A.H. Ho

Interferometer mirrors with holes on-axis

Abstract Placing holes on-axis in the mirrors of an optical interferometer used for a free-electron laser (FEL) oscillator has several desirable effects. For a given amount of power generated at saturation, the peak power density on the mirrors is decreased, thereby ameliorating the problem of mirror damage; the electron beam can be introduced into the cavity in a collinear fashion; and the small-signal gain can be increased. Surprisingly, these holes may be quite large without a significant increase in cavity loss. This is a consequence of the fact that higher-order modes are generated which sum to a null of the field at the position of the holes.

A novel wiggler design for use in a high-efficiency free-electron laser

Abstract A wiggler, for use in a high-efficiency FEL system utilizing microwave acceleration to maintain synchronism, is proposed. The wiggler not only provides a periodic transverse magnetic field, but also acts as a loaded waveguide capable of supporting microwaves. Measurements on the wiggler confirm that it can be utilized in a high-efficiency design to attain conversion efficiencies of approximately 42% in a 2 m length at 10 μm wavelength.

High efficiency energy conversion of microwave to optical power within a free-electron laser oscillator

A microwave field can be used to accelerate electrons as they lose energy to radiation in an FEL, thereby maintaining synchronism and, used in conjunction with an optical klystron, providing high conversion efficiencies. The microwaves can be programmed to increase in time with the optical power level in the FEL so that high gain is maintained over a wide range of power levels. In this paper, parameter constraints for such an FEL are discussed, leading to a structure design that integrates a wiggler with a linac. It is shown that conversion ettlciencies of 50 percent at \lambda = 10 \mu m with a 2 m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain.

Interferometer Mirrors With Holes On-axis

Placing holes on-axis in the mirrors of an optical interferometer used for a free-electron laser (FEL) oscillator has several desirable effects. For a given amount of power generated at saturation, the peak power density on the mirrors is decreased, thereby ameliorating the problem of mirror damage; the electron beam can be introduced into the cavity in a collinear fashion; and the small-signal gain can be increased. Surprisingly, these holes may be quite large without a significant increase in cavity loss. This is a consequence of the fact that higher-order modes are generated which sum to a null of the field at the position of the holes.

A solenoid-derived wiggler

A novel wiggler design for use in free-electron lasers (FELs) is proposed, consisting of a staggered array of magnetic poles situated inside the bore of a solenoid. The resultant field pattern consists of a periodic transverse magnetic field on axis, as well as a longitudinal guide field. Such a wiggler has several advantages: the longitudinal field acts to confine the electrons near the FEL axis, high fields can be attained at short wiggler periods, the field strength is easily varied, and fabrication and testing of the wiggler are relatively easy. It is planned to use this wiggler design in a far infrared FEL to be built at Stanford University. >

Novel approaches to FEL operation: The gas-loaded FEL and a high efficiency FEL design

Two novel methods for improving free-electron laser (FEL) oscillator performance are discussed: (a) The gas-loaded FEL (GFEL) allows operation at snorter wavelengths for a given accelerator energy and wiggler. Experimental results of laser operation with a gas retention foil in the electron beam line and with the introduction of gas to the wiggler cavity are presented, (b) An FEL design utilizing a time-ramped microwave field to accelerate electrons as they lose energy to radiation allows for high conversion efficiencies. Parameter constraints for such an FEL are discussed, leading to a structure that integrates a wiggler with a linac. It is shown that conversion efficiencies of 50% at λ = 10 μm with a 2m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain

Topics in a high-efficiency FEL design

Abstract Various topics concerning a high-efficiency free-electron laser (FEL) design are discussed. The concept of applying an accelerating microwave field to the electrons as they pass through the wiggler to maintain constant electron energy is reviewed. Efficiencies of up to 48%, for practical values of FEL parameters, are calculated, and advantages of the scheme in comparison with tapered wiggler FELS are discussed. A unique structure for use in the high-efficiency scheme, combining the functions of a linear accelerator and a wiggler, and including a pre-bunching section, is reviewed. The effect of optical field strength variation along the resonator axis as well as the effect of operation at different wavelengths on the attainable efficiency are investigated.

Effects of introducing a gas into the free-electron laser

Many interesting applications of the free-electron laser (FEL) require the extension of the operating wavelength into the ultraviolet region of the spectrum. The introduction of a gas into the wiggler section of a FEL alters the phase velocity of the electromagnetic wave and so changes the synchronism condition relating wavelength to wiggler parameters and beam energy. This provides a means for tuning the frequency of an oscillator, and with the addition of 200 Torr of hydrogen gas the wavelength of a FEL operating in the near infrared without gas was reduced by 0.73 μm. The plasma generated from ionization of the hydrogen molecules by collisions with the electron beam diminished the oscillator gain, but this effect was eliminated by the addition of less than 0.1% of an electron attachment gas. Gain is also reduced by multiple scattering of the beam electrons, but this effect is not severe for a 1-m wiggler length. When hydrogen is used, a FEL with fixed wiggler parameters and electron energy can be tuned from the near infrared to ≈1200 A, and with helium the wavelength can be reduced to 600 A.

Observations of gain and pressure tuning in a gas-loaded FEL

Abstract The addition of gas to a free electron laser allows oscillation at shorter wavelengths with a given accelerator and wiggler. Stanfords Mark III near-infrared (vacuum) FEL has been modified to permit operation as a gas-loaded FEL (GFEL). By adding up to 100 Torr of H2, the emission wavelength was tuned over a range of 0.4 μm, in agreement with the GFEL synchronism condition incorporating the index of refraction of the gas. The variation of gain with pressure was consistent with calculations including multiple small-angle scattering of the electron beam by the gas. Extension to higher pressures and shorter wavelengths is discussed.

Design of a gas-loaded free electron laser experiment

Abstract An infrared, vacuum FEL designed to operate at 2.6 μm will be modified to permit the introduction of hydrogen gas into the wiggler section. This gas will be separated from accelerator vacuum using a 9 mm diameter, 1 μm thick BN foil, and tuning will be obtained from 2.6 to 0.40 μm by the addition of up to 1 atm of gas. The Rayleigh range in the optical cavity will be reduced to compensate for the additional emittance produced by multiple scattering. Calculated small-signal gains with the gas are comparable to the gain at the longer wavelength in vacuum. A prior experiment demonstrated propagation of the 43 MeV electron beam through 1 m of H 2 at pressure of 10 −3 to 1.25 atm, with full current transmission and no plasma instabilities.