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AAPPS-DPP2020 Poster Award Winners
Abhijit Sen
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AAPPS-DPP2020 Poster Award Winners

Abhijit Sen
Chair, Selection Committee

 

The AAPPS-DPP annual conference has celebrated good poster presentations since 2018. AAPPS-DPP20 attracted a large number (45) of very high quality poster presentations on the various sub-topics of the conference. A selection committee, comprising of eminent international experts (A. Sen (Chair), ZM Sheng, SSH Chen, S. Sengupta, H. Shiraga, V. Tikhonchuk, T. Murphy, R. Hatakeyam, C. Chang, H. Akatsuka, K. Takahasshi, A. Misra, D. Verma, M. Hoshino, G. Lakhina, S. Liu, R. Matsumoto, C. Chrabtree, P. Mantica, H. Jhang, G. Zhuang, T. Hoang, J. Dong, M. Hole, K. Ida), judged these posters and selected the award winning ones in the various categories. Despite the virtual nature of the conference the poster sessions were a great success as the posters were available for display (and downloading) throughout the week for participants to look at and post questions to the authors. Eleven posters by N. Imagawa, G. Yu, P. Adulsiriswad, C.W. Domier, M.S. Hussain, J.X. Ji, W. Tan, H. Miura, S. Barman, S.S. Mishra and D. Behmani were selected for awards and it is a great pleasure to have the opportunity to publish the abstracts of their prize winning posters in the AAPPS Bulletin. Thanks to APCTP executive director Woo-Sung Jung. Sincere thanks to the selection committee members for their time and expert help and hearty congratulations to the winners.


Characterization of fluctuations in atmospheric pressure micro-plasma jets

Deepika Behmani, Kalyani Barman, and Sudeep Bhattacharjee
Department of Physics, Indian Institute of Technology Kanpur, Kanpur (India)
e-mail (speaker): deepika@iitk.ac.in

Atmospheric pressure micro-plasmas are non-equilibrium plasma with widely different ion (~0.026 eV) and electron temperature (~0.5 eV) [1]. These plasmas are popularly known as "cold" plasmas and used in several varieties of applications in fields such as surface functionalization and modifications, biology, medicine, environmental, and cell or tumor treatments [2]. The strength of potential (or electric field) penetrating onto the target is one of the most important parameters that can affect these applications. A minor fluctuation in the potential and electric field can affect these processes drastically due to transport and heating of active species. Therefore, it is important to investigate the fluctuations of these parameters in the plasma jet.

In our laboratory, a plasma jet has been created by dielectric barrier discharge inside a tapered glass capillary tube having inlet outer and inner diameter of 4.14 mm and 2.5 mm respectively, and outlet outer and inner diameters of 2.06 mm and 0.8 mm respectively [3]. Helium is used as a primary working gas. The plasma is ignited by applying a sine wave of maximum amplitude of 15 kV pp and of frequency 10 kHz. A ring to ring electrode configuration having a gap of 10 mm between the electrodes, has been used in this experiment. The charged particles emerge from the capillary tube into the ambient air as a fine plasma jet of length ~10 mm and diameter ~ 0.8 mm as shown in Fig. 1. A double pin probe having a diameter of 0.18 mm and length of 2 mm each, and 0.267 mm distance between them, has been employed to measure the floating plasma potential at spatial points in the jet at a downstream distance of 5 mm from the orifice of the capillary. The probe is kept inside a metallic aluminum box to shield it from the high voltage signal. The floating potential (amplitude-time series) has been captured by a digital oscilloscope through a double pin probe inside the jet, and their frequency characteristics have been investigated by employing classical tools such as fast Fourier transform (FFT) and time-frequency analysis (TFA) [4].

 

Fig.1: Schematic diagram of experimental set-up with the digital picture of plasma jet. (at an applied voltage 10 kV and flowrate 2 L/min)

 

Fig. 2: Time frequency analysis results at a fixed voltage of 14 kV and flow rate of 1 L/min for (i) helium plasma jet and (ii) argon plasma jet.

In the present work, the dependence of floating potential fluctuations on the operating parameters (applied voltage, gas flow rate, and mixing of other gas (Ar) with the main He gas) has been studied. It has been found that at a fixed flow rate (1 L/min), fluctuation increases with applied voltage (from 7 kV to 11 kV), then attains a maximum value at 11 kV because of high discharge current at this particular voltage and thereafter decreases. For a fixed applied voltage 14 kV, the plasma jet becomes turbulent after a flow rate of 3 L/min. The fluctuation analysis confirms that the turbulent jet is less stable than the laminar one. In the case of mixing argon, different general properties of argon gas e.g. low thermal conductivity and ionization potential as compared to helium gas, make argon jet highly unstable than helium jet. Time-frequency analysis further helps in understanding the stability of the plasma jet during different operating conditions. TFA plot (Fig.2) shows that a narrow frequency band (~6 kHz) almost disappears in case of argon but frequencies in another wide band (~2 to 4 kHz) increase, indicating the instability the instability of argon jet. Hence, this research is helpful to choose suitable operating parameters and gas as per the requirement of the application.

