E5 Particle Sources

Topic

Everything described so far—radio-frequency acceleration, magnetic guidance, optics, and bunching—acts upon particles that must first be produced by a source. For electrons, the most common source extracts electrons from a material's surface by heating it or by directing a laser pulse onto a photoemissive surface; the latter method can produce extremely short bunches that are well-synchronized with the rest of the machine. For protons, the typical source ionizes hydrogen gas via an electric discharge or microwave plasma, extracting the resulting nuclei using an electric field. For heavier ions, more sophisticated plasma sources strip atoms of several electrons to yield particles with higher electric charges; this is advantageous because a more highly charged particle gains more energy at each acceleration stage described in the first module of this catalog. Regardless of the particle type, the source determines properties that propagate throughout the subsequent acceleration chain: the available initial current, the initial beam emittance, and the initial temporal structure of the bunches. No subsequent stage can improve beam quality beyond what the source initially delivered; thus, a limitation in the source design propagates as a limitation across the entire machine. For this reason, this module—which begins by noting that all the previously described systems require particles to act upon—concludes by establishing that the quality of those particles, set at the source, acts as a ceiling that no subsequent stage can surpass.

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