Photon Acceleration-Based Radiation Sources

The acceleration and deceleration of photons m a plasma provides the means lbr a series of new radiation sources. Previous work on a DC to AC Radiation Converter (DARC source) has shown variable acceleration of photons having zero frequency (i.e., an electrostatic field) to belx~een 6 and 100 GHz (I-3). These sources all had poor guiding characteristics resulting in poor power coupling froln the source to the load. Continuing research has identilied a novel way to integrate the DARC source into a waveguide. The so called "'pin structure" uses stainless steel pins inserted through the narrm~ side of an X band waveguide to form the electrostatic field pattern (k ,, 0, m = 0). The pins are spaced such that the absorption band resulting from this additional periodic structure is outside of the X band range (8-12GHz), in which the normal waveguide characteristics are left unchanged. The power of this X band source is predicted theoretically to scale quadratically with the pin bias voltage as -800W,'(kV)= and have a pulse width of ~ Ins. Cold tests and experimental results are presented. Applications tor a high power, short pulse radiation source extends to the areas of landminc detection, improved radar resolution, and experimental investigations of molecular systents. INTRODUCTION Three methods of photon acceleration have been proposed in tile past: the photon acceleralor, which uses a plasma wakefield to acceleratc a photon bunch: fi'eqttency upshifting by over and underdense ionization fronts: and DC to AC radiation conveners (see figure 1). In the first scheme called the photon accelerator, an initial laser pulse moves through a plasma producing a plasma wake which accelerates a trailing laser pulse. The amount of acceleration, or equivalently, the frequency upshift of trailing photons is given by ~ is the frequency of the incident photon, and E is the plasma wave amplitude (typically, 0.1 0.5). For typical values of E. an upshift factor of about 5 can be achieved(4). In the second scheme, a traveling electromagnetic wave is incident upon a moving ionization front (plasma/gas boundary usually produced by a laser), and is either reflected, if the plasma density is such that the corresponding plasma frequency is larger than the double Doppler shifted frequency (overdense), or is transmitted, if the plasma density is below the transmitted frequency (underdense). The frequency CP472, Advanced Accelerator Concepts: Eighth Workshop, edited by W. Lawson, C. Bellamy, and D. Brosius © 1999 The American Institute of Physics 1-56396-889-4/99/$15.00 544 A)Photon Accelerator: lasma de-~s~ty B)lonization Fronts~ I/~ plasma density ~x,~ [ C