Disruptive Instability Research Articles

Avoiding fusion plasma tearing instability with deep reinforcement learning

For stable and efficient fusion energy production using a tokamak reactor, it is essential to maintain a high-pressure hydrogenic plasma without plasma disruption. Therefore, it is necessary to actively control the tokamak based on the observed plasma state, to manoeuvre high-pressure plasma while avoiding tearing instability, the leading cause of disruptions. This presents an obstacle-avoidance problem for which artificial intelligence based on reinforcement learning has recently shown remarkable performance1–4. However, the obstacle here, the tearing instability, is difficult to forecast and is highly prone to terminating plasma operations, especially in the ITER baseline scenario. Previously, we developed a multimodal dynamic model that estimates the likelihood of future tearing instability based on signals from multiple diagnostics and actuators5. Here we harness this dynamic model as a training environment for reinforcement-learning artificial intelligence, facilitating automated instability prevention. We demonstrate artificial intelligence control to lower the possibility of disruptive tearing instabilities in DIII-D6, the largest magnetic fusion facility in the United States. The controller maintained the tearing likelihood under a given threshold, even under relatively unfavourable conditions of low safety factor and low torque. In particular, it allowed the plasma to actively track the stable path within the time-varying operational space while maintaining H-mode performance, which was challenging with traditional preprogrammed control. This controller paves the path to developing stable high-performance operational scenarios for future use in ITER.

Self-organized magnetic equilibria in tokamak plasmas with very low edge safety factor

Tokamak plasmas often exhibit self-organizing behavior in which internal modes shape the toroidal current density profile, a common example being the sawtooth instability. However, such behavior has not been studied in detail for edge safety factor below 2 due to disruptive kink instabilities that typically prevent operation in this regime. Now, steady tokamak plasmas with an edge safety factor down to 0.8 have been created in the Madison Symmetric Torus, where disruptions are prevented due to a thick, conductive wall and a feedback power supply that sustains the plasma current. Internal measurements and nonlinear magnetohydrodynamic modeling reveal a family of safety factor profiles with a central value clamped near unity as the edge safety factor decreases, indicating current profile broadening through a relaxation process. As the safety factor decreases, the magnetic fluctuations become irregular, and the electron energy confinement time decreases.

Hobbes’s On the Citizen: A Critical Guide ed. by Robin Douglass and Johan Olsthoorn

Reviewed by: Hobbes’s On the Citizen: A Critical Guide ed. by Robin Douglass and Johan Olsthoorn Daniel Collette Robin Douglass and Johan Olsthoorn, editors. Hobbes’s On the Citizen: A Critical Guide. Cambridge Critical Guides. Cambridge: Cambridge University Press, 2020. Pp. xi + 251. Hardback, $99.99. Robin Douglass and Johan Olsthoorn’s edited critical guide grew from a European Hobbes Society meeting themed on Hobbes’s On the Citizen (De Cive). Hobbes intended On the Citizen to be the final treatise of his tripartite Elements of Philosophy. Sociopolitical forces demanded that he publish On the Citizen first, and he only later completed the trilogy with two preceding volumes: On the Body (De Corpore) and On Man (De Homine). Despite On the Citizen’s significance, it is often overlooked in scholarly work and in classroom instruction favoring Hobbes’s better-known Leviathan. Douglass and Olsthoorn augment the intellectual landscape by providing an engaging collection of articles that make an important and needed contribution to fill this void. The guide contains twelve chapters authored by well-known scholars such as S. A. Lloyd and A. P. Martinich. Mirroring the order of presentation in On the Citizen, these chapters are organized into three themes: liberty, government, and religion. The contents of these themes are weighed differently in this volume, however. Where over half of On the Citizen is devoted to discussions on government, these editors provide a more in-depth exploration of liberty and religion without neglecting political themes more often associated with Hobbes studies, such as sovereignty and authorization. The volume commences with five chapters on liberty. Deborah Baumgold and Ryan Harding begin this section strongly, comparing On the Citizen to Elements of Law (Hobbes’s first political treatise) in chapter 1. They leverage differences between those two treatises to establish that Hobbes’s geometrical method is analytic, not formalist or synthetic. This is important because Hobbes’s method is one of discovery; it was an empirical approach that led him to narrower questions and, ultimately, to political philosophy. The remaining chapters on liberty cover chiefly moral philosophy. The second chapter provides a defense of Hobbes’s strong rejection of Aristotle’s definition of humans as natural political animals, the third considers the role that self-admiration plays in conflict (and hence, its suitability for regulation), the fourth admirably appraises Hobbes’s thought experiment of parricide to stress the limits of political obligation, and the fifth contains an exploration of the sincerity of Hobbesian right reason. Hobbes rarely strays from the topic of government in On the Citizen; the next three chapters explicitly address this by returning to central political concerns. Authors of these chapters consider despotic sovereignty, liberal principles, and property rights (chapter 6); sociopolitical forces, lordship, and slavery (chapter 7); and Hobbes’s notion of corporate persons, which is presented here as a precursor to authorization (chapter 8). The guide’s last four chapters focus on religion. Hobbes viewed religion as a destabilizing force and political threat since citizens who are torn between the dualism of religious duties and civic duties, or those who are arbiters of their own religious convictions, are never fully committed to the state; such split allegiances can cause disruptive conflicts and instability. [End Page 505] For these reasons, religion is significant for Hobbes’s political program, and it receives a refreshing amount of attention in the volume. The authors here engage with Hobbes’s conflation of the love of God with the laws of nature, which ultimately collapses into obedience to the sovereign (chapter 9); his softened rhetoric against Rome for publishing in France, which changed superficial aspects of church-state relations, but not its fundamental structure (chapter 11); and his selective use of biblical models for grounding contracts, covenants, and sovereign making, as well as why he is selective (chapter 12). Alison McQueen (chapter 10) provides an especially persuasive analysis of Hobbes’s religious concerns, responding to previous scholarship by Jeffrey Collins. Collins argues that there was a shift in Hobbes’s religious focus in the period between Elements of Law and On the Citizen: where in Elements Hobbes was concerned with private inspiration and religious enthusiasm, in On the Citizen he addressed...