References

[1] Chang Z, Zhang G, Shao X, and Zhang H, Phys. Plasmas 2012 19 073513.
[2] Tian W, Lietz A M, and Kushner M J 2016 Plasma Sources Sci. Technol. 25 055020.
[3] Barman K, Behmani D, Mudgal M, Bhattacharjee S, Rane R and Nema S K 2020 Plasma Res. Express 2 025007.
[4] Tu X, Yan J, Yu L, Cen K and Cheron B 2007 Appl. Phys. Lett. 91 13.


Observation of nonlinear demagnification in plasma-based ion beam optics

Sushanta Barman, Sanjeev Kumar Maurya, and Sudeep Bhattacharjee
Department of physics, Indian Institute of Technology Kanpur, India
e-mail (speaker): sushanta@iitk.ac.in

In conventional optics, the magnification of a lens depends upon the lens material and the geometry of the lens. The magnification is a constant for optical lenses as well as for electrostatic lenses for a set of fixed potentials applied to the electrodes. In an earlier work [1], it is reported that the demagnification factor (DM) of an Einzel lens system employed to focus ion beams extracted from a plasma-based ion source, varies nonlinearly when the object size (beam source size) is reduced to below the plasma Debye length (λd). The results indicate that the reason for nonlinearity results from the non-uniform penetration of electric fields in the plasma sheath region through the plasma electrode (PLE) aperture, from where the beams are extracted. There are no systematic studies on the effect of plasma parameters on the DM factor of plasma-based electrostatic focusing devices. This study for the first time shows the effect of plasma-parameters on the DM, in plasma-based ion beam focusing systems.

A multi-element focused ion beam (MEFIB) system [2] developed in our laboratory, consists of four major parts: plasma column, beam column, experimental chamber and stage manipulator. In the plasma column, an electron cyclotron resonance plasma is produced using microwaves of 2.45 GHz and confined in an octupole magnetic multicusp [2, 3]. A compact electrostatic lens system is employed in the beam column to extract the ions from the plasma and focus the beam with the required energy (~ 30 KeV).

In order to calculate λd at various operating pressures, the electron temperature (Te) and density (ne) in steady state plasma are calculated numerically by solving the growth equation for Te and particle balance equation [4] in the cylindrical geometry of the multicusp. The numerical results show excellent agreement with experimental ones, measured by Langmuir probes. It is found that λd has a minimum value (~55 μm) at an optimum pressure ~ 0.4 mTorr, where the plasma density is maximum (~1.3× 1017 m-3).

 

Fig. 1: Region of interest of the numerical calculation. φp and V1 are the plasma potential and applied voltage to the first electrode of the first Einzel lens, respectively.

To obtain the potential (V) and fields near the plasma electrode (PLE) aperture (Fig. 1, with a magnified view in Fig 2(a)), Poisson's equation is solved using the successive over relaxation method. The variation of V and electric field (Ez) along the axis (z) for a 100 μm PLE aperture is shown in Fig. 2(b). It is observed that Ez has a maximum value near the PLE aperture, which is responsible for the extraction of ions from plasma. The axial electric field near the PLE aperture depends upon the size of the aperture.

 



Fig. 2: (a) PLE aperture, (b) Variation of V and Ez along the z axis. The focal length of an aperture lens is given by [1]

(1)

where q and T are the charge and kinetic energy of the ions respectively, Ez1 and Ez2 are the axial electric fields on either side of the aperture. The DM of the combined lens system in the MEFIB is calculated using optical methods where the main lenses are considered as thick lenses, and the beam blanking aperture as a thin lens. The DM is calculated for different PLE aperture sizes and for different λd, which can be realized by varying the discharge power and pressure. The results will be presented in the conference. Work supported by SERB (DST), Government of India.

References

[1] S. K. Maurya et al., Phys. Plasmas 26, 063103 (2019).
[2] J. V. Mathew et al., Appl. Phys. Lett., 91, 041503 (2007).
[3] S. K. Maurya et al., J. Appl. Phys, 121, 123302, (2017).
[4] S. Bhattacharjee et al., J. Appl. Phys. 89, 3573 (2001).


Molecular dynamics simulation of collisional cooling of He and
its binary mixtures with Ne, Ar, Kr and Xe in Nose-Hoover thermostat
for creating strongly coupled cryo-plasmas

Swati Swagatika Mishra and Sudeep Bhattacharjee
Department of Physics, IIT Kanpur, India
e-mail (speaker) : swatis@iitk.ac.in

Cryo-plasmas are micro-plasmas created at extremely low temperatures (below room temperature to 4 K) and usually at atmospheric pressure. They provide an excellent platform for investigating the physics physics of strongly coupled systems, where the plasma in the ambient gas can be taken to extremely low temperatures and dust added. Together, both these effects are expected to increase the coupling parameter (ratio of the typical potential energy to kinetic energy of nearest neighbors) significantly and take it to regimes not investigated earlier. In these plasmas, neutral gas temperature dependent dynamics control the energetics of electrons and ions, owing to their weakly ionized nature [1]. Most of the studies in this field are focused on the production and properties of pure He cryo-plasma [1, 2]. The first step for generating such a plasma is to cool the gas inside a vessel connected to a cryostat. One of the most significant findings from such studies, is the dependence of electron density, temperature and hence the particle coupling parameter on neutral gas temperature. Such a dependence primarily originates from the the inter-particle interactions in neutral gas [1].

