The Physics
Opus in profectus


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  1. The US Department of Energy's Linac Coherent Light Source (LCLS) in Menlo Park, California was the world's most powerful x-ray laser when it opened for business in 2009. The LCLS generates x-rays from a beam of electrons accelerated from zero to 99.9999999 per cent the speed of light in 3.2 km. The central tube of this linear accelerator or linac is the straightest object in the world and lies beneath the longest building in the United States. The beamline ends at a 100 m long series of alternating magnets called an undulator where the electrons slalom back and forth in a synchronized fashion generating coherent x-rays. The pulses are quick enough (t < 100 fs) that ultrafast events can be captured and the x-rays are short enough (λ = 0.15 nm) that ultrasmall objects can be imaged. By stringing together sequences of ultrafast, ultrasmall images the LCLS is effectively a movie camera for individual molecules.

    Determine the following quantities for the LCLS…

    1. the length of a pulse
    2. the number of wavelengths in a pulse
    3. the energy of a single x-ray photon in the laser beam
    4. the energy of a single electron in the linac
    5. the number of photons produced by one electron assuming 100% efficiency
    6. the power of a single x-ray pulse if it contains about a trillion photons (n = 1012)
    7. the average power of the x-ray beam if it pulses 120 times per second
  2. Write something.
  3. Write something.
  4. Write something completely different.


  1. A typical cathode ray tube (CRT) television or computer display manufactured at the end of the 20th century accelerated electrons across a 10,000 V potential difference in a space of 0.5 m. Most of the energy of these electrons was converted into visible light, but a small portion of the beam's energy went into generating x-rays. Determine the minimum wavelength of the x-rays unintentionally produced by this once common household device.