Beam Breakup Instability Research Articles

Propagation of wiggler focused relativistic sheet electron beams

A recent design concept for millimeter-wave free-electron lasers [J. Appl. Phys. 60, 521 (1986)] would require the stable propagation of a sheet electron beam through a narrow waveguide channel. Experimental results reported in this article support the feasibility of such a configuration by demonstrating the stable propagation of relativistic sheet electron beams through a narrow waveguide gap (3.2 mm) using focusing by a short-period electromagnet wiggler. 90% of the electron current in a 100-keV sheet electron beam was transmitted through a 5-cm-long channel with peak wiggler fields of 800 G. Almost 80% of a 400-keV beam was similarly confined with a 1600-G wiggler field. The data were consistent with single electron trajectory models, indicating that space-charge effects were minimal. No evidence of beam breakup or filamentation instabilities was observed.

The effect of accelerating gap geometry on the beam breakup instability in linear induction accelerators

The electron beam in a linear induction accelerator is generally susceptible to growth of the transverse beam breakup instability. In this paper we analyze a new technique for reducing the transverse coupling between the beam and the accelerating cavities, thereby reducing beam breakup growth. The basic idea is that the most worrisome cavity modes can be cutoff by a short section of coaxial transmission line inserted between the cavity structure and the accelerating gap region. We have used the three-dimensional simulation code SOS to analyze this problem. In brief, we find that the technique works, provided that the lowest TE mode cutoff frequency in the coaxial line is greater than the frequency of the most worrisome TM mode of the accelerating cavity.

Laser guiding of electron beams in the advanced test accelerator.

The recently demonstrated technique of laser guiding has been used to propagate successfully a 10-kA relativistic electron beam 95 m through the Advanced Test Accelerator and postaccelerator beamline. The maximum transverse displacement of the beam at the end of the Advanced Test Accelerator was 1 mm while the maximum beam breakup amplitude was 0.1 mm. The use of laser guiding constitutes a breakthrough in accelerator technology in that it is able to reduce greatly or even suppress the beam breakup instability, which is the most serious obstacle to high-current beam transport in linear induction accelerators.

Initial Measurements of Beam Breakup Instability in the Advanced Test Accelerator

This paper reports the measurements of beam breakup (BBU) instability performed on the Advanced Test Accelerator (ATA) up to the end of February, 1984. The main objective was to produce a high current usable electron beam at the ATA output. A well-known instability is BBU which arises from the accelerator cavity modes interacting with the electron beam. The dominant mode is TM/sub 130/ at a frequency of approximately 785 MHz. It couples most strongly to the beam motion and has been observed to grow in the Experimental Test Accelerator (ETA) which has only eight accelerator cavities. ATA has one hundred and seventy cavities and, therefore, the growth of BBU is expected to be more severe. In this paper, BBU measurements are reported for ATA with beam currents of 4 to 7 kA. Analysis showed that the growth of the instability with propagation distance was as expected for the lower currents. However, the high-current data showed an apparent higher growth rate than expected. An explanation for this anomaly is given in terms of a ''corkscrew'' excitation. The injector BBU noise level for a field emission brush cathode was found to be an order of magnitude lower than for a cold plasma more » discharge cathode. These injector rf amplitudes agree very well with values obtained using the method of differenced B sub solar loops. « less

Beam Breakup Instability in RF Linacs

The Beam Breakup (BBU) instability appears to be the principal limitation to high current operation of RF linacs. This instability results in the loss of part or all of a beam bunch above a certain current or beyond a certain number of bunches. Even when the bunch is not lost it may be deflected sufficiently far off its axis to become useless for its intended application. This paper presents analytical and numerical calculations of the BBU instability in RF linac cavities which should be useful in scaling RF linacs to high current operation. Analytical calculations for single bunch BBU in pillbox cavities show that the beam deflection scales with the initial deflection, the charge per bunch, the accelerating gradient, the accelerator length, and the cavity wavelength in a manner that can be generalized to include the effects of periodic focusing on transverse emittance growth. Numerical simulations of multibunch BBU show the growth of beam deflection amplitude from wake fields generated by preceding bunches. Wake field calculations using a transverse beam cavity interaction code are presented.