The Mechanical Analysis of ELM Joint under Coupling Field

The Edge Localized Mode coil is the key component to prohibit the phenomena of disruptive instability occur-ring in the edge of Tokamak plasma. And the coil is made of Stainless Steel Jacketed Mineral Insulated Con-ductors. The different pieces of conductor are connected by joints. During the normal operation of Tokamak device, the joint will be shocked by electromagnetic and thermal loads. Thus, it is necessary to perform the mechanical analysis to verify whether or not the ELM joint has sufficient safety margin to resist the impact of coupling field. In order to obtain the load boundary condition for mechanical analysis, the electromagnetic and thermal analysis are launched first. Then the temperature and electromagnetic force density are inserted into the mechanical analysis model. And the equivalent stress is calculated. The analysis results indicate there is stress intensity at the component of supporting rail. To mitigate the stress intensity, the local structural optimi-zation design is employed. Finally, the stress evaluation is carried out based on analytical design. The assess-ment results demonstrate the optimized model has sufficient safety margin to withstand the combined action of multiple loads.

Progressive Thermalization Fusion Reactor Able to Produce Nuclear Fusions at Higher Mechanical Gain

In the standard fusion reactors, mainly tokamaks, the mechanical gain obtained is below 1. On the other hand, there are colliding beam fusion reactors, for which, the not neutral plasma and the space charge limit the number of fusions to a very small number. Consequently, the mechanical gain is extremely low. The proposed reactor is also a colliding beam fusion reactor, configured in Stellarator, using directed beams. D+/T+ ions are injected in opposition, with electrons, at high speeds, so as to form a neutral beam. All these particles turn in a magnetic loop in form of figure of “0” (“racetrack”). The plasma is initially non-thermal but, as expected, rapidly becomes thermal, so all states between non-thermal and thermal exist in this reactor. The main advantage of this reactor is that this plasma after having been brought up near to the optimum conditions for fusion (around 68 keV), is then maintained in this state, thanks to low energy non-thermal ions (≤15 keV). So the energetic cost is low and the mechanical gain (Q) is high (>>1). The goal of this article is to study a different type of fusion reactor, its advantages (no net plasma current inside this reactor, so no disruptive instabilities and consequently a continuous working, a relatively simple way to control the reactor thanks to the particles injectors), and its drawbacks, using a simulator tool. The finding results are valuable for possible future fusion reactors able to generate massive energy in a cleaner and safer way than fission reactors.

