linac is identical but reversed in direction. The two beams collide head-on at the IP. The design luminosity is ~10^34 cm^-2 s^-1, achieved by high repetition rate (10 kHz) and micro-bunch trains.

• Laser damage: plasma mirrors have limited lifetime (~100 shots); need robust replaceable optics or thin-film tapes.
• Beam emittance growth: wakefield acceleration can increase emittance due to transverse fields; active feedback needed.
• Stage coupling: alignment of electron beam to laser axis across 1000 stages is extremely tight (micron-level); active steering using BPMs and correctors.
• Plasma instabilities: self-modulation or hosing can disrupt acceleration; mitigation via transverse laser shaping or plasma density ramps.
• Positron acceleration: positrons behave differently in plasma wakes; may require separate design or use of electron-driven wake.
• Cost and funding: a full 10 TeV version would still be expensive (~$500M); need incremental demonstration to attract funding.

• Can we achieve the required emittance (< 1 mm-mrad) for high luminosity without active cooling of the plasma?
• What is the maximum practical length of a single stage before dephasing or diffraction limits the energy gain?
• Can we use a single laser pulse per stage, or do we need multiple pulses (e.g., a pulse train) to maintain a stable wake?
• Is it better to use a gas jet or a heat pipe for the plasma source? Which offers better uniformity and density control?
• How do we efficiently generate and inject positrons into the plasma accelerator? A conventional source may limit the efficiency.

The project was conceived as a decade-long roadmap during a sabbatical at a national lab, but the principal investigator took a position at a startup commercializing LWFA for medical applications and never secured the consortium funding needed. The plan remains on a personal website and a few grant proposals that were politely declined.

This design is inspired by the 'plasma wakefield accelerator' concept originally proposed by Tajima and Dawson (1979). Recent experiments at SLAC (FACET) and LBNL (BELLA) have shown single-stage gradients >50 GeV/m. The ambitious leap here is the staging and the extension to collider parameters. A similar approach is being pursued by the AWAKE collaboration (proton-driven), but we prefer laser-driven for better control and compactness. The project would benefit from international collaboration with groups in the US, Europe, and Asia.