Studies of Space-Charge Dominated Beams at the University of Maryland

M. Reiser    mreiser@glue.umd.edu
3mm J. G. Wang    jgwang@plasma.umd.edu
Institute for Plasma Research
Univ. of Maryland
College Park, MD 20742, USA3mm

1 Introduction\

An active research program has been conducted in the Institute for Plasma Research, University of Maryland, to study the physics of space-charge dominated beams for advanced accelerator applications. We have been employing low energy (up to 10 keV), high perveance (up to 1.4 AV) electron beams in experiments to investigate the space-charge effects in beams. The main facility has been a 5-m long transport channel focused by 36 short solenoids. \

In the past our theoretical investigations of the collective effects in high current beams included the thermodynamic beam model, the emittance growth due to free energy (beam mismatch, etc.), the longitudinal instability, the image effect and geometry factor in bunched beams, etc. [1-7]\

In experiments on the transverse beam dynamics we launched a five-beamlet configuration and studied the emittance growth and halo formation due to charge homogenization and rms mismatch. [8-10] Comparison of the measurements with theoretical predictions and computer simulations showed excellent agreement. \

The longitudinal beam dynamics experiments involved pulse compression and the propagation of space-charge waves. We investigated the longitudinal particle distribution in parabolic and rectangular bunches, the reconstruction of an eroded rectangular bunch, