Far Tech (United States)

Electron beams emitted from a cold cathode are thermally stable and mono-energetic with a small phase-space volume*. We have been developing a field-emission type RF-gun system for high brightness electron source applications, including electron scattering/diffraction and tunable coherent X-ray/THz generation. The system consists of a single-gap gun-cavity and an S-band klystron/modulator capable of powering the gun with up to 5.5 MW peak (PRR = 1 Hz, duration = 2.5 μs). The designed gun built with the symmetrised side-couplers has surface field on the cathode ranging 50 – 100 MV/m with 1.3 – 1.7 MW klystron-power and 1.2 field ratio (HFSS). ASTRA simulations also indicate that the gun produces the beam with transverse emittances of less than 1 mm-mrad with 10 – 20 pC bunch charge at 500 keV beam energy. Under the gun operating condition, particle tracking/PIC simulations (CST) show that a single-tip CNT field-emitter** produces short pulsed bunches (~ 1/10 RF-cycle) with small emittance ( 0.01 mm-mrad) and high peak current density ( 10,000 kA/cm²). After the gun is fully installed and commissioned, a CNT-tip cathode will be tested with RF-field emission.

The Electron Beam Ion Source particle-in-cell code (EBIS-PIC) tracks ions in an EBIS electron beam while updating electric potential self-consistently and atomic processes by the Monte Carlo method. Recent improvements to the code are reported in this paper. The ionization module has been improved by using experimental ionization energies and shell effects. The acceptance of injected ions and the emittance of extracted ion beam are calculated by extending EBIS-PIC to the beam line transport region. An EBIS-PIC simulation is performed for a Cs charge-breeding experiment at BNL. The charge state distribution agrees well with experiments, and additional simulation results of radial profiles and velocity space distributions of the trapped ions are presented.

Recent experiments on DIII-D have provided the first systematic data on the impact of the current profile on the transport and stability properties of high-performance, steady-state scenario plasmas. In a future tokamak, to achieve 100% noninductive conditions and produce net power, the current profile J must be sustained by a large fraction of bootstrap current JBS, which is nonlinearly coupled with the kinetic profiles. Systematic scans of qmin and q95 were performed to determine empirically the best alignment of the noninductive currents with J and the variation of the transport properties with q. Transport analysis indicates that χe and χi are sensitive to the details of J in a way that makes the pressure profile peaking and JBS scale nonlinearly with both q and β in the experiment. Drift wave stability analysis yields linear growth rates that do not reproduce experimental trends in χ with qmin and q95. At high beta, necessary to maximize fBS, the plasma duration is often limited by n=1 tearing modes, whose stability also depends on the J profile. Broadly deposited electron cyclotron (EC) current at mid-radius was found to supply part of the required noninductive current and to positively affect the tearing stability. The modes appear when JEC is turned off for stable cases and always appear when the EC deposition is shifted outwards. The variation in the EC scan results is consistent with PEST3 calculations, showing that the tearing stability becomes extremely sensitive to small perturbations of the equilibrium in wall-stabilized plasmas run close to the ideal MHD limit. These modeling results are being used to design new experiments with higher ideal and tearing limits. A new capability for off-axis neutral beam injection system will be used to explore higher qmin scenarios and different current alignments.

Significant improvement of plasma confinement in the Madison Symmetric Torus reversed field pinch (RFP) has been routinely achieved by applying an inductive electric field at the plasma boundary in the direction parallel to the equilibrium magnetic field at the plasma edge. An auxiliary edge current is driven by this electric field with the goal of replacing the dynamo-driven current and modifying the parallel current profile to reduce current-driven instabilities. This current-drive technique is called pulsed parallel current drive (PPCD) in RFP. During PPCD plasma fluctuations are reduced everywhere resulting in tokamak-like confinement parameters, while the edge density profile steepens significantly and plasma beta increases. A steep edge plasma pressure profile, a relatively high plasma beta and a strong unfavourable curvature of equilibrium magnetic field near the edge in RFP could excite pressure-driven fluid turbulence near the edge and worsen plasma confinement, opposite to the experimental observations. In this study stability analysis of edge pressure gradient driven ideal modes in standard-like and in PPCD-like plasma equilibria is performed. An ideal magnetohydrodynamic plasma model in cylindrical RFP equilibrium with a step function plasma pressure profile and a vacuum layer between the plasma boundary and the conducting shell is used. Standard-like and PPCD-like plasma equilibria in the model are defined by the direction of the surface current at the plasma–vacuum interface. The results show that while in standard-like equilibrium the edge pressure gradient driven modes are highly unstable in this model, the transition to PPCD-like equilibrium completely stabilizes these modes. The modes stabilization is primarily due to strengthening of magnetic shear at the location of the pressure gradient during the drive and due to the proximity of this location to the conducting wall. This stabilization mechanism is not specific to RFPs, making PPCD a general method of stabilization of the edge pressure gradient driven instabilities which could be applied in other magnetic confinement systems. Application of PPCD to stabilize the edge localized modes in tokamaks is proposed.

Advanced tokamaks (AT) are distinguished from conventional tokamaks by their high degree of shaping, achievement of profiles optimized for high confinement and stability characteristics, and active stabilization of MHD instabilities to attain high values of normalized beta and confinement. These high performance fusion devices thus require accurate regulation of the plasma boundary, internal profiles, pumping, fueling, and heating, as well as simultaneous and well-coordinated MHD control action to stabilize such instabilities as tearing modes and resistive wall modes. Satisfying the simultaneous demands on control accuracy, reliability, and performance for all of these subsystems requires a high degree of integration in both design and operation of the plasma control system in an advanced tokamak. The present work describes the approach, benefits, and progress made in integrated plasma control with application examples drawn from the DIII-D tokamak. The approach includes construction of plasma and system response models, validation of models against operating experiments, design of integrated controllers which operate in concert with one another as well as with supervisory modules, simulation of control action against off-line and actual machine control platforms, and iteration of the design-test loop to optimize performance.