Amy Regan

Total Lightning Observations with the New and Improved Los Alamos Sferic Array (LASA)

Abstract Since 1998, Los Alamos National Laboratory (LANL) has deployed an array of fast electric field change sensors in New Mexico and Florida in support of LANL’s satellite lightning observations. In April 2004, all the sensors were significantly upgraded and improved, and a new array was deployed in north-central Florida. This paper describes the operations of the new array and reports the first 12 months of lightning observations. The new array is about 10 times more sensitive than the previous one and can capture millions of discharge events during a stormy day in Florida. In this paper, the array’s lightning location accuracy, minimum detectable peak current, and ratio of intracloud-to-cloud-to-ground flashes are analyzed. Some case studies that illustrate the storm evolution, lightning classification, and radar comparisons are presented. A new three-dimensional capability of the array is demonstrated.

Katrina and Rita were lit up with lightning

Hurricanes generally produce very little lightning activity compared to other noncyclonic storms, and lightning is especially sparse in the eye wall and inner regions within tens of kilometers surrounding the eye [Molinari et al., 1994, 1999]. (The eye wall is the wall of clouds that encircles the eye of the hurricane.) Lightning can sometimes be detected in the outer, spiral rainbands, but the lightning occurrence rate varies significantly from hurricane to hurricane as well as within an individual hurricanes lifetime. Hurricanes Katrina and Rita hit the U.S. Gulf coasts of Louisiana, Mississippi, and Texas, and their distinctions were not just limited to their tremendous intensity and damage caused. They also differed from typical hurricanes in their lightning production rate.

The SNS linac RF control system

Development of the Spallation Neutron Source (SNS) continues, with operations beginning in 2004. The SNS is a 1 GeV machine consisting of a combination normal-conducting and super-conducting linac as well as a ring and target area. The RF control system (RFCS) for the linac has been under development for two years, with the first versions of the individual modules under construction and system integration begun. This paper will discuss this stage of the design and its implementation. In addition, it will report on the module variations required to use similar hardware for both the normal conducting (NC) and superconducting (SRF) portions of the linac.

The Los Alamos VXI-based modular RF control system

This paper describes the design and implementation of the Los Alamos modular RF control system, which provides high-performance feedback and/or feedforward control of RF accelerator cavities. This is a flexible, modular control system that has been realized in the industry-standard VXI card-modular format. A wide spectrum of system functionality can be accommodated simply by incorporating only those modules and features required for a particular application. The fundamental principles of the design approach are discussed. Details of the VXI implementation are given, including the system architecture and interfaces, performance capabilities, and available features.<<ETX>>

Frequency shift observer for an SNS superconducting RF cavity

In contrast to a normal conducting RF cavity, a superconducting RF cavity is very susceptible to shifts in its resonance frequency. The main sources of the shift are Lorentz force detuning and microphonics. In a spallation neutron source, to compensate for the frequency shift, a feedforward control is to be applied. In this paper, as an initiative step, a frequency shift observer is proposed which is simple enough to be implemented with a digital signal processor in real time. Simulation results of the proposed frequency shift observer show reliable performance and acceptable computation time for the real time implementation.

SNS Superconducting RF cavity modeling-iterative learning control

The Spallation Neutron Source (SNS) Superconducting RF (SRF) linear accelerator is operated with a pulsed beam. For the SRF control system to track the repetitive electromagnetic field reference trajectory, both feedback and feedforward controllers have been proposed. The feedback controller is utilized to guarantee the closed loop system stability and the feedforward controller is used to improve the tracking performance for the repetitive reference trajectory and to suppress repetitive disturbances. As the iteration number increases, the feedforward controller decreases the tracking error. Numerical simulations demonstrate that inclusion of the feedforward controller significantly improves the control system performance over its performance with just the feedback controller.

APT LLRF control system model results

The low-level RF (LLRF) control system is an essential component of the RF system for the Accelerator Production of Tritium (APT). Requisite for good performance at a reasonable cost is system modeling prior to actual hardware build. Models have been created to help establish the LLRF control system baseline design. These models incorporate common signal processing functions and control functions as well as mixed continuous and discrete-time analysis. Components include klystron saturation curves, waveguide delays, realistic resonant cavity equivalents, and LLRF proportional, integral, and differential (PID) control transfer functions. They predict the performance of the LLRF system in the presence of beam noise, excitation of non-fundamental modes which occur in the superconducting cavities, and pulsed beam situations. This paper describes the basic model and presents results for a variety of operating scenarios.

Uncertain system modeling of SNS RF control system

This paper addresses the modeling problem of the linear accelerator RF system for SNS. The cascade of the klystron and the cavity is modeled as a nominal system. In the real world, high voltage power supply ripple, Lorentz force detuning, microphonics, cavity RF parameter perturbations, distortions in RF components, and loop time delay imperfection exist inevitably, which must be analyzed. The analysis is based on the accurate modeling of the disturbances and uncertainties. In this paper, modern control theory is applied for modeling the disturbances, uncertainties, and for analyzing the closed loop system robust performance.

Commissioning of the Low Energy Demonstration Accelerator (LEDA) radiofrequency quadrupole (RFQ)

Initial commissioning of a 6.7-MeV 100-mA RFQ is underway. The RFQ is part of LEDA, the He injector for the Accelerator Production of Tritium (APT) project. To benchmark the RFQ performance, beam physics experiments will be done with low and high current beams for both pulsed and cw beam operation. Commissioning efforts thus far have been limited to low-current pulsed beam LEDA operation. Measurements to fully characterize the RFQ will ultimately include the dependence of RFQ beam transmission on RFQ vane voltage, input beam energy, input match, and input transverse centroids. Other commissioning measurements for the RFQ will include output beam energy, phase, noise, transverse profiles, and transverse RMS emittances. This paper contains initial LEDA RFQ commissioning results, including RFQ pulsed output beam currents up to 40 mA.

Results of adaptive feedforward on GTA

This paper presents the results of the adaptive feedforward system in use on the Ground Test Accelerator (GTA). The adaptive feedforward system was shown to correct repetitive, high-frequency errors in the amplitude and phase of the RF field of the pulsed accelerator. The adaptive feedforward system was designed as an augmentation to the RF field feedback control system and was able to extend the closed-loop bandwidth and disturbance rejection by a factor of ten. Within a second implementation, the adaptive feedforward hardware was implemented in place of the feedback control system and was shown to negate both beam transients and phase droop in the klystron amplifier.<<ETX>>