In order to be able to predict how the interactions of neutral gas molecules control the plasma dynamics, two studies are vital: (i) a proper knowledge of the correct interaction potential acting between the gaseous atoms, and (ii) the efficiency of collisional cooling of gas atoms and eventually the cooling of plasma electrons (and ions) through interaction with neutral atoms in such low temperatures (~10 K). Although the Lennard-Jones (LJ) (6-12) potential is conventionally used to model real gases and fluids [6], there arise several discrepancies at cryogenic temperatures, due to the emergence of quantum effects, which puts the validity of this potential under scrutiny, in the aforementioned temperature range.

In order to investigate the cooling process, the effect of gas mixing of He with other gases, such as Ne, Ar, Kr and Xe, a 3D molecular dynamics simulation has been set up using the LAMMPS package [3]. The monatomic gases are chosen based upon their polarizability, atomic radius and mass. The simulation box has periodic boundary conditions in all directions. To replicate the experimental method of cooling, the experimental chamber has been assumed to be a thermostat. For this purpose, a Nose-Hoover type of thermostat has been used, which gives rise to a statistical canonical ensemble [4]. The temperature damping parameter is set to be 0.1ps for all the processes. The cooling process is carried out starting from 300 K to 10 K, with a starting pressure of 1atm and a time step of 1fs. The chosen pair potential (Lennard-Jones), has the form: , where 𝜖 is the interaction strength and σ is the distance parameter. To guide the interactions between unlike atoms, two types of combining rules are used, such as Lorentz-Berthelot (LB) and Fender-Halsey (FH) [7]. For the binary mixtures, the gas mixing ratio is taken to be 1:1.

The initial simulations have been carried out for a pure He system. The gas particles are cooled, starting from 300 K, to 10 K. The speed (velocity) distribution functions are obtained for 200 K, 100 K and 10 K respectively. The velocity distributions are acquired for an equilibration time period of 20ns, with 2ns intervals. Final distribution plots are generated using time averaging method. Figure 1 shows a comparison of velocity distributions from the MD simulations, with the theoretically predicted Maxwell-Boltzmann distribution curves. The excellent agreement between the simulation and theoretical results confirms the successful implementation of the simulation system.

In the conference, the cooling rate of such pure He system and the mixtures will be presented. The effect of mass, size and interaction strength of the secondary gas on the cooling rate of He will be be discussed. The performance of the LJ potential, and both the mixing rules, will be ascertained by comparing the time dependent transport properties with the available experimental results [5, 6]. Finally, the onset of quantum effects will be investigated by comparing the inter-particle distance and the de-Broglie wavelength.

 



Fig. 1: Velocity distributions at various temperatures.

References

[1] Y. Noma et al., J. Appl. Phys. 109, 053303 (2011).
[2] S. Stauss et al., Plasma Sources Sci. Technol. 27, 023003 (2018).
[3] Plimpton S, J. Comp. Phys 117, 1 - 19 (1995).
[4] Evans et al., J. Chem. Phys., 83, 8 (1985).
[5] J. Kestin et al., J. Phys. Chem. Ref. Data, 13, 1 (1984).
[6] A. Rahman, Physical Review, 2A, 136 (1964).
[7] Arjan Frijns et. al., Micromachines, 11, 319 (2020).


Dual-angle Thomson scattering diagnostic experiment design
and preliminary results

Weiqiang Tan1, Yaoyuan Liu1, Xinyan Li1, Peng Yuan1, Jian Zheng1,2,3
1 Department of Plasma Physics and Fusion Engineering, University of Science and Technology of China
2 CAS Center for Excellence in Ultra-intense Laser Science
3 IFSA Collaborative Innovation Center, Shanghai Jiao Tong University
e-mail: wqtan@mail.ustc.edu.cn

Coherent Thomson scattering (TS) is one of the most accurate methods methods for measuring local plasma properties. The detected spectrum includes two regimes, the low frequency regime (IAW) and the high frequency regime (EPW). In the EPW regime, the density and temperature of the plasma can be determined, while in the IAW regime, one can obtain the ion flow velocity, ion temperature and relative drift between electrons and ions in the plasma. Recently Yaoyuan, Liu [1] proposed that by fitting spectra collected at two angles, the accuracy of Te and ne can be drastically increased, assuming that good signal-to-noise ratios can be achieved. To verify this propose, we designed and carried out a dual-angle proposal Thomson scattering diagnostic experiment on Magnetized Laser Plasma Device (MLPD) at University of Science and Technology of China (USTC). The MLPD has a frequency-doubled heating laser (532 nm), with maximum energy output of 4J and pulse width of 7 ns (FWHM). The heating beam enters the target chamber from north, perpendicularly irradiating an aluminum foil with thickness of 100 μm to produce a blow-off plasma, and the heating beam also acts as the probe beam. The TS signal is collected at 90° and 135° relative to the heating beam via four achromatic lenses with total magnification ratio of 1. The scatter volume is designed to be located 400 μm in front of the target, and is imaged onto a multimode fiber, with NA of 0.15 and core diameter of 105 μm. The fiber transmits the scattered signal to a 750 mm spectrometer (PI, SP-2750) equipped with a 1800 LP/mm reflection grating, thus in this experiment, we only measure the IAW regime spectrum. After the signal is dispersed, the spectrum is relayed to the ICCD with a pair of achromatic lenses, and a blackened knife-edge is placed at the image plane of the spectrometer to block un-shift green light. The gate time of the ICCD is set to be 3.5 ns. The spectrum resolution of this system is 0.6Å. The acquired spectra show good SNR and the two peaks of IAW feature can be clearly distinguished. Fitting the spectrum with the method proposed by Liu, the fitting result shows that Te is about 176 eV, Ti about 111 eV, with uncertainty of 11% and 17% respectively. Moreover, the fitting returns ne with a value of 6.5×1018 cm-3, which is consistent with an earlier interferometric measurement. The fitting method used above does not take density and temperature gradients into account, which may be the cause of the inconsistency between the fitted spectrum and the acquired spectrum.

