The fundamental difference between an undulator and a wiggler lies in their K value, also known as the deflection parameter, which dictates the electron beam's trajectory and, consequently, the properties of the emitted synchrotron radiation.
Understanding Undulators
An undulator is a type of insertion device used in synchrotron light sources that forces an electron beam to oscillate gently. This gentle oscillation is achieved because undulators operate with a low K value, typically less than 3 (K < 3).
- Radiation Properties: Due to the small deflection angle, the radiation emitted at each bend along the electron's path constructively interferes. This results in highly collimated (narrow beam), quasi-monochromatic (single wavelength or narrow band of wavelengths), and coherent X-ray light. The spectrum consists of a series of narrow peaks.
- Number of Periods: Undulators are characterized by a large number of periods, meaning the electron beam undergoes many gentle wiggles. This large number of periods enhances the constructive interference, leading to the undulator's unique radiation properties.
- Applications: Undulators are preferred for experiments requiring high spectral brightness, coherence, and tuneability, such as in spectroscopy, imaging, and advanced materials science, where precise wavelength control is crucial.
Understanding Wigglers
In contrast, a wiggler is another insertion device that causes a much larger deflection of the electron beam. Wigglers operate with a large K value, typically greater than 3 (K > 3).
- Radiation Properties: The significant deflection in a wiggler means that the radiation emitted at each bend adds up incoherently or with limited constructive interference. This produces a broad, continuous spectrum of X-rays with higher photon energies compared to undulators. While not as coherent or monochromatic as undulator radiation, wigglers provide higher total flux (more photons) over a wider energy range.
- Number of Periods: Wigglers generally have a lower number of periods compared to undulators, as their primary goal is to produce high-energy, high-flux radiation across a broad spectrum rather than exploiting interference effects.
- Applications: Wigglers are valuable for experiments needing high flux over a broad energy range, such as in X-ray absorption spectroscopy (XAS), protein crystallography, and some types of imaging that can benefit from a white beam.
Key Differences at a Glance
The table below summarizes the primary distinctions between undulators and wigglers:
| Feature | Undulator | Wiggler |
|---|---|---|
| K Value (Deflection) | Low (K < 3) | Large (K > 3) |
| Electron Path | Gentle, small oscillations | Large, wide oscillations |
| Number of Periods | Large number of periods | Lower number of periods |
| Radiation Properties | Quasi-monochromatic, coherent, highly collimated, narrow spectral peaks | Broad spectrum, high total flux, incoherent, less collimated |
| Brightness | Very High | Lower than undulators (but higher total flux) |
| Typical Applications | High-resolution spectroscopy, coherent imaging, resonant scattering | X-ray absorption, protein crystallography, general high-flux applications |
Both undulators and wigglers are essential components of modern synchrotron facilities, providing different types of X-ray beams optimized for various scientific and industrial applications.