Avoidance of vertical displacement events in DIII-D using a neural network growth rate estimator

Robust disruption avoidance techniques are critical for the development of reliable fusion reactor devices. A viable reactor will require non-disruptive, long pulse operation where simply shutting down a discharge is undesirable. To achieve such performance, the plasma must be controlled to continuously avoid hazardous regimes instead of asynchronously aborting. A recent experiment on DIII-D demonstrated for the first time real-time control of proximity to a disruptive instability boundary. In particular, the vertical growth rate, an eigenvalue that characterizes the degree of instability of the plasma's vertical position, was regulated so as not to exceed DIII-D's vertical controllability limit. The open-loop growth rate was estimated in real time on the DIII-D plasma control system using a neural network model trained with tens of thousands of DIII-D shots. The model was trained to replicate the results of RZRIG [1], a rigid displacement code for calculating the growth rate. Once trained, producing an estimate using the neural network is multiple orders of magnitude faster than RZRIG, thereby making the calculation suitable for real-time execution. The control system regulated the estimated growth rate by adjusting plasma elongation and distance to the inner wall of the vessel, and this regulation was shown to reliably avoid vertical displacement event disruptions (i.e. uncontrolled vertical oscillations) of the plasma. This work presents these experimental results, including the dynamic performance and the effectiveness of the control technique. Details are presented on the training of the neural network model, including concerns such as hyperparameter tuning and uncertainty quantification. Additionally, the methodology for embedding the neural network into the control system is discussed.

Magnetorotational core collapse of possible GRB progenitors – III. Three-dimensional models

ABSTRACT We explore the influence of non-axisymmetric modes on the dynamics of the collapsed core of rotating, magnetized high-mass stars in three-dimensional simulations of a rapidly rotating star with an initial mass of $M_{\rm {\small ZAMS}} = 35 \, \mathrm{M}_{\odot }$ endowed with four different pre-collapse configurations of the magnetic field, ranging from moderate to very strong field strength and including the field predicted by the stellar evolution model. The model with the weakest magnetic field achieves shock revival due to neutrino heating in a gain layer characterized by a large-scale, hydrodynamic m = 1 spiral mode. Later on, the growing magnetic field of the proto neutron star launches weak outflows into the early ejecta. Their orientation follows the evolution of the rotational axis of the proto neutron star, which starts to tilt from the original orientation due to the asymmetric accretion flows impinging on its surface. The models with stronger magnetization generate mildly relativistic, magnetically driven polar outflows propagating over a distance of 104 km within a few $100 \, \textrm {ms}$. These jets are stabilized against disruptive non-axisymmetric instabilities by their fast propagation and by the shear of their toroidal magnetic field. Within the simulation times of around $1 \, \textrm {s}$, the explosions reach moderate energies and the growth of the proto neutron star masses ceases at values substantially below the threshold for black hole formation, which, in combination with the high rotational energies, might suggest a possible later proto-magnetar activity.

First wall energy deposition during vertical displacement events on ITER

The beryllium (Be) first wall energy deposition and melt damage profiles resulting from the current quench phase of an unmitigated, 5 MA/1.8 T upward vertical displacement event for ITER are investigated. Time dependent 2D magnetic flux profiles are calculated with the DINA code and used as input for the SMITER 3D field line tracing software. 3D maps of the wetted area and perpendicular heat flux show that the majority of the energy deposition occurs on the upper first wall panels #8 and #9. SMITER simulations predict on the surfaces of upper FWPs #8 and #9 at the end of the ∼450 ms current quench. The surface heat flux maps generated by SMITER are used as input in the MEMOS-U code, which models Be melt formation and dynamics. Simulations reveal peak surface temperatures of ∼2200 K, inward surface damage of ∼0.5 mm in depth, and average melt velocities of ∼2 m s−1. Although VDEs are in principle the easiest disruptive instability to avoid, the analysis demonstrates that any non-mitigated events or intentional VDEs taking place during low Ip, early operational phases of ITER for the purposes of estimating disruption forces, must be kept to a low number.