 



Fig. 1: a) Experiment setup; b) illustration of scattering volume; c) illustration of differential scattering vector.



Fig. 2: top: raw data of TS spectrum from ICCD; bottom: fitting result for the dual-angle TS spectrum.

 

Table 1. fitting results with uncertainties.

No.

ne

Te

Ti

Unit
Param
σ

cm-3
6.5×1018
~16%

eV
176
~11%

eV
111
~17%

References

[1] Yaoyuan Liu, Yongkun Ding, and Jian Zheng, Review of Scientific Instruments 90 (8), 083501 (2019).


Characterization of Isotope Effect on Particle Transport
in Large Helical Device

Naoto Imagawa1, Hiroshi Yamada1, Tatsuya Yokoyama1, Katsumi Ida2, Ryuichi Sakamoto2,3, Keisuke Fujii4,
Mikirou Yoshinuma2, Gen Motojima2,3, Kenji Tanaka2,5
1 Graduate School of Frontier Sciences, The University of Tokyo,
2 National Institute for Fusion Science/NINS, 3 SOKENDAI, 4 Kyoto University, 5Kyushu University
e-mail (speaker): imagawa.naoto19@ae.k.u-tokyo.ac.jp

Particle transport in magnetically confined plasmas has been investigated in the Large Large Helical Device (LHD). This study aims at characterization of the isotope effect on particle transport.

The most promising fusion reaction for a fusion reactor is the reaction of deuterium (D) and tritium (T). Both deuterium and tritium are isotopes of hydrogen, and the control of their concentration of 50/50 is required to maximize fusion power output. Nonetheless, difference and similarity of particle transport of these isotopes have not been identified experimentally yet.

Recently, a deuterium plasma experiment has begun in LHD, and measurements of the hydrogen (H) and deuterium(D) profiles have been made using bulk charge exchange recombination spectroscopy (b-CXRS) [1]. Global particle confinement time in the steady state, as well as transient decay time after pellet injection, have been analyzed for H, D and H/D mixture plasmas in order to characterize the isotope effect on particle transport.

In the steady state, the global particle confinement time τp is obtained by the following relation between the flux of particles flowing into the plasma ninflow and the plasma electron density [2]. The index as a measure of τp in arbitrary unit is defined by

 

Here, ninflow is evaluated by the following equation using the emission line intensities of Hα, Dα and HeI.

 

Statistical regression analysis of dataset for ranging
B = 1.4 T or 2.4 T, 0.87 MW < P < 12.5 MW, 1.0×1019m-3 < < 5.7 × 1019m-3 has been conducted in terms of the averaged particle mass number of the plasma constituents m, the magnetic field B, the plasma electron density , and the absorbed heating power by NBI or ECH power P. It has been found, in particular, that the global particle confinement time deteriorates as the mixture changes from pure H to pure D.

The transient decay time of H/D concentration after H or D pellet injection has been also analyzed.

Figure 1 shows (a) temporal evolution of and (b) concentration of H ions in the peripheral region which is measured by b-CXRS when a H pellet was injected into D dominant plasma. It shows a sudden increase in the concentration of H and at 4.46s due to H pellet injection.

 

Fig.1: The temporal evolution with pellet injection. (a) Line averaged electron density in the vicinity of the center chord. (b) Concentration of H ions in the peripheral region measured by b-CXRS.

The concentration of H decreases exponentially after pellet injection. This decay time τdecay has been surveyed for cases with H and D pellet injection into target plasmas with different H/D mixtures and analyzed using statistical regression analysis in the same manner as the global particle confinement time in the steady state. Provisional analysis suggests no significant difference in the decay time between H and D particles in contrast to the result from the global particle confinement time.

The difference and similarity of these observations in steady-state and transient response is discussed in detail in terms of parameter dependence.

This work is supported by the JSPS KAKENHI Grant Number 17H01368 and the National Institute for Fusion Science Grant administrative budgets NIFS18KLPP051.

References

[1] K. Ida et al., Rev. Sci. Instru. 90 (2019) 093503.
[2] P. C. Stangeby and G. M. McCracken 1990 Nucl. Fusion 30 (1990) 1225.


