D. P. Hutchinson and R. K. Richards
The USDOE, the European community, Japan, and the Russian Federation are
jointly designing the world's first fusion reactor, the International Thermonuclear
Experimental Reactor (ITER). One of the primary purposes of ITER is the study of confined
alpha particles produced in the fusion nuclear reaction by combining deuterium and tritium
at high temperature. The alpha particles, created with an energy of 3.5
million-electron-volts, must be trapped in the magnetic field of ITER long enough
(~1 sec) to heat incoming cold fuel to ignition temperatures (~ 100 million degrees). The
study of the trapping and slowing down of these alpha particles is a major goal of the
ITER device. We have developed a diagnostic technique to determine the density and
velocity distribution of the alpha particles, based on the scattering of 10 micron light
generated by a CO2 laser from alpha particles in the plasma. A
proof-of-principle experiment successfully completed on the Advanced Toroidal Facility in
the Fusion Energy Division in 1991, has led to the selection of our technique as the one
of the primary alpha particle diagnostics for ITER1.
The diagnostic technique used for the measurement is called Collective Thomson Scattering (CTS). This technique works by scattering a high-power CO2 laser beam from clouds of electrons surrounding ions in a high temperature plasma. In a high temperature plasma all of the atoms of the filling gas, deuterium and tritium in the case of a fusion reactor, are completely ionized and will remain so if the temperature is maintained. The ions attract the fast moving electrons but recombination into atoms is prohibited by the high energy of the particles. If a probing wavelength is chosen that is comparable in size to or larger than this cloud of electrons hovering around each ion, then the spectrum of the scattered laser light, Doppler-shifted by the motion of the cloud, will be characterized by the ion velocity distribution. The criterion used to establish the proper conditions for such ion measurements in a plasma is known as the Salpeter parameter, alpha, given by
In order to measure the ion spectra in a plasma, must be greater than one. For conditions typical in a fusion plasma, the scattering angle theta must be less than 1-degree. A schematic diagram of the experimental set-up is shown in Figure 1.
Ion scattering in the ATF experiment was not feasible because of the low ion temperature. To properly simulate CTS for alpha particle measurements, a proof-of-principle test was performed which relies on scattering from plasma electrons at large shifts from the pulsed laser frequency near the electron plasma resonance. These resonances are only weakly dependent on electron temperature and have a scaling of ne1/2. Figure 2 contains a summary of data from scattering experiments that were performed on the ATF device as the plasma density was varied from 0 to about 7 x 1019 m3. The solid and dotted curves in the figure represent theoretical calculations of the scattered spectrum as a function of plasma density.
1 R. K. Richards, D. P. Hutchinson, C. A. Bennett, H. T. Hunter, and C. H. Ma,
"Measurement of CO2 Laser Small-Angle Thomson Scattering on a Magnetically
Confined Plasma," Applied Physics Letters, 62, P. 28-30, January 1993.