Power-Dominant Regime in High-Field Physics

• Power-dominant regime is defined as conditions where energy conversion in high-intensity laser–matter interactions is governed by nonlinear radiation friction and QED effects.
• Experimental evidence shows that, at intensities above 10^23 W/cm², electron damping leads to efficient gamma-ray production with conversion efficiencies reaching tens of percent.
• Theoretical models using normalized laser field amplitude, radiation friction thresholds, and QED parameters guide the optimization of ultra-intense laser experiments.

A power-dominant regime refers to conditions in which a particular physical, mathematical, or network effect is governed by mechanisms associated with “power,” typically in the sense of energy emission, dissipation, or dominance in dynamical processes. In high-intensity laser–matter interactions, the power-dominant regime is specifically defined by a transition in the behavior of electrons subjected to extreme electromagnetic fields—where nonlinear radiation losses (“radiation friction”) and quantum electrodynamics (QED) effects play crucial roles in the conversion of electromagnetic energy into hard gamma radiation.

1. Regime Definitions and Transition Criteria

The central dimensionless control parameter is the normalized laser field amplitude: $a = \frac{e E \lambda}{2\pi m_e c^2}$ where $E$ is the electric field magnitude, $\lambda$ the wavelength, $m_e$ the electron mass, $e$ the elementary charge, and $c$ the speed of light. For $a > 1$, electron motion is relativistic. Power-dominant effects arise when two additional thresholds are crossed:

• Radiation friction threshold: $$a > a_{\mathrm{rad}} = \epsilon_{\mathrm{rad}}^{-1/3}$$, with $$\epsilon_{\mathrm{rad}} = \frac{4 \pi r_e}{3 \lambda}$$ and $r_e = e^2/(m_e c^2)$ the classical electron radius.
• QED effect significance: Characterized by the dynamical parameter

$\chi_e \approx 2 \frac{a}{a_S} \gamma_e$

with $a_S = \frac{m_e c^2}{\hbar\omega}$ and $\gamma_e$ the Lorentz factor. QED corrections are important for $\chi_e \gtrsim 1$.

These thresholds delineate distinct physical domains (see Fig. 2 and Fig. 5 of (Bulanov et al., 2013)):

• Domain I: Purely relativistic regime, negligible radiation friction and QED.
• Domain II: Radiation friction–dominant, efficient total electromagnetic energy conversion into radiation.
• Domain III: Onset of QED corrections without dominant radiation friction.
• Domain IV: Both QED and radiation friction are significant, limiting achievable electron energy and setting the gamma-ray spectrum.

2. Electron Dynamics and Energy Conversion

When entering the radiation friction–dominated regime ($a > a_{\mathrm{rad}}$), the electron’s energy absorption from the EM field is rapidly reradiated as high-energy photons. The total radiated power scales strongly: $$P_\gamma \approx \epsilon_{\mathrm{rad}} m_e c^2 \omega \gamma_e^4$$ Energy balance analysis reveals that at these intensities, radiative losses can damp electron oscillations within one laser period ($\sim$ fs-scale), causing most incoming laser energy to be rerouted to high-frequency emission. The nonlinear Thomson/Compton scattering mechanism is central; the characteristic emission frequency is: $\omega_c \approx 0.3 \omega \gamma_e^3$ For typical $\gamma_e \sim a$, $\omega_c$ reaches the gamma-ray regime for field intensities above $10^{22}$–$10^{23}$ W/cm$^2$.

Quantum corrections—when included via the factor $G_e(\chi_e)$,

$$G_{R}(\chi_e) \approx [1 + 8.93\chi_e + 2.41\chi_e^2]^{-2/3}$$

weaken classical radiation reaction at large $\chi_e$. This modifies both the total emission and the emission spectrum, introducing non-deterministic recoil ("straggling") and momentum diffusion in electron trajectories.

