range are needed. Energy absorption is achieved by electron cyclotron resonance, which requires wavelengths in the range of 0.5 to 2 µ m. In some current experiments, it would be desirable to be able to tune the frequency within this range of wavelengths so as to heat plasmas to fusion temperatures, for different magnetic fields, or to move the absorption position from place to place inside the plasma. Sources used for heating should be quite efficient (about 50%) and able to sustain long pulses (several seconds).
Proposals have been made for using localized heating as a means of controlling plasma instabilities. This approach would probably require the ability to change frequency rapidly so as to heat the critical region of the plasma. The requirements for control of plasma instability would involve the ability to deliver high peak power quickly and for a short time. FELs are one of the few sources with the potential to meet the above requirements.
Plasma diagnostics is the second major use of far-infrared radiation in plasma physics. Such diagnostics involve either measuring the phase shifts of waves sent through the plasma or measuring the reflection of waves from critical density regions. For typical fusion plasmas the wavelengths required for phase shift measurements are in the range of centimeters to millimeters. More information about the state of a plasma can be obtained if it can be probed at two or more frequencies.
For reflection measurements of plasma properties, the wavelength must be roughly in the same wavelength range as that required for transmission diagnostics. In some reflection experiments, multiple frequencies are used to map the plasma density using reflections from regions of different density. For this application, having a single source whose frequency could be changed on a millisecond time scale, such as a far-infrared FEL, would be quite valuable.
Studies of low-temperature plasmas such as are used in plasma processing (plasma etching, plasma chemistry, and so on) would benefit greatly from tunable sources in the infrared for detecting the various molecular species and radicals that exist in a discharge. Since the chemical species give a signature of the processes taking place and also play a central role in those processes, such information is vital to understanding and controlling the chemical species.
The scientific case for a tunable, short-pulse (picosecond) source in the far infrared is compelling, but at present there are no picosecond far-infrared FEL user facilities in the United States. This is the spectral region where molecule-surface vibrations, intermolecular cluster vibrations, and transitions in semiconductor quantum wells can be excited. It is also the region for probing transitions between adjacent high-lying Rydberg states of atoms and low-frequency motions in large biomolecules. There is sufficient scientific interest to efficiently use a far-infrared user facility capable of producing picosecond pulses, and the committee believes that the scientific opportunities justify the establishment of such a facility. Operation of