Abstract
A wave detector, a newly designed magnetic probe, is installed in the large helical device (LHD). This wave detector is a 100-turn loop coil with electrostatic shield. Comparing a one-loop coil to this detector, this detector has roughly constant power coupling in the lower frequency range of 40 MHz, and it can easily detect magnetic wave in the frequency of a few megahertz. During high-harmonic fast wave heating, lower frequency waves (<10 MHz) were observed in the LHD for the first time, and for the power density threshold of lower frequency wave excitation (7.5 MHz) the power density of excited pumped wave (38.47 MHz) was approximately -46 dBmHz. These lower frequencies are kept constant for electron density and high energy particle distribution, and these lower frequency waves seem to be ion cyclotron waves caused by nonlinear wave-particle interaction, for example, parametric decay instability.
Highlights
High-harmonic fast wave 共HHFW兲 heating has been investigated in high beta regime, and the accessibility and electron absorption have been inspected in torus plasmas such as spherical tokamak
The excited wave frequency is typically ten times larger than the ion cyclotron range of frequencies 共ICRF兲, and ion cyclotron damping is negligible in the experimental region
Three pairs of ICRF heating antennas are installed at three toroidal sections in the LHD 共3.5 U & L, 4.5 U & L, and 7.5 U & L兲, and two pairs of them 共3.5 U & L and 7.5 U & L兲 are used for ICRF
Summary
High-harmonic fast wave 共HHFW兲 heating has been investigated in high beta regime, and the accessibility and electron absorption have been inspected in torus plasmas such as spherical tokamak. In electron heating experiments using. The excited wave frequency is typically ten times larger than the ion cyclotron range of frequencies 共ICRF兲, and ion cyclotron damping is negligible in the experimental region. A HHFW electron heating experiment was attempted in the LHD with normalized beta of 1% 共magnetic field B of 1.5 T at major radius R of 3.6 m兲, and central electron heating was clearly achieved when additional electron cyclotron heating was injected at the core region of plasma.. Second ion cyclotron resonances of hydrogen exists at near r / a ⬃0.6, and second hydrogen cyclotron damping is a strong candidate to be the ion heating mechanism, rather than nonlinear wave-particle interaction like PDI.. It is important to clarify the parasitic ion heating mechanism in HHFW electron heating experiments, and a high-sensitivity wave measurement system using a newly designed wave detector to measure these waves was installed in the LHD Second ion cyclotron resonances of hydrogen exists at near r / a ⬃0.6, and second hydrogen cyclotron damping is a strong candidate to be the ion heating mechanism, rather than nonlinear wave-particle interaction like PDI. It is important to clarify the parasitic ion heating mechanism in HHFW electron heating experiments, and a high-sensitivity wave measurement system using a newly designed wave detector to measure these waves was installed in the LHD
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