3. Mathematical Framework for Electron Motion

The evolution of the normalized electron momentum vector $\mathbf{q}$ in a rotating electric field, including both radiation friction and QED effects, is governed by a Landau–Lifshitz-type equation: $$\frac{d\mathbf{q}}{d\tau} = -\mathbf{a} - \frac{\epsilon_{\mathrm{rad}} G_{e}(\chi_e)}{\gamma_e} \left\{ \gamma_e^2 \frac{d\mathbf{a}}{d\tau} - \mathbf{a}(\mathbf{q} \cdot \mathbf{a}) + \mathbf{q}\big[(\gamma_e \mathbf{a})^2 - (\mathbf{q} \cdot \mathbf{a})^2\big] \right\}$$ with $\tau = \omega t$ and $\mathbf{a}$ the normalized field vector. Analysis in a basis aligned/orthogonal to the field demonstrates rapid damping and dominant gamma-ray emission at $a > a_{\mathrm{rad}}$.

4. Experimental and Simulation Evidence

Simulations demonstrate that for ultra-high intensities, conversion efficiency from laser energy to hard gamma photons can reach tens of percent, especially within the power-dominant (radiation friction) regime. Including QED effects (via $G_e(\chi_e)$) slightly decreases the yield, but overall efficiency remains high. Fast damping of electron energy, strong spectral broadening, and ultra-short emission durations (femtoseconds or less) are hallmarks.

5. Practical Implications in Extreme Laser-Plasma Physics

In high-power laser–matter experiments (multi-petawatt to exawatt class), the regime dictates:

• Fast and efficient hard photon (gamma-ray) flash generation.
• The necessity to account for radiative loss when designing plasma targets and density profiles for optimized conversion.
• Accurate modeling of emission spectra and electron energy evolution requires inclusion of both nonlinear classical damping and quantum modifications.

Relevant intensities ($I_{\mathrm{rel}}$) are given by: $$I_{\mathrm{rel}} = 1.37 \times 10^{18} a^2 (\lambda / 1\,\mu\text{m})^2\,\text{W/cm}^2$$ Domain transitions across $a_{\mathrm{rad}}$ and $\chi_e \sim 1$ typically occur for intensities above $10^{23}$ W/cm$^2$ (for $\lambda \sim 1\,\mu$m).

6. Relation to Advanced Applications

The understanding of the power-dominant regime is central for:

• Laser-driven gamma-ray source development.
• Design and interpretation of experiments probing strong-field QED.
• Next-generation high energy density physics (HEDP) applications where rapid, energetic dissipation is required.

Strong interaction between high-field quantum and classical radiation physics defines design constraints and expected performance for future ultra-intense laser facilities.

7. Summary of Key Formulas

Quantity	Formula	Physical Meaning
Normalized amplitude	$a = \frac{eE\lambda}{2\pi m_e c^2}$	Relativistic electron threshold
Power-dominated emission	$$P_\gamma \approx \epsilon_{\mathrm{rad}} m_e c^2 \omega \gamma_e^4$$	Loss rate with radiation friction
Dominance of radiation friction	$a_{\mathrm{rad}} = \epsilon_{\mathrm{rad}}^{-1/3}$	Damping threshold
QED parameter	$\chi_e \approx 2(a/a_S)\gamma_e$	Quantum correction threshold
Photon frequency	$\omega_c \approx 0.3 \omega \gamma_e^3$	Hard gamma-ray scaling
QED weakening factor	$$G_R(\chi_e) \approx [1 + 8.93\chi_e + 2.41\chi_e^2]^{-2/3}$$	Reduction in classical reaction

Conclusion

The power-dominant regime in high-intensity laser–matter interactions is defined by dominant dissipation via nonlinear radiation friction and QED-corrected emission. This regime enables extremely efficient transformation of electromagnetic energy into ultra-short, ultra-intense gamma-ray flashes, with strong implications for advanced physical experiments and the understanding of strong-field quantum processes. Transition into and across this regime is governed by the normalized field amplitude and the dynamical QED parameter, shaping the electron dynamics, emission characteristics, and experimental constraints for extreme plasma and radiation physics (Bulanov et al., 2013).