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Shevchenko, V.

Publications and source records attributed to Shevchenko, V..

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Multi-harmonic electron cyclotron heating and current drive scenarios for non-inductive start-up and ramp-up in high field ST-40 spherical tokamak

We report non-inductive start-up and ramp-up is an important topic for spherical tokamak reactor design as the central solenoid implementation is highly restrictive particularly for the low-aspect-ratio tokamak configuration. In the high field spherical tokamak (ST), ST-40 with B T0 ≤ 3 T, a preparation is underway for high power ECH and ECCD current start-up/ramp-up experiments utilizing two MW-class 140/105 GHz gyrotrons. Here, we explored various ECH/ECCD scenarios for a low-field-side (LFS) launch-angle steerable waveguide launcher placed near the mid-plane region. Due to the large toroidal field variation of ST configuration, multiple cyclotron harmonic resonance layers could exist within the plasma. In this start-up and ramp-up regime, both fundamental and second harmonic ECH resonances must be considered. We find that even with the presence of X-II resonance layer in the plasma, an efficient X-I ECH and ECCD regime can be accessed for the low electron temperature T e0 as low as 200 eV which is a typical starting temperature of ECH heated plasmas in an open-field-line configuration. The presence of X-II resonance could become significant at higher T e0 as X-II absorption increases with T e0 which could reduce the current ramp-up efficiency as the power reaching X-I is reduced. Finally for the pure X-I regime where the 2Ω e resonance is moved outside the plasma with B T0 ~ 3.4 T, we find that it is possible to reach the full current of I p ~ 1 MA fully non-inductively with the ECH power of ~1 MW at n e0 ~ 1.0 × 10 19 m -3 using 105 GHz frequency gyrotron. By reducing the outer limiter position R L ~ 78 cm to 70 cm, the pure X-I regime is recovered at the rated ST-40 magnetic field of B T0 ~ 3.0 T. This X-I regime is accessible with a relatively broad range of launched n II or the launching angles. A survey of X-mode X-II ECH and ECCD at higher density regimes is also shown for completeness.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Efficient electron cyclotron current drive regime for plasma current start-up in fusion reactors

We report an accessibility enhanced efficient fundamental X-mode electron cyclotron heating (ECH) current start-up regime was identified for a reactorlike toroidal magnetic field range which has more than 100 times higher current drive efficiency compared to more conventional ECH methods for the relevant start-up temperature range. Very high current drive efficiency is possible due to the strong cyclotron interaction only with unidirectional passing electrons constrained by the wave accessibility conditions. This efficient electron cyclotron current drive regime may help facilitate the design of innovative economical solenoid-free tokamak fusion reactor systems.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2011

Lunar Reconnaissance Orbiter (LRO) Observations with the Lunar Exploration Neutron Detector (LEND): Neutron Suppression Regions (NSR) and Polar Hydrogen

Orbital detection of neutrons has become the dominant remote sensing technique for detecting and inferring H concentrations and its spatial distribution beneath planetary surfaces [Lawrence et al, (2010) Icarus, 205, pp. 195-209, Mitrofanov et al (2007) Science 297(5578), 78-81]. Indications for the presence of localized and relatively high water content was provided by LRO and LCROSS. LEND identified Cabeus, as the most promising LCROSS impact site [Mitrofanov I. et al. (2010) Science, 330, 483], and instruments onboard LRO and LCROSS have measured signatures of water, H2 and other volatiles in the impact plume [Colaprete A. et al. (2010) Science, 339,463, Gladstone R. et al. (2010) Science, 330, 472].

Chin, G.↗
NASA NTRS2009

Medical Operational Challenges in the Expedition 16 Landing and Recovery

On April 19, 2008 the crew of Expedition 16 left the International Space Station and returned to earth via their Soyuz TMA-11 capsule after 192 days on orbit. Their capsule experienced the second consecutive and third ballistic reentry in the last 10 TMA recoveries and landed approximately 260 miles (420 km) from the prime landing site. Issues: The purpose of this presentation will be to describe, not only the typical medical operational challenges faced by Flight Surgeons recovering a long duration crew from space, but also address the unique challenges that existed with the Expedition 16 landing and crew recovery. Nominal Soyuz recovery challenges include remote recovery sites with crew exposures to sleep shifting and fatigue, dehydration, hypothermia and hyperthermia, and rotational, sustained, and impact g-forces. These environmental factors coupled with the patho-physiologic neuro-vestibular and orthostatic intolerance changes that occur secondary to the crews reintroduction into the earth s gravity field will be detailed. Additional challenges that were unique to this expedition included a ballistic reentry with higher g-loads, the presence of fire outside of the capsule on landing, a contingency medical event of a ground support personnel, and loss of communications with the crew just prior to landing and during recovery operations. Conclusions: In spite of these unique challenges the Russian Search and Rescue Forces and Medical Support personnel along with U.S. Medical Support performed well together. Possible improvements in training and coordination will be discussed.

Moynihan, S.↗
NASA NTRS1994

Acceleration of electrons and ions by strong lower-hybrid turbulence in solar flares

One of the outstanding problems in solar flare theory is how to explain the 10-20 keV and greater hard x-ray emissions by a thick target bremsstrahlung model. The model requires the acceleration mechanism to accelerate approximately 10(exp 35) electrons sec(exp -l) with comparable energies, without producing a large return current which persists for long time scales after the beam ceases to exist due to Lenz's law, thereby, producing a self-magnetic field of order a few mega-Gauss. In this paper, we investigate particle acceleration resulting from the relaxation of unstable ion ring distributions, producing strong wave activity at the lower hybrid frequency. It is shown that strong lower hybrid wave turbulence collapses in configuration space producing density cavities containing intense electrostatic lower hybrid wave activity. The collapse of these intense nonlinear wave packets saturate by particle acceleration producing energetic electron and ion tails. There are several mechanisms whereby unstable ion distributions could be formed in the solar atmosphere, including reflection at perpendicular shocks, tearing modes, and loss cone depletion. Numerical simulations of ion ring relaxation processes, obtained using a 2 1/2-D fully electromagnetic, relativistic particle in cell code are discussed. We apply the results to the problem of explaining energetic particle production in solar flares. The results show the simultaneous acceleration of both electrons and ions to very high energies: electrons are accelerated to energies in the range 10-500 keV, while ions are accelerated to energies of the order of MeVs, giving rise to x-ray emission and gamma-ray emission respectively. Our simulations also show wave generation at the electron cyclotron frequency. We suggest that these waves are the solar millisecond radio spikes. The strong turbulence collapse process leads to a highly filamented plasma producing many localized regions for particle acceleration and resulting in approximately 10(exp 17) electron 'beamlets' of width approximately equal to 10 lambda sub De which eliminates the production of large magnetic fields. In this paper, we demonstrate that the model produces an energetic electron spectrum with the right flux to account for the hard x-ray observations.

Spicer, D. S.↗