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Ground Test Accelerator [GTA]

The Ground Test Accelerator [GTA] Project was a 100-MeV, 100-mA pulsed-ion-beam accelerator for the Neutral Particle Beam Program of the Strategic Defense Initiative. The 40-foot-long Ground Test Accelerator was a ground based high power accelerator program used as a test bed for space based strategic defense initiative concepts where Northrop Grumman was selected as the accelerator industrial support contractor. The Ground Test Accelerator incorporated new concepts of accelerator technology developed by Los Alamos National Laboratory, AT-Division. The modulator is driven on/off by RF switching power supply to achieve 1-MW RF accelerator energy from the klystron and klystrode at 850 MHz. The advantages are: (1) system cost savings up to 70%, (2) operating cost savings up to 60%, (3) weight reduction from 24000 lbs to 700 lbs, (4) size reduction from 512 to 12 ft3 and (5) efficiency improved from 75% to 97%.

The Ground Test Accelerator RF system consists of a low-level RF (LLRF) control system that uses a tetrode as a high-power amplifier (HPA) as part of its plant to deliver up to 300 kW of peak power to the RFQ at a 2 percent duty factor. The LLRF control system implements in-phase and quadrature (I&Q) control to maintain the cavity field within tolerances of 0.5 percent in amplitude and 0.5 degrees in phase in the presence of beam-induced instabilities.

The Ground Test Accelerator (GTA) had the objective of verifying much of the technology (physics and engineering) required for producing high-brightness, high-current H{minus} beams.GTA commissioning is staged to verify the beam dynamics design of each major accelerator component as it is brought on-line. The commissioning stages were the 35 key H{minus} injector, the 2.5 MeV Radio Frequency Quadrupole (RFQ), the Intertank Matching Section (IMS), the 3.2 MeV first 2ß? Drift Tube Linac (DTL-1) module, the 8.7 MeV 2ß? DTL (modules 1--5), and the 24 MeV GTA; all 10 DTL modules.

The Intertank Matching Section (IMS) of the Ground Test Accelerator (GTA) contains four variable-field quadrupoles (VFQs) and is designed to match beam exiting the RadioFrequency Quadrupole to the first tank of the Drift-tube LINAC (DTL-1). By varying the VFQ field strengths to create a range of beam mismatches at the entrance to DTL-1, one can test the sensitivity of the DTL-1 output beam to variations in the DTL-1 input beam. Experimental studies made during commissioning of the GTA indicate an unexpected result: the beam exiting DTL-1 shows little variation for a range of mismatches produced at the entrance. Initial commissioning tests on the first Drift-Tube Linac section (DTL-1) of the accelerator produced a beam with an energy of 3.2 MeV.

The ground test accelerator program at Los Alamos National Laboratory utilizes a radio frequency quadrupole (RFQ) through which a particle beam passes. A large quantity of RF power is added to this RF cavity, which is operated at cryogenic temperatures (20-50 K.). It is desirable to fabricate the RFQ of aluminum having a thin coating of copper. However, the coefficient of thermal expansion of aluminum is different from that of copper, so that cooling an aluminum article having a copper coating from room temperature to a cryogenic temperature and back to room temperature promotes delamination at the aluminum/copper interface. Aluminum articles having dimensions similar to those of components of the RFQ were coated with thin layers of copper by prior art electrodeposition processes and subjected to cycling between room temperature and the temperature of liquid helium (35 K.). It was observed that the copper layers separated from the aluminum substrates. Bubbles formed on the surface of the copper and the copper layer detached from the aluminum at the edges of the articles, even though no mechanical stresses were applied to the coated test articles. Thus, it was necessary to develop a process which provides better adherence between a substrate and a layer of electrodeposited copper. Articles which were copper coated using the process did not delaminate when subjected to cycling between room temperature and 20 K. and scratch and peel tests did not result in separation of the copper layer from the substrate.

There are many applications where a phase-stable RF signal must be transported some distance from the signal source to a load. For example, in the Ground Test Accelerator (GTA) signals must be transported between a RF master reference generation oscillator and each individual control rack (RF Reference Transport) as well as between RF cavities and the associated control racks located away from the cavities (Field Sense Transport), where phase errors contribute directly to errors in cavity-to-cavity phase of the accelerator. The GTA system requires an insertion phase tolerance of .+-.0.15 degrees for each transport cable. This system is a fixed frequency system and frequency-dependent effects are not of concern. Temperature changes, however, affect the electrical length and transmission delay time of the coaxial cable, with concomitant changes in the signal insertion phase. For example, over a 250-foot run of cable, an ambient temperature range of 23.degree. to 43.degree. C. can introduce as much as 8 degrees of electrical phase change at 425 MHz in phase stabilized coaxial cable, more than 50 times the tolerable limit for GTA.

The Accelerator Technology (AT) Division continued in fiscal year 1990 to fulfill its mission of developing accelerator science and technology for application to research, defense, energy, and other problems of national interest. Highlights for the year included successful operation of the first cryogenically cooled radiofrequency quadrupole (RFQ). The RFQ accelerated a beam of negative hydrogen ions to the design energy of 2.5 MeV with high transmission and beam quality. This is the first acceleration stage for the Ground Test Accelerator (GTA), being developed for the Neutral Particle Beam (NPB) program sponsored by Strategic Defense initiative Organization (SDIO) and the US Army Strategic Defense Command.

From 1992 throught 1994, the Ground Test Accelerator (GTA) used a variety of off- and on-line beam diagnostic measurements to understand and verify the transverse and longitudinal phase space characteristics of a 35-mA, low-energy (2.5- to 3.2-MeV) H--beam. For the transverse phase-space characterization measurements, a slit and collector device samples of the x-x and y-y phase space, to determine the transverse emittance and Courant-Snyder parameters. The longitudinal phase-space data were acquired by a laser neutralization technique developed at Los Alamos know as the laser induced neutralization diagnostics approach (LINDA). The transverse and longitudinal phase-space centroids of the low-energy, 425-MHz-bunched beam were directly measured using the microstrip probe systems. Beam current and transmission are measured by various toroid systems. Beam-loss-detection techniques were installed and a non-interceptive beam-profile measurement was commissioned. All of these measurement systems had their share of successes and challenges. For example, while the microstrip-system's energy, phase, and intensity measurements operated successfully, their beam-position-measurement calibrations did not agree with either wire calibration data or measured slit-and-collector beam-centroid data due to perturbations to the beam's image-current distributions and low- effects of the probe's position-detection sensitivity.

The Indiana University Cyclotron Facility obtained the prototype ground test accelerator from Los Alamos National Laboratory along with a number of klystrons, tubes that will be the source of RF power used by the accelerator. The year 2004 saw the construction of the first phase of the Low Energy Neutron Source. Although it cost millions of dollars to make almost two decades earlier, the device was given to the cyclotron free of charge.