Daniel E. Rees

High power RF conditioning of the LEDA RFQ

We are preparing the radio frequency quadrupole (RFQ) for the Low Energy Demonstration Accelerator (LEDA) to accelerate beam. The LEDA RFQ accelerates a 100-mA CW proton beam from 75 keV to 6.7 MeV. We report our experience with high-power RF conditioning the RFQ, first with one klystron and then with two klystrons. The RFQ will dissipate 1.2 megawatts of RF power at design fields. This 350-MHz CW RFQ has peak fields on the vane tips of 33 MV/m. The average power dissipation is 13 watts/cm/sup 2/ on the outer walls of the RFQ near the high energy end. The power from each klystron is split 4 ways to lower the stress on the RF windows. Each klystron can produce 1.3 megawatts of RF power.

Design of 250-MW CW RF system for APT

The design for the RF systems for the APT (Accelerator Production of Tritium) proton linac is presented. The linac produces a continuous beam power of 130 MW at 1300 MeV with the installed capability to produce up to a 170 MW beam at 1700 MeV. The linac is comprised of a 350 MHz RFQ to 7 MeV followed in sequence by a 700 MHz coupled-cavity drift tube linac, coupled-cavity linac, and superconducting (SC) linac to 1700 MeV. At the 1700 MeV, 100 mA level the linac requires 213 MW of continuous-wave (CW) RF power. This power will be supplied by klystrons with a nominal output power of 1.0 MW. 237 klystrons are required with all but three of these klystrons operating at 700 MHz. The klystron count includes redundancy provisions that are described which allow the RF systems to meet an operational availability in excess of 95 percent. The approach to achieve this redundancy is presented for both the normal conducting (NC) and SC accelerators. Because of the large amount of CW RF power required for the APT linac, efficiency is very important to minimize operating cost. Operation and the RF system design, including in-progress advanced technology developments which improve efficiency, are discussed. RF system performance is also predicted. Because of the simultaneous pressures to increase RF system reliability, reduce tunnel envelope, and minimize RF system cost, the design of the RF vacuum windows has become an important issue. The power from a klystron will be divided into four equal parts to minimize the stress on the RF vacuum windows. Even with this reduction, the RF power level at the window is at the upper boundary of the power levels employed at other CW accelerator facilities. The design of a 350 MHz, coaxial vacuum window is presented as well as test results and high power conditioning profiles. The transmission of 950 kW, CW, power through this window has been demonstrated with only minimal high power conditioning.

Calculation of beam loading using the induced-current method in passive cavities

A method for calculating the loaded Q and beam detuning of a passive (gain or idler) cavity in an electron device is presented. It is based on the determination of induced current by a technique often referred to as Ramos theorem. The induced current is used in conjunction with a lumped equivalent circuit representing the cavity. This leads to a solution for the self-consistent cavity fields. Although the induced-current expression is usually developed from low-frequency models, it is shown that Ramos theorem is valid for high-frequency, steady-state analysis when the unloaded passive cavity Q is high. The method is used to calculate the loaded Q of the passive cavity of a new type of gyro-resonant electron device, the magnicon. >

The SNS linac high power RF system design, status, and results

The Spallation Neutron Source being built at the Oak Ridge National Lab in Tennessee requires a 1 GeV proton linac. Los Alamos has responsibility for the RF systems for the entire linac. The linac requires 3 distinct types of RF systems: 2.5-MW peak, 402.5 MHz, RF systems for the RFQ and DTL (7 systems total); 5-MW peak, 805 MHz systems for the CCL and the two energy corrector cavities (6 systems total); and 550-kW peak, 805 MHz systems for the superconducting sections (81 systems total). The design of the SNS Linac RF system was presented at the 2001 Particle Accelerator Conference in Chicago. Vendors have been selected for the klystrons (3 different vendors), circulators (1 vendor), transmitter (1 vendor), and high power RF loads (3 different vendors). This paper presents the results and status of vendor procurements, test results of the major components of the Linac RF system and our installation progress.

An overview of the Low Energy Demonstration Accelerator (LEDA) project RF systems

Successful operation of the Accelerator Production of Tritium (APT) plant will require that accelerator downtime be kept to an absolute minimum. Over 230 separate 1 MW RF systems are expected to be used in the APT plant, making the efficiency and reliability of these systems two of the most critical factors in plant operation. The Low Energy Demonstration Accelerator (LEDA) being constructed at Los Alamos National Laboratory will serve as the prototype for APT. The design of the RF systems used in LEDA has been driven by the need for high efficiency and extremely high system reliability. We present details of the high voltage power supply and transmitter systems as well as detailed descriptions of the waveguide layout between the klystrons and the accelerating cavities. The first stage of LEDA operations will use four 1.2 MW klystrons to test the RFQ and supply power to one test stand. The RFQ will serve as a power combiner for multiple RF systems. We present some of the unique challenges expected in the use of this concept.