Progress in disruption prevention for ITER

Key plasma physics and real-time control elements needed for robustly stable operation of high fusion power discharges in ITER have been demonstrated in recent research worldwide. Recent analysis has identified the current density profile as the main drive for disruptive instabilities in discharges simulating ITER’s baseline scenario with high and low external torque. Ongoing development of model-based profile control and active control of magnetohydrodynamic instabilities is improving the stability of multiple scenarios. Significant advances have been made toward real-time physics-based prediction of instabilities, including path-oriented analysis, active sensing, and machine learning techniques for prediction that are beginning to go beyond simple disruption mitigation trigger applications. Active intervention contributes to prevention of disruptions, including forced rotation of magnetic islands to prevent wall locking, and localized heating/current drive to shrink the islands. Stable discharge rampdowns have been achieved with the fastest ITER-like scaled current ramp rates, while maintaining an X-point configuration. These elements are being integrated into stable operating scenarios and new event-handling systems for off-normal events in order to develop the physics basis and techniques for robust control in ITER.

Predicting disruptive instabilities in controlled fusion plasmas through deep learning.

Nuclear fusion power delivered by magnetic-confinement tokamak reactors holds the promise of sustainable and clean energy1. The avoidance of large-scale plasma instabilities called disruptions within these reactors2,3 is one of the most pressing challenges4,5, because disruptions can halt power production and damage key components. Disruptions are particularly harmful for large burning-plasma systems such as the multibillion-dollar International Thermonuclear Experimental Reactor (ITER) project6 currently under construction, which aims to be the first reactor that produces more power from fusion than is injected to heat the plasma. Here we present a method based on deep learning for forecasting disruptions. Our method extends considerably the capabilities of previous strategies such as first-principles-based5 and classical machine-learning7-11 approaches. In particular, it delivers reliable predictions for machines other than the one on which it was trained-a crucial requirement for future large reactors that cannot afford training disruptions. Our approach takes advantage of high-dimensional training data to boost predictive performance while also engaging supercomputing resources at the largest scale to improve accuracy and speed. Trained on experimental data from the largest tokamaks in the United States (DIII-D12) and the world (Joint European Torus, JET13), our method can also be applied to specific tasks such as prediction with long warning times: this opens up the possibility of moving from passive disruption prediction to active reactor control and optimization. These initial results illustrate the potential for deep learning to accelerate progress in fusion-energy science and, more generally, in the understanding and prediction of complex physical systems.

Health effects of work and family transitions

Disruptive life events, including transitions in work or family structure, affect health. Research often focuses on one transition rather than thinking of an event framework in which respondents experience multiple transitions across qualitatively distinct domains. This paper contributes original evidence on the effects of event interaction, transition timing, and multiple occurrences of events on health outcomes. I look at employment loss, employment gain, marriage, and divorce as instances of disruptive transitions or instability in the life course; I analyse these events’ effects on self-rated health and depression at ages 40 and 50. I show that employment losses and divorces have significant negative effects on health, and employment gains and marriages show smaller positive effects or null effects. Higher counts of transitions lead to stronger effects on health. Respondents who are older at event occurrence show larger negative effects, suggesting that work and family instability at early ages is not as detrimental to health as such instability at later ages. These results show that there are similarities across work and family domains in effects on health outcomes; moreover, experiencing several transitions can lead to overlaps in effects that might lessen or worsen health outcomes overall.

The causes of the disruptive tearing instabilities of the ITER Baseline Scenario in DIII-D

Analysis of the evolving current density (J), pedestal and rotation profiles in a database of 200 ITER Baseline Scenario (IBS) demonstration discharges in the DIII-D tokamak identifies the current profile as a key determinant of the disruptive instability limiting both high and low torque operation of these q95 = 3 plasmas. The m = 2/n = 1 tearing modes, occurring after several pressure-relaxation times, are linked to the shape of the current profile in the outer region of the plasma. The q = 2 surface is located at ρ ~ 0.78, near a minimum in J. A steeper ‘well’ around the q = 2 surface is correlated with the instabilities, and this finding is consistent for modes that occur both early (<1 τR) and late (>2 τR) on the βN flattop, at high and low input torque, for discharges with and without direct electron heating. Low rotation or differential rotation in the core plasmas is shown to not be the direct cause of the onset of these instabilities. The current profile trends identified in this work indicate that the modes limiting the Q = 10 operation in the ITER demonstration plasmas are likely due to the slow evolution of the current profile, and reveal which shape of the current profile is more prone to the 2/1 tearing instability.

