Fast-ion Velocity Distribution Research Articles

Measurement of the Dα spectrum produced by fast ions in DIII-D

Fast ions are produced by neutral beam injection and ion cyclotron heating in toroidal magnetic fusion devices. As deuterium fast ions orbit around the device and pass through a neutral beam, some deuterons neutralize and emit D(alpha) light. For a favorable viewing geometry, the emission is Doppler shifted away from other bright interfering signals. In the 2005 campaign, we built a two channel charge-coupled device based diagnostic to measure the fast-ion velocity distribution and spatial profile under a wide variety of operating conditions. Fast-ion data are acquired with a time resolution of approximately 1 ms, spatial resolution of approximately 5 cm, and energy resolution of approximately 10 keV. Background subtraction and fitting techniques eliminate various contaminants in the spectrum. Neutral particle and neutron diagnostics corroborate the D(alpha) measurement. Examples of fast-ion slowing down and pitch angle scattering in quiescent plasma and fast-ion acceleration by high harmonic ion cyclotron heating are presented.

Fast-Ion Dynamics in the TEXTOR Tokamak Measured by Collective Thomson Scattering

Here we present the first measurements by collective Thomson scattering of the evolution of fast-ion populations in a magnetically confined fusion plasma. 150 kW and 110 Ghz radiation from a gyrotron were scattered in the TEXTOR tokamak plasma with energetic ions generated by neutral beam injection and ion cyclotron resonance heating. The temporal behavior of the spatially resolved fast-ion velocity distribution is inferred from the received scattered radiation. The fast-ion dynamics at sawteeth and the slowdown after switch off of auxiliary heating is resolved in time. The latter is shown to be in close agreement with modeling results.

Observation of compressional Alfvén eigenmodes (CAE) in a conventional tokamak

Fast-ion instabilities with frequencies somewhat below the ion cyclotron frequency occur frequently in spherical tokamaks such as the National Spherical Torus Experiment (NSTX). NSTX and the DIII-D tokamak are nearly ideal for fast-ion similarity experiments, having similar neutral beams, fast-ion to Alfvén speed vf/vA, fast-ion pressure, and shape of the plasma but with a factor of two difference in major radius. When DIII-D is operated at low field (0.6 T), compressional Alfvén eigenmode (CAE) instabilities appear that closely resemble the NSTX instabilities. In particular, the mode frequencies, polarization and beam-energy threshold are nearly identical to NSTX. CAE in high-field discharges and emission at cyclotron harmonics are also observed. As on NSTX, the basic stability properties are consistent with the idea that the instability is driven by anisotropy in the fast-ion velocity distribution and is damped predominantly by Landau damping of electrons. The results suggest that these modes might be excited in ITER.

A filter bank system for scattered spectrum analysis in collective Thomson scattering diagnostic on JT-60

In JT-60, a collective Thomson scattering (CTS) diagnostic system based on a pulsed CO2 laser (wavelength 10.6 μm, beam energy 15 J, pulse width 1 μs) has been developed to measure ion temperature and velocity distribution of fast ions to demonstrate the feasibility of measurements of confined alpha particles. A high power pulsed CO2 laser and heterodyne receiver system using a quantum-well infrared photodetector (QWIP) has been developed and installed in the diagnostic room. In order to resolve frequency distribution of pulsed scattered light (1 μs), we developed a wideband filter bank system (0.4–4.5 GHz) for scattered spectrum analysis in JT-60 CTS diagnostic. The detector bandwidth and frequency resolution are both determined by the calculation results of expected scattered spectrum. In order to realize wideband multichannel detection, cascade connected seventh order simultaneous-Chebyshev (elliptic) bandpass filters were adopted. Signal to noise ratio based on the noise equivalent power of the QWIP receiver and laser pulse length are also evaluated.

Fast-Ion Velocity Distributions in JET Measured by Collective Thomson Scattering

Results on fast ion diagnosis by collective Thomson scattering at JET are presented. 400 kW, 2 mm radiation was scattered in plasmas with energetic proton minority populations heated by 4--8 MW ion cyclotron resonance heating. There is clear evidence of scattering from plasma fluctuations driven by the fast ion population. In a few cases ion velocity distributions with energies in the MeV range could be inferred.

Characteristics of fusion reaction rate and neutron yield in deuterium plasma

Characteristics of the fusion reaction rate and the fusion neutron yield have been investigated taking into account the effect of fast ions. The dependences of the fusion reactivity (〈σν〉B) and normalized fusion reaction rate (QB) on the fast ion energy and target plasma temperature are clarified. The following reactions are considered: ( 1 ) D 0 → D + ( D ( d , n ) 3 He ) , ( 2 ) D 0 → D + ( D ( d , p ) T ) , ( 3 ) D 0 → T + ( T ( d , n ) 4 He ) , ( 4 ) T f → D + ( T ( d , n ) 4 He ) , ( 5 ) D 0 → H 3 e + ( H 3 e ( d , p ) 4 He ) , ( 6 ) H 3 e f → D + ( H 3 e ( d , p ) 4 He ) . The mutual relation of 〈σν〉B and QB for each reaction is also clarified. Furthermore, the fusion neutron yield (Fn) has been evaluated using a simple steady-state plasma model in which the radial profiles of the plasma temperature and density or the fast-ion velocity distribution are the same as those observed in recent experiments. The results of this model agree with those of the other calculational model. We have studied the following dependence for JT-60 plasma size: F n vs. n ¯ e , F n vs. τ E , F n vs. n ¯ e ( using ⁡ Kaye − Goldston or INTOR scaling ) , F n vs. n D / ( n D + n H ) , F n vs. E B , and F n vs. T i / T e .