Techniques for maintaining design efficiency when operating klystron amplifiers at levels below the maximum output power

Some accelerator designs require that the amount of RF power supplied to the accelerating cavities be varied along the accelerator length. In order to minimize operating expense for a high-power, CW system, it is also desirable to maximize RF-generator efficiency. Klystrons are the traditional source used for RF-accelerator applications, but a high-efficiency klystron design is optimized to provide high efficiency at only one operating point, usually at saturation. Different klystron designs could be used for different power levels to maintain high efficiency; however, this approach would increase the capital costs of the accelerator. We discuss several methods of varying the output power of a klystron and show semi-empirically a way to preserve klystron efficiency. We derive two semi-empirical methods to predict the variations in power and efficiency under nonoptimum operating conditions. Experimental data from a 1.25 MW klystron are also shown. These data support our hypothesis and our models. >

Accelerator production of tritium 700 MHz and 350 MHz klystron test results

The Accelerator Production of Tritium project (APT) utilizes a 1,700 MeV, 100 mA proton Linac. The radio frequency (RF) power is provided by 244 continuous wave (CW) klystron amplifiers at 350 MHz and 700 MHz. All but three of the klystrons operate at a frequency of 700 MHz. The 350 MHz klystrons have a nominal output power of 1.2 MW at a DC-to-RF conversion efficiency of 65%. They are modulating-anode klystrons and operate at a beam voltage and current of 95 kV and 20 A. The design is based on the CERN klystron. The 700 MHz klystron is a new development for APT. Three 700 MHz klystrons are currently under development. Two vendors are each developing a baseline klystron that has a nominal output power of 1.0 MW at a DC-to-RF conversion efficiency of 65%. A 700 MHz klystron is also under development that promises to provide an efficiency in excess of 70%. The 700 MHz klystrons operate at a maximum beam voltage of 95 kV and a maximum beam current of 17 A. The test results of these klystrons will be presented and the design features will be discussed.

Lansce RF System Refurbishment

The Los Alamos Neutron Science Center (LANSCE) is in the planning phase of a refurbishment project that will sustain reliable facility operations well into the next decade. The linear accelerator was constructed in the late 1960s and commissioned as the Los Alamos Meson Physics Facility (LAMPF) in 1972. As the mission changed, LANSCE became a national user facility that provides pulsed protons and spallation neutrons for defense and civilian research and applications. The upgrade will replace all of the 201.25 MHz RF systems and a substantial fraction of the 805 MHz RF systems and high voltage systems. This paper will provide the design details of the new RF and high voltage systems.

High-power RF operations studies with the CRITS RFQ

High-current, cw linear accelerators have been proposed as spallation neutron source drivers for applications to tritium production, transmutation of nuclear waste, and safe nuclear power generation. Key features of these accelerators are high current (100 mA) low emittance-growth beam propagation, cw or high duty-factor operation, high efficiency, and minimal maintenance downtime. A 267.1 MHz, cw RFQ and klystrode based RF system were obtained from CRL and installed at LANL to support these next generation accelerator studies. The reconditioning of the RFQ accelerator section to its design power of 150 kW at 100% duty factor is being accomplished with studies focusing on the details of high-power RF structure operation, personnel and equipment safety systems integration, and RF control integration.

LANSCE 201 MHZ and 805 MHZ RF System experience

The LANSCE RF system consists of four RF stations at 201 MHz and forty-four klystrons at 805 MHz. In the LANSCE accelerator, the beam source is injected into the RF system at 0.75 MeV. The beam is then accelerated to 100 MeV in four drift tube linac (DTL) tanks, driven at 201.25 MHz. Each 201 MHz RF system consists of a train of amplifiers, including a solid state amplifier, a tetrode, and a triode. After the DTL, the beam is accelerated from 100 MeV to 800 MeV in the forty-four coupled cavity linac (CCL) tanks at 805 MHz. The machine operates with a normal RF pulse width of 835 microseconds at a repetition rate up to 120 Hz, and sometimes operates with a pulse width up to 1.2 milliseconds at 1 Hz. This RF system has been operating for about 37 years. This paper summarizes the recent operational experience. The reliability of the 805 MHz and 201 MHz RF systems is discussed, and a summary the lifetime data of the 805 MHz klystrons and 201 MHz triodes is presented.