H. A. Hogg

Superconducting Niobium Cavity Measurements at SLAC

The program of measurements at SLAC on superconducting niobium cavities is described. Results for TE and TM mode X-band cavities are presented. An RF magnetic breakdown field of 960 gauss and Q values greater than 1011 were measured for an electron beam welded TE011 mode cavity at 10.5 GHz. The best result for a TM mode cavity was a Q of 1.4 × 109 and a breakdown field of 360 gauss, limited probably by the quality of the electron beam weld. Data on the effect of exposure to air and to nitrogen are presented.

Recent Progress on Sled, The SLAC Energy Doubler

From the following description, it will be seen that SLED raises the accelerator peak energy by 40% if the present 2.7 1s RF pulse length is used. The energy increase ” is 80% if the pulse length is extended to 5 ps. TO do this, however, changes have to be made in the modulators and trigger system, and the maximum repetition rate has to be halved to maintain the present average power level. In addition, more extensive switchyard modifications are required for~handling the higher energy beams. For these reasons, the SLED system will be initially installed and run at the present 2.7 ns pulse length. Performance at both pulse lengths is discussed in this paper.

Microwave Beam Position Monitors at SLAC

The design and performance of three types of nonintercepting microwave beam position monitors are described. The monitors locate the transverse centroid of the bunched beam. Monitors of the first two types each consist of two orthogonal position-sensing cavities and one reference cavity. The microwave signals induced by the beam in these cavities are combined in two phase bridges and linearly detected. The video signals are normalized with respect to beam current and transmitted to two control buildings where they are displayed. Thirty-three monitors along the linear accelerator have 0.8-inch apertures, and six special monitors at the end of the machine have 2.0-inch apertures. A third type of monitor has been installed in one experimental area. Waveglide couplers on a 3-inch diameter beam drift tube feed signals to a phase bridge. The bridge sum and difference signals are amplified, detected, and differentially displayed on an oscilloscope. The device can detect 0.004-inch changes in position of a 0.01 mA beam with a response time of 50 ns.

Recent Measurements at SLAC on Superconducting Niobium X-Band Cavities

Measurements at 8.6 GHz on TM mode superconducting niobium cavities have been carried out at SLAC in an attempt to establish definitive conditions for reproducibly attaining high peak electric and magnetic fields and high residual Qs. Four cavities, processed by techniques which insure the presence of an oxide layer on the niobium surface before final high temperature outgassing, have given peak magnetic fields exceeding 1000 G and corresponding peak electric fields in excess of 56 MV/m. From this and related experience it is speculated that, in order to achieve high peak fields, carbon present on the niobium surface must be removed through the formation of volatile compounds with oxygen or fluorine during high temperature processing. Data are also presented on the effect on rf properties of exposure at room temperature to various gases. Measurements on anodized cavities are briefly discussed.

Recent Beam Performance and Developments at SLAC

The purpose of this paper is to describe recent developments at SLAC which have contributed to improvements in beam operation. The paper will be divided into two parts. The first will summarize overall beam performance and operational efficiency in delivering beams to various experiments. The second will be devoted to specific developments such as the achievement of higher energies, increasingly narrow energy spectra, higher beam breakup current thresholds, chopped beams and improvements in pulse-to-pulse operation. The discussion will include a description of various new pulsed devices such as pulsed quadrupoles, improvements in the positron source and new beam loading measurements obtained for very short pulses.

Feasibility Study of a Two-Mile Superconducting Linac

Assuming that investigations now underway will result in superconducting accelerator structures capable of withstanding gradients of 33 MeV/m, a feasibility study of a two-mile 100 GeV superconducting electron linac with 6% duty cycle has been made at frequencies of 1428 and 2856 MHz. Tentative machine parameters and the preliminary design of components and systems have been examined. These studies are based upon a traveling-wave accelerator structure with RF feedback in each 20-ft section.

Experiments with Very High Power RF Pulses at SLAC

Experiments in which the powers of two SLAC klystrons were combined and fed into a resonant cavity pulse-compression system (SLED) are described. Pulse powers up to 65 MW into SLED were reached. The corresponding instantaneous peak power out of SLED was 390 MW. After normal initial aging, no persistent RF breakdown problems were encountered. X-radiation at the SLED cavities was generally less than 400 mR/hr after aging. The theoretical relationship between x-radiation intensity and RF electric field strength is discussed.

Microwave Developments at SLAC

Three microwave development projects presently undertaken by the Accelerator Physics Department at SLAC are reviewed. The first is SLED (SLAC Energy Development). Some theoretical work is mentioned, and a few details of installation and control are given. The latest beam-test results are presented. Microwave beam-position monitors are the second project to be discussed. Details of two very sensitive systems which have been developed and tested are given. Finally, the use of a fast digital phase-shifter to compensate for beam-loading transients is described.

Transient High-Field Behavior of Niobium Superconducting Cavities

Tests have been performed on the breakdown behavior of a TM010 mode, S-band niobium cavity at low temperatures. Unloaded Qs of 9 × 107 at 4.2 K and of 7 × 109 at 1.35 K were measured during several tests performed using pulses long enough for the cavity to reach steady state. The breakdown field at 1.35 K was increased from 15 to 20 MV/m by processing the cavity at room temperature using 1 MW, 2.5 ps pulses. The response of the cavity at 4.2 K to 1 MW, 2.5¿s pulses was also tested in several cool-downs. In these tests the cavity was heavily overcoupled to lower its time constant to a value of 0.80 timnes the RF pulse length of 2.5 ¿s. This condition maximizes the energy transfer from the klystron source to the cavity. Measurements made during these experiments clearly indicated that fields of about 50 MV/m were being reached in the cavity without breakdown.

Improving the Phase Stability of the SLAC RF Driveline Network for SLC Operation

Successful operation of the Stanford Linear Collider (SLC) will require greater phase stability from the two-mile long RF drive network than previous linac operation did. This paper discusses four proposed modifications of the present system that should help achieve the general objective to reduce all long term temperature and atmospheric pressure induced phase variations to less than 20° at 2856 MHz, so that the phase/amplitude detector subsystems, which will control the network output phases relative to a beam reference, will operate within their most accurate ranges.