Results from the low-power 60 GHz gyrotron collective Thomson scattering diagnostic on TFTR

Abstract

A low-power 60 GHz gyrotron collective Thomson scattering diagnostic has been operating on TFTR to test the feasibility of detecting alpha particles when scattering perpendicular to the magnetic field. An enhanced scattered signal is predicted to result from the interaction of the energetic ions with plasma resonances in the lower hybrid frequency range. Millimeter-wave power levels at the plasma were approximately 200 W for typical pulse lengths of 50 ms. Deuterium and possible fusion product ion cyclotron frequencies and their harmonics were observed during neutral beam heating. These spectra are similar to ion cyclotron emission spectra which are detected with radio-frequency probes on TFTR at the plasma edge. Also, ion cyclotron resonance heating fundamental and harmonic fluctuations were observed. However, a signal has not been definitively detected in the lower hybrid frequency range which correlates to alpha particles. Broadband noise was observed during neutral beam heating which is greater than the predicted alpha particle signal.

Recent development of collective Thomson scattering for magnetically confined fusion plasmas

“…Here the scattered radiation was expected to be enhanced by the lower hybrid wave. However, development in the scattering theory [40] found that this enhancement would be cancelled by the magnetic field fluctuations as later reported experimentally [41]. In 1999, the first fast ion CTS measurements of ICRH heated ions in JET were reported [42].…”

Ion Cyclotron Emission from JET D-T Plasmas

“…ICE spectra from discharges with high-power NBI also show evidence of ion hybrid wave excitation by beam ions, relevant to a channeling. Collective effects driven by fusion a particles in deuterium-tritium (D-T) plasmas have been a primary research objective of JET [1] (the Joint European Torus) and TFTR [2,3] (the Tokamak Fusion Test Reactor). The most easily excited phenomenon is spectrally structured, suprathermal ion cyclotron emission (ICE): this was observed from the outer edge regions of the earliest D-T plasmas, in JET hot ion H-modes [1], and TFTR supershots [2] (indeed, ICE driven by fusion products was observed in JET before the use of T [4], and ICE driven by beam ions has been observed in TFTR [5]).…”

“…The signal at n Ӎ 100 MHz may incorporate second harmonic emission from the ICRH source [13]. ICE spectra obtained previously in JET [1,4] and TFTR [2,3,5] show emission at cyclotron harmonics of fusion products and beam ions in the outer midplane edge. Combined T and D beam injection was used in pulse 42697, the beam T fraction being 30%, so one would expect the intensity of T beam ICE to be comparable to that of n D͞a harmonics if the latter were produced by beam D. In fact, there is little or no evidence of emission at T harmonics other than those coinciding with D harmonics.…”

Alpha particle physics experiments in the Tokamak Fusion Test Reactor

“…The visible helium light emitted by doubly charge exchanged alphas inside the pellet cloud was measured, but the signal/background level was too low to detect alphas [33]. A system to detect fast ions via the scattering of microwaves was installed, but significant hardware and modeling difficulties were encountered [34]. A detector was installed for measuring nuclear gamma emission from resonant nuclear reactions [35], but the signal/background ratio was too low to observe alphas in DT.…”