Super-Alfvénic translation of a field-reversed configuration into a large-bore dielectric chamber.

An experimental device to demonstrate additional heating and control methods for a field-reversed configuration (FRC) has been developed. The newly developed device, named FRC Amplification via Translation (FAT), has a field-reversed theta-pinch plasma source and a low-elongation dielectric (transparent quartz) confinement chamber with quasi-static confinement field. In the initial experiments on the FAT device, FRC translation and trapping were successfully demonstrated. Although the typical elongation of the trapped FRC in the confinement region was roughly three, no disruptive global instability, such as tilt, was observed. The FAT device increases the latitude to perform translation-related experiments, such as those concerning inductive current drive, equivalent neutral beam injection effects, and wave applications.

Plasma heating and acceleration in current sheets formed in discharges in argon

According to present notion, flares on the sun and other stars, substorms in magnetospheres of Earth and other planets, and disruptive instabilities in tokamak plasma are connected to development of current sheets in magnetized plasma. Therefore, current sheet dynamics and magnetic reconnection processes were studied actively during the last several decades. This paper presents the results of experimental studies of plasma heating and acceleration in current sheets formed in discharges in argon. The temperature and energy of directed motion of argon ions of different degrees of ionization were measured by spectroscopic methods. It was found that Ar II, Ar III and Ar IV ions are localized in different regions of the sheet. It was shown that Ampere forces applied to the sheet can accelerate the argon ions to observed energies.

The external kink mode in diverted tokamaks

An explanation is provided for the disruptive instability in diverted tokamaks when the safety factor$q$at the 95 % poloidal flux surface,$q_{95}$, is driven below 2.0. The instability is a resistive kink counterpart to the current-driven ideal mode that traditionally explained the corresponding disruption in limited cross-sections (Shafranov,Sov. Phys. Tech. Phys., vol. 15, 1970, p. 175) when$q_{edge}$, the safety factor at the outermost closed flux surface, lies just below a rational value$m/n$. Experimentally, external kink modes are observed in limiter configurations as the current in a tokamak is ramped up and$q_{edge}$decreases through successive rational surfaces. For$q_{edge}&lt;2$, the instability is always encountered and is highly disruptive. However, diverted plasmas, in which$q_{edge}$is formally infinite in the magnetohydrodynamic (MHD) model, have presented a longstanding difficulty since the theory would predict stability, yet, the disruptive limit occurs in practice when$q_{95}$, reaches 2. It is shown from numerical calculations that a resistive kink mode is linearly destabilized by the rapidly increasing resistivity at the plasma edge when$q_{95}&lt;2$, but$q_{edge}\gg 2$. The resistive kink behaves much like the ideal kink with predominantly kink or interchange parity and no real sign of a tearing component. However, the growth rates scale with a fractional power of the resistivity near the$q=2$surface. The results have a direct bearing on the conventional edge cutoff procedures used in most ideal MHD codes, as well as implications for ITER and for future reactor options.

A Review of the Gas-Puff &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$Z$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Pinch as an X-Ray and Neutron Source

Fisher and collaborators at the University of California, Irvine, invented the gas puff $Z$ -pinch in the late 1970s using a 200-kA generator. The implementation of gas puffs as a copious source of X-rays has encountered major challenges, such as disruptive instabilities and the quest for long implosion times. During nearly four decades of experimental and theoretical efforts, those challenges have been successfully met to a great extent. This success is a result of the efforts of a large number of researchers. Today, the gas-puff $Z$ -pinch has evolved into a powerful source of X-rays and neutrons, and is fielded on multi-Mega-Ampere generators. The basic force imploding a pinch is straightforward, but the operation of a gas puff requires specialized hardware and a thorough understanding of the radiation physics involves magnetohydrodynamics coupled with nonequilibrium ionization kinetics. The goal of this review is to document the experiments and theory that have led to the success of the gas puff as a K-shell X-ray and neutron source. Consequently, this review takes a historical approach to the covered material, but also provides a broad introduction to relevant scientific concepts.

Achieving a long-lived high-beta plasma state by energetic beam injection.