Pedestal ECE data interpretation for turbulence
characterization with a synthetic diagnostic

G. Yu1, R. Nazikian2, Y. Zhu1, Z. Yan3, G. Kramer2, A. Diallo2, G.R McKee3, N.C. Luhmann Jr1
1University of California, Davis
2Princeton Plasma Physics Laboratory
3University of Wisconsin-Madison

Although a powerful local electron temperature fluctuation diagnostic, ECE (Electron Cyclotron Emission) data interpretation is complicated by insufficient optical depth and non-local radiation effect when being used for pedestal turbulence characterization. Consequently, the diagnostic data are a mixture of density fluctuations δne and temperature fluctuations δTe. Forward modeling of the ECE radiation at the pedestal is thus essential in interpreting the measurements. Here, synthetic ECE [1] is applied to enhance the capability of ECEI (ECE-Imaging [2]) in characterizing an ion scale turbulence, which occurs during ELM (Edge Localized Modes) suppression with RMP (Resonant Magnetic Perturbation) in the DIII-D tokamak.

The synthetic ECE is first benchmarked to prove its capability in simulating radiation near the separatrix. The anomalous radiation is robustly observed with ECEI in the presence of a strong core MHD, as shown in Fig 1(a). Assuming the core MHD perturbs the plasma edge with a periodic rigid displacement, the ECE radiation temperature Te,rad profile is then modelled by a rigid shift of the temperature and density profiles. Due to the nonlocal radiation effect, the radiation profile shows an anomalous increase outside the separatrix. This anomalous radiation results in a phase inverted structure in the radial profile of the radiated power near the separatrix (Fig1(b)). This benchmark using core MHD is crucial as it shows that our synthetic diagnostic is in quantitative agreement with the radiation profile at the pedestal foot or even outside the separatrix [3].

 

Fig. 1: The ECE radiation profile due to the inward shift of the ne and Te profile is consistent with the phase inverted structure that often appears on ECEI in presence of strong rotating core MHD. (a) Phase inverted structure observed with ECEI (b) The ECE radiation profile modelled with the synthetic diagnostic

Fundamental and quantitative understanding are achieved with the synthetic modeling of radiation at two ECE frequencies in response to analytical δTe and δne at the pedestal top (at rho~0.95). The two ECE frequencies point to cold resonances at the pedestal foot (rho~0.97) and Scrape of Layer (rho~1.03), but their hot resonances are all centered at rho~0.95 due to the nonlocal radiation effect in the pedestal. The density and temperature have opposite effects on the radiated power as the electron temperature enhances the local emission, while electron density produces radiation absorption. Quantitatively, we found the ECE radiation from the location rho~0.97 is 5.5 times more sensitive to the δTe than δne at the pedestal top (rho≈0.95), while the radiation at rho~1.03 is equally sensitive to density and temperature fluctuations at the pedestal top (rho≈0.95).

Combining the findings in the last paragraph and ECEI observations, the source of radiation fluctuation and cross phase between δTe and δne are inferred. Experimentally in shot 179328, we found the pedestal top turbulence during RMP ELM suppression displays 1.55δ0.08% δTe,rad / Te,rad, and the cross phase between the two ECEI channels at rho~0.97 and rho~1.03 on the midplane is ~3.01 ± 0.1 rad. Consequently, we deduce that the radiation fluctuation is dominantly caused by δTe instead of δne. In addition, the δTe and δne fluctuation are ~ in-phase at the pedestal top.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under Award(s) DE-FC02-04ER54698, DE-FG02-99ER54531, and DE-AC02-09CH 11466, DE-FG02-08ER54999. The author is partially supported by Chinese Scholar Council.

References

[1] Shi, L., Review of Scientific Instruments 87.11 (2016): 11D303.
[2] Zhu, Y., Review of Scientific Instruments 89.10 (2018): 10H120.
[3] Tobias Benjamin John, Review of Scientific Instruments 83.10 (2012): 10E329.


Simulation Study of Energetic Particle-driven MHD Modes and Energetic Particle Redistribution in Heliotron J

P. Adulsiriswad1, Y. Todo2, S. Kado3, S. Yamamoto4, S. Kobayashi3, S. Ohshima3, H. Okada3, T. Minami3, Y. Nakamura1,
A. Ishizawa1, S. Konoshima3, T. Mizuuchi3, K. Nagasaki3
1Graduate School of Energy Science, Kyoto University, Japan
2National Institute for Fusion Science, National Institutes of Natural Sciences, Japan
3Institute of Advanced Energy, Kyoto University, Japan
4Naka Fusion Institute, National Institutes for Quantum and Radiological Science and Technology, Japan
e-mail (speaker): adulsiriswad.panith.85x@st.kyoto-u.ac.jp

A sufficiently long energetic particle (EP) confinement time is important for realizing self-sustainable plasma; however, these EPs can resonate with the shear Alfvén waves (SAW) through fundamental and sideband resonances in the magnetic confinement fusion devices (e.g. tokamak and stellarator/heliotron). In this study, we analyzed the stability of the EP-driven MHD instabilities and their effects on the EPs confinement in Heliotron J, a low shear helical axis stellarator/heliotron device. The three-dimensional magnetic field of Heliotron J is mainly composed of helical and toroidal magnetic fields characterized by bumpy Fourier components. This creates additional interactions between EPs and SAW [1-2]. In this study, the stabilities and EPs interactions are analyzed by MEGA [3-4]. Fixed and free MHD boundary conditions are utilized. It has been shown in Ref. [5] that the free MHD boundary condition can have a significant impact on the stability of the low-n mode in the strongly shaped plasma. The simulation is conducted for three main magnetic configurations: low (𝜖01 = 0.01), medium (𝜖01 = 0.06) and high (𝜖01 = 0.15) bumpiness configurations. Based on a CX-NPA measurement [6], EP confinement is improved in the medium and high bumpiness configurations.