Beam Breakup Instabilities in High Current Electron Beam Racetrack Induction Accelerators

Beam breakup and negative mass instability growth rates for a 1 kA, 40 MeV electron beam racetrack induction accelerator are computed. The device is taken to have four acceleration gaps, each with 0.2 MeV applied voltage and 15 ohm transverse impedence; the guide field is 2 kg. We find that the total amplification of the beam breakup mode is limited to five e-foldings provided that the cavity mode quality factor Q is 6. Thus, the negative mass instability, which grows several times faster, is the dominant consideration. However, we also find that the energy range over which the negative mass instability occurs can be narrowed substantially by reducing the guide field strength after the beam has been accelerated to about 12 MeV. This approach, coupled with beam thermal effects, not considered here, probably is sufficient to limit negative mass growth to acceptable levels in the racetrack accelerator.

Beam Breakup (BBU) Instability Experiments on the Experimental Test Accelerator (ETA) and Predictions for the Advanced Test Accelerator (ATA)

In linear accelerators the maximum achievable beam current is often limited by the Beam Breakup (BBU) instability. This instability arises from the interaction of a transversely displaced beam with the dipole modes of the acceleration cavities. The modes of interest have non-zero transverse magnetic fields at the center of the cavity. This oscillating field imparts a time varying transverse impulse to the beam as it passes through the accelerating gap. Of the various modes possible only the TM/sub 130/ mode has been observed on the Experimental Test Accelerator (ETA) and it is expected to surface on the Advanced Test Accelerator (ATA). The amplitude of the instability depends sensitively on two cavity parameters; Q and Z/sub perpendicular//Q. Q is the well-known qualtiy factor which characterizes the damping rate of an oscillator. Z/sub perpendicular//Q is a measure of how well the beam couples to the cavity fields of the mode and in turn, how the fields act back on the beam. Lowering the values of both these parameters reduces BBU growth.

Alternate Transport Techniques for Electron Induction Linacs

Alternate focusing methods will have significant cost and design advantages if they are effective. In this paper the authors outline results and prospects for several of these techniques. The most obvious of the alternate techniques is to use gas cells between accelerating gaps. The pinch force acts to confine the beam. The use of gas cells has also been proposed to damp oscillations due to the beam breakup instability. Experimental results on a particular facet of this problem-extraction from a magnetic region into a gas cell -- are discussed. Results from the first experimental demonstration of the 'image' or 'foil' focusing technique are presented.

The Beam Breakup Instability in Pulsed Transmission Line Linear Accelerators

The transverse Beam Breakup Instability (BBI) can result in reduced pulse length in linear accelerators. A brief summary of the instability mechanism and a gain formula are presented. Test bench measurements of the two relevant parameters, transverse shunt impedance Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , and cavity Q have been made. These measurements illustrate the importance of transmission line dielectric and cavity geometry to the gain. Results indicate that Q may vary from 10 - 75, and that Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> /Q is in the range 4 - 70 Ω.

Beam Transport Issues in High Current Linear Accelerators

Stable beam transport may be the limiting factor in the development of a new generation of high current linear induction accelerators. In this paper we analyze several important beam stability topics, including radial oscillations induced by an accelerating gap, the diocotron, resistive wall, and cyclotron maser instabilities, and the transverse beam breakup and image displacement instabilities. At present image displacement appears to represent the most serious limitation to high current beam transport in linear accelerator structures.

M=1Cyclotron-Mode Klystron Instability

The transverse beam breakup instability, for a uniform axial focusing magnetic field, has been investigated utilizing the approach of the well-known klystron instability. An expression for the gain is obtained. It is proportional to the beam current and the cavity equivalent shunt resistivity at the radius of the beam surface and inversely proportional to the cyclotron frequency and the cavity frequency, resulting in instability at high beam current, in contrast to the standard klystron instability which occurs at low currents and is self-stabilized at high beam currents.