Developing a stable plasma state with high-beta (ratio of plasma to magnetic pressures) is of critical importance for an economic magnetic fusion reactor. At the forefront of this endeavour is the field-reversed configuration. Here we demonstrate the kinetic stabilizing effect of fast ions on a disruptive magneto-hydrodynamic instability, known as a tilt mode, which poses a central obstacle to further field-reversed configuration development, by energetic beam injection. This technique, combined with the synergistic effect of active plasma boundary control, enables a fully stable ultra-high-beta (approaching 100%) plasma with a long lifetime.

"Queer Aswang Transmedia: Folklore as Camp"

In recent years, the aswang—a supernatural creature of Philippine folklore that is often associated with female monstrosity and patriarchal misogyny—is being flamboyantly queered across a range of media. The aswang is a centuries-old transmedial, transgeneric figure whose monstrosity has been interpellated by gender-essentialist agendas while nonetheless epitomizing disruptive gender instabilities. In the handful of texts that comprise queer aswang transmedia—a 2011 Filipino novel (Ricky Lee’s Si Amapola sa 65 na Kabanata [Amapola in 65 Chapters]), mainstream film (Mga Bata ng Lagim [Children of Terror], dir. Mar S. Torres, 1964), and amateur digital video (Amabilis 2, 2011)—the aswang, an iconic female monster, is being destabilized and re-imagined. Gay men (or more accurately, bakla subjects) are occupying the place formerly reserved for monstrous women. This queering of aswang transmedia is a forceful, funny, yet undeniably risky reapproriation lodged in language (“swardspeak”) and a kind of pinoy [Filipino] camp style. This essay attempts to theorize a distinctly Filipino camp sensibility in relation to queer time. It wrestles with queer aswang transmedia’s implications for both temporality (since anachronism underpins the cultural figures of both bakla and aswang) and visibility (queer scholars argue that the bakla, stigmatized as effeminate and lower class, is increasingly the object of forcible bourgeois erasure in the face of the urban gay scene’s aspirations toward an imagined gay globality).

Queer Aswang Transmedia: Folklore as Camp

In recent years, the aswang—a supernatural creature of Philippine folklore that is often associated with female monstrosity and patriarchal misogyny—is being flamboyantly queered across a range of media. The aswang is a centuries-old transmedial, transgeneric figure whose monstrosity has been interpellated by gender-essentialist agendas while nonetheless epitomizing disruptive gender instabilities. In the handful of texts that comprise queer aswang transmedia—a 2011 Filipino novel (Ricky Lee’s Si Amapola sa 65 na Kabanata [Amapola in 65 Chapters]), mainstream film (Mga Bata ng Lagim [Children of Terror], dir. Mar S. Torres, 1964), and amateur digital video (Amabilis 2, 2011)—the aswang, an iconic female monster, is being destabilized and re-imagined. Gay men (or more accurately, bakla subjects) are occupying the place formerly reserved for monstrous women. This queering of aswang transmedia is a forceful, funny, yet undeniably risky reapproriation lodged in language (“swardspeak”) and a kind of pinoy [Filipino] camp style. This essay attempts to theorize a distinctly Filipino camp sensibility in relation to queer time. It wrestles with queer aswang transmedia’s implications for both temporality (since anachronism underpins the cultural figures of both bakla and aswang) and visibility (queer scholars argue that the bakla, stigmatized as effeminate and lower class, is increasingly the object of forcible bourgeois erasure in the face of the urban gay scene’s aspirations toward an imagined gay globality).

Electron Cyclotron Power Source System For ITER

Electron cyclotron (EC) heating and current drive is a well-established auxiliary heating mechanism for tokamak plasmas, which is also effective in assisting plasma breakdown and controlling disruptive plasma instabilities. ITER requires 20 MW of power at 170-GHz frequency to be coupled into the plasma for EC radio-frequency (rf) applications. Gyrotrons are rf/microwave oscillators capable of delivering high continuous-wave power in the microwave and millimeter-wave frequency range (a few to hundreds of gigahertz). An EC system with 26 gyrotron sources at 170 GHz, with a typical unit power of 1 MW each, and a total installed power capacity of 24 MW is planned for ITER. As a part of the in-kind contributions, the Indian domestic agency is responsible for two sets of EC sources that provide 2 MW (∼8%) of the EC power at 170-GHz frequency. Here, we provide an overview of the gyrotron source system, its basic concepts and main features, design aspects, auxiliary requirements, performance issues, and future research and development goals.