The fixed boundary simulation results show that the n/m=2/4 GAE is a dominant mode for all the magnetic configurations, while the n/m=1/2 mode is observed as a recessive component. The time evolution of the n/m=1/2 mode is obscured due to the large difference in the mode amplitude between n/m=1/2 and n/m=2/4 modes. This contradicts with the experiments, where the n/m=1/2 mode is the dominant mode. These modes have a global structure. It is shown that the EPs in the core region with sufficiently large orbit width interact with these modes in the peripheral region. The linear growth rate for the n/m=2/4 GAE is highest for the low bumpiness configuration (See Fig 1). This is due to the difference in the local magnetic shear (MHD dissipation) and EP-SAW interactions (EP drive). For the EP interactions, the majority of the resonances were intermediated by the interaction between high velocity EPs and toroidicity-induced resonances. The interactions become weaker for the medium and high bumpiness configurations. For the free boundary case, the mode profiles radially shift toward the edge region, and their linear growth rates increase for all configurations. This increase is due to the stronger EP-SAW interaction in the peripheral region, while the MHD dissipation rate remains almost constant. The apparent linear growth of the n/m=1/2 mode can also be observed. This suggests the importance of the boundary condition for simulating the low-n instability in Heliotron J. A systematic parametric scan for the occurence of the n/m=1/2 will be made in the future.

This work was supported by 'PLADyS', JSPS Core-to-Core Program, A. Advanced Research Networks and Future Energy Research Association.

 

Fig. 1: The logarithmic time evolution of the radial velocity amplitude for n/m=2/4 GAE between low, medium and high bumpiness configurations for the fixed and free boundary MHD conditions.

References

[1] S. Yamamoto, et al. Phys. Rev. Lett. 91 245001 (2003).
[2] Y.I. Kolesnichenko and V.V. Lutsenko, Phys. Plasmas 9 5 (2002).
[3] Y. Todo and T. Sato, Phys. Plasmas 5 1321 (1998).
[4] P. Adulsiriswad et al, Nucl. Fusion 60 096005 (2020).
[5] E.Y.Chen, et al, Phys. Plasmas 18 052503 (2011).
[6] M. Kaneko, et al., Fus. Sci. Technol. 50 428-433 (2006).


A Next Generation Ultra Short Pulse Reflectometer

Calvin W. Domier, Yilun Zhu, Jon Dannenberg, N.C. Luhmann, Jr.
University of California at Davis, Davis, CA 95616 USA
e-mail (speaker): cwdomier@ucdavis.edu

Ultrashort Pulse Reflectometry (USPR) is a diagnostic technique involving the propagation of a number of ultra short duration (~few nsec) chirps which contain frequency components spanning large portions of the desired plasma density profile [1]. Here, each frequency component in the wave packet reflects from a different density layer in the edge plasma. The reflected wave packet is down-converted and passed through a multichannel filter bank where it is detected, with time-of-flight measurements made on each of the filtered wave packets. With sufficient time-of-flight (TOF) data, it is then possible to reconstruct the electron density profile of the target plasma.

One key advantage of USPR is that the diagnostic measurement takes place during such a short time (~nsec) that density fluctuations are essentially frozen in place. This technique was applied in the past to the Sustained Spheromak Physics Experiment (SSPX), where a 24-channel system operated spanning a frequency range of 33 to 75 GHz [2]. UC Davis is now extending this technique with higher power (>10X) sources, enhanced channel count (~60), and higher speed TOF electronics.

A schematic illustration of this next generation USPR system is illustrated in Fig. 1. A microwave chirp is upconverted to mm-wave frequencies by high power active multiplier chains (AMCs); three AMCs provide coverage extending from 26.5 to 75 GHz. The reflected waves are collected by the same microwave horns (i.e., a monostatic horn configuration), and down-converted to microwave frequencies (2.55 to 18.5 GHz). TOF data are collected by a field programmable gate array (FPGA) based controller which will collect and process all USPR data in addition to generating all required control signals.

 



Fig. 1: Illustration of the USPR technique, showing advances over the previous USPR implementation on the SSPX device.

Output from a Picosecond Pulse Labs 3500C impulse generator is dispersed into a monotonically decreasing frequency chirp using a length of WRD475 waveguide, and high pass filtered to form a 5.0 to 9.5 GHz chirp. This low frequency chirp is amplified, frequency doubled, and amplified once more to form a 10.0 to 19.0 GHz Baseband Transmitter Chirp which is then upconverted to mm-wave frequencies (see Fig. 2). An SP3T switch connects this chirp to one of three waveguide assemblies, while a similar switch connects the USPR receiver to the same assembly. With a pulse repetition rate of 1 MHz, electron density profiles may be obtained in as little as 3 μsec.