Enhancement of fusion power multiplication factor

Enhancement of the fusion power multiplication factor, Q , has been investigated using a simple plasma model, in which the radial profiles of the plasma temperature and density or the fast-ion velocity distribution are the same as those observed in recent experiments. The break-even condition is relaxed by a peaked density profile, beam-plasma reactions, and the fast ions produced by ICRF heating at the second harmonic of deuterium in a D-T thermal plasma. The minimum n τ T required for break-even has also been lowered. Furthermore, it is possible to enhance Q through local thermal instability near the plasma center using a simple transport simulation, in which the profile of the plasma density does not change during the simulation and only the thermal conduction loss is considered. Finally, the operational plasma parameters for JT-60 in the n τ vs. T diagram have been investigated.

Selective Acceleration of NBI-injected Fast Ions by Ion Cyclotron Wave in a Tokamak Plasma

The acceleration of NBI-injected fast ions by ICRF waves has been studied in the TORIUT-5 tokamak. The velocity distribution of fast ions has been measured by a neutral-particle analyzer. The number of fast ions with energies higher than the injected energy is found to be increased by the ICRF acceleration effect. The experiment is consistent with numerical simulations.

Analytical expressions for fusion spectra produced in ‘‘beam-target’’ fusion reactions

For ‘‘beam-target’’ fusion reactions, collimated measurements of the energy spectrum of one of the reaction products can provide information on the degree of anisotropy of the reacting beam ions. Analytical expressions relating the fusion spectrum to the velocity distribution of the beam ions are given. Application of the expressions to measurements of the spectrum of 15-MeV protons produced by reactions between energetic 3He ions and relatively cold deuterons during ICRF heating in the PLT tokamak indicate that the velocity distribution of fast 3He ions is peaked perpendicular to the tokamak magnetic field.

Analysis of fast-ion velocity distributions in laser plasmas with a truncated Maxwellian velocity distribution of hot electrons

Fast-ion production in laser plasmas with a truncated Maxwellian velocity distribution of hot electrons is investigated by using a new numerical method, namely the systematic Newton’s iteration method which has been applied to nonlinear equations. The analysis of the temporal evolution of coronal plasma expansion discloses that the ion-front velocity approaches a constant value within thirty ion plasma oscillation periods, when the high-energy tail is truncated. In such a case, the maximum ion velocity is found to be proportional to the maximum energy of hot electrons. Consequently, the ion velocity distributions are steepened and truncated. Furthermore, it is found that the energy partition of the absorbed laser energy to fast ions is appreciably reduced when the maximum electron energy is below a few times the hot-electron temperature.

Instability of Alfvén Waves by Fast Ions in a Plasma in a Magnetic Mirror Field

The instability of Alfven waves by fast ions in a plasma in a magnetic mirror field is investigated theoretically taking explicitly into account the bounce motion of the fast ions between turning points. The linear growth rate of instability is calculated and the result is compared with the result obtained by the usual theory which assumes free motion of particles. In the case of a nearly mono-energetic velocity distribution of fast ions, with the velocity spread equal to the thermal velocity of the host plasma ions, the maxumum growth rate by the bounce-motion theory is reduced by the factor (fast-ion energy/host-ion temperature) 1/4 from that by the free-motion theory. The difference is less remarkable in the case of a broad distribution of fast ions.

Irradiance scaling of fast ion expansion for CO2 laser produced plasmas

Faraday collector and calorimeter measurements of the ion emission are obtained for short CO2 laser pulses irradiating planar polyethylene targets at intensities between 1012 and 2×1013 W cm−2. The fast ion angular distribution is more peaked towards target normal than the thermal ion distribution. The hot electron temperature inferred from both the slope of the fast ion velocity distribution and the maximum ion velocity scales as I0.25 and I0.9 for irradiances respectively above and below a critical value of Icr = 5×1012 W cm−2. The high irradiance scaling agrees with that of the hot electron temperature deduced from x-ray measurements. At low irradiance, the discrepancy between the ion and x-ray hot electron temperature scaling indicates that the expansion is nonisothermal resulting in a cooling of the hot electrons and in a colder fast ion velocity distribution.

Fast-Ion Generation by Ion-Acoustic Turbulence in Spherical Laser Plasmas

Numerical hydrodynamic simulations which include absorption and transport modifications due to ion-acoustic turbulence have been performed. These simulations can reproduce the important features of the fast-ion velocity distributions as measured with biased charge collectors and a magnetic spectrograph in spherically illuminated microballoon experiments. Experiments with hemispherical targets show that a substantial amount of fast-ion energy is directed inward and may contribute to driving the implosion.