 



Fig. 2: Schematic diagram of the USPR mm-wave configuration.

Work is now underway on the critical TOF electronics, which is illustrated schematically in Fig. 3. The input signal is amplified, rectified by a high speed detector, and amplified once more before passing through a Constant Fraction Discriminator (CFD) which generates a trigger signal whenever the input signal exceeds the detection threshold limit. A high speed flip-flop generates a pulse that in turn charges up a capacitor whose voltage is proportional to the length of the pulse. This is amplified, sampled by a high speed sample-and-hold (S/H), and finally digitized by the FPGA.

 



Fig. 3: Schematic layout of the time-of-flight circuit to be used by USPR.

The prototype TOF circuitry is scheduled to be completed by late August, and the entire system as illustrated in Fig. 1 by early October. Detailed descriptions of the USPR system, along with laboratory character-ization results, will be presented.

The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0001164.

References

[1] C.W. Domier et al., "Rev. Sci. Instrum. 66, 399 (1995).
[2] Y. Roh et al., "Rev. Sci. Instrum. 74, 1518 (2003).


The investigation of QCM on EAST using DR diagnostics

J.X. Ji, C.Zhou, A.D.Liu, X. Feng, J. Zhang, Z.Y. Liu, X. M. Zhong, H. R. Fan, G. Zhuang, J. L. Xie,
T. Lan, W. Z. Mao, W. X. Ding, H.Li, Z. X. Liu, and W. D. Liu
School of Nuclear Sciences and Technology, University of Science and Technology of China, Hefei, Anhui 230026, China
e-mail (poster):jijiaxu@mail.ustc.edu.cn

A low frequency electrostatic Quasi Coherent Mode (QCM) with frequency around 5-80kHz has been investigated by Doppler Reflectometry (DR) on Experimental Advanced Superconducting Tokamak (EAST) H-mode operation. QCM is a kind of instability, which is relevant to the Dissipative Trapped Electron Mode (DTEM) [1], and plays an essential part in the transportation process in H mode discharges.

Fig.1 is a typical H-mode discharge with the parameters: IP ~ 502 KA, BT ~2.4 T, q95 ~5.3, κ~1.6. It's a RF-dominant discharge with LHCD~1.7 MW, and ECRH ~ 0.35 MW. Around 2.68s, the Da emission features a sharp decrease, the stored energy and density increasing quickly, indicating an L-H transition. After the L-H transition, the QCM appears at around 2.7s in the spectrum of velocity fluctuations measured by the DR system.

The eight-channel V-band DR and five-channel W-band DR systems are used for the QCM investigation in EAST, and they can cover the radial region rho =0.1~1. In many discharges, the QCM was observed in both the V-band and W-band DR systems, indicating that the QCM exists from the edge to the core plasma, which is quite different from the Edge Coherent Mode (ECM) [2]. The radial distribution of QCM intensity is demonstrated here, which gives more information about the stimulation and spreading of QCM. By analysis of the evolutions of the poloidal velocity and the frequency of QCM during NBI modulation, we found that the QCM rotates poloidally in the electron diamagnetic direction relative to the poloidal direction. The ELM behavior with relative to without QCM has been investigated in the experiments, indicating that the QCM relates to outward transportation of particles.

 

Fig.1: Example of QCM in H-mode (#93358). Evolution of (a) the parameters of the heating: LHCD, ECRH; (b) the average chord density and the stored energy; (c) XUV radiation ; (d) Da radiation. (e) The spectrogram of the fluctuations of the poloidal velocity detected by DR.

References

[1] Arnichand H, Sabot R, Hacquin S, et al. Quasi-coherent modes and electron-driven turbulence[J]. Nuclear Fusion, 2014, 54(12):123017.1-123017.7.
[2] A stationary long-pulse ELM-absent H-mode regime in EAST[J]. Nuclear fusion, 2017, 57(8):086041.1-086041.19.


Direct measurement of a thrust induced by sputtered materials
in magnetron-type plasma sources

Hidemasa MIURA, Kazunori TAKAHASHI, and Akira ANDO
Department of Electrical Engineering, Tohoku University
e-mail (speaker):hidemasa_miura@ecei.tohoku.ac.jp

Electric propulsion is one of the important technologies for space missions by spacecraft and artificial satellites. These days, small satellites called CubeSats, packed in a 10×10×10 cm3 cubic unit, have attracted much attention. When the space mission is accomplished, the small satellites have to be removed from Earth orbit, to avoid increasing the number objects in Earth orbit. The increasing number of space debris has been recognized as a serious problem, because such objects collide with other other spacecraft causing damage. Therefore development of a compact electric propulsion device mountable on the small satellite is required to prevent it from become space debris and contribute to making space activities sustainable. It would be useful to eliminate a high-pressure gas tank for the propellant storage, yielding a compact system. An ion gridded thruster operated with water propellant has been proposed and is under development [1]. Here a new concept for the compact electric propulsion device using a metallic propellant is proposed, where the thrust is generated by mass ejection due to sputtering of a metallic material. This This preliminary experiment is performed with argon gas for plasma production and the thrust imparted by sputtering of target material is demonstrated.

"Sputtering" is a phenomenon that occurs when ions in plasmas impinge a target surface. The ions give their momentum to target material and the metallic particles are released from the target surface with a certain kinetic energy. As the thrust is equivalent to the momentum flux exhausted from the system, the ejection of the sputtered material is expected to contribute to the thrust generation. A well-known magnetron sputtering system is chosen here for the sputtering system since its high density plasma is confined. Since the charged particles are confined by the magnetic field lines terminating on the target surface in the magnetron source, it is expected that the plasma itself has only a small contribution to the thrust generation. Furthermore, two different magnetrons sources are tested in the present experiment employing Direct Current Magnetron Sputtering (DCMS) and High Power Impulse Magnetron Sputtering (HiPIMS). Since the HiPIMS can supply a high power in a few tens of microseconds, a current magnitude in the HiPIMS source is typically about 500 times greater than that in the DCMS source. When HiPIMS is used, sputtering of the target by the ionized target material often occurs, which is called as "self sputtering" [2].

The magnetron sputtering source, shown in Fig.1(a), is attached to a pendulum thrust balance installed in a 26-cm diameter and 73-cm long vacuum chamber evacuated by a turbo-molecular pumping system. The thrust balance consists of two flexible plates suspended from the top side of the chamber; this structure enables the plasma source to move axially with a pendulum motion[3]. A displacement induced the plasma production and sputtering is measured using a commercial displacement sensor. By measuring a calibration coefficient relating the displacement to the force before pumping down the chamber, the absolute value of the thrust can be obtained.

The thrust obtained with a copper target and a DC supply is about 400-600 μN for the discharge power of 40-60 W as shown in Fig.1(b), while the thrust is undetectable for a carbon target even for similar discharge power (not shown here). Under the same conditions with the HiPIMS, the obtained thrust is about 200-300 μN. The thrust reduction for HiPIMS can be attributed to self sputtering. Since the sputtered particles are ionized and are accelerated to the target in the self-sputtering process, the exhausted particles decrease. These results demonstrate thrust generation due to mass ejection resulting from sputtering. A detailed discussion will be shown in the presentation.

 

Fig. 1: (a) Schematic diagram of the magnetron sputtering source.(b) Measured thrust with a Cu target for the DCMS (red dots) and the HiPIMS (black dots).

References

[1] H. Koizumi,et al.,JSASS Aerospace Tech.12,19-24 (2014).
[2] Andre Andres,Journal of Appl Phys., 121, 171101 (2017).
[3] K.Takahashi, Rev. Mod. Plasma Phys., 3,3 (2019).


Effects of energetic particles on the collisionless trapped-electron-mode instability in tokamak plasmas

M. S. Hussain1,2, Weixin Guo1, Lu Wang1
1International Joint Research Laboratory of Magnetic Confinement Fusion and Plasma Physics,
State Key Laboratory of Advanced Electromagnetic Engineering and Technology,
School of Electrical and Electronic Engineering, Huazhong University of Science
and Technology, Wuhan, Hubei 430074, People's Republic of China.
2School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei 430074,
People's Republic of China.
e-mail (poster presenter): sadamhussaingcu@gmail.com

Abstract

Effects of energetic particles (EPs) on the microturbulence driven by temperature gradient and/ or density gradient, such as ion temperature gradient mode and trapped electron mode turbulence are important to study for controlling magnetic magnetic confinement fusion in tokamak plasmas. [1, 2] ICRH driven EPs (He-3) have strong stabilizing effects on the ITG linear growth rate as compared to NBI driven fast ions (deuterium). [2] EPs have stabilizing effects on ion temperature gradient instability with low magnetic shear in JET-like plasmas. [3] For a steep density profile of EPs (fusion born alpha particles) with a slowing-down distribution, the trapped electron drift waves are stabilized by the presence of EPs. [4] Recent GENE gyrokinetic simulations show slight destabilization of the trapped electron mode (TEM) by NBI driven EPs. [5] In this work, the influence of EPs on collisionless TEM instability (CTEM) is analyzed using linear gyrokinetic theory and bounce kinetic theory for tokamak plasmas. The EPs under consideration are fusion born alpha particles and NBI beam ions. The effects of these EPs on the CTEM instability are investigated by comparing a trace model and a dynamic model with slowing-down and equivalent Maxwellian distribution functions for the equilibrium EP distribution functions. It is shown that the CTEM instability can be destabilized by the presence of EPs, which is mainly attributed to the downshift of real frequency. In particular, the destabilizing effects of fusion born alpha particles on the CTEM instability are found to be greater than the energetic beam ions effects. Moreover, the difference between growth rates calculated through slowing-down and equivalent Maxwellian distribution functions for EPs are negligible but for a higher number density of beam ions the trace model shows slightly higher mode growth rate as compared to the dynamic model. By increasing the fraction of tritium with fixed EP fraction, the destabilizing effects become stronger.

References

[1] A. Di Siena et al., Nucl. Fusion 58, 054002 (2018).
[2] G. J. Wilkie et al., J. Plasma Physics 81, 905810306 (2015).
[3] A. Di Siena et al., Nucl. Fusion 59, 124001 (2019).
[4] G. Rewoldt and W. M. Tang, Phys. Fluids 26, 3619 (1983).
[5] S. Mazzi et al., Nucl. Fusion 60, 046026 (2020).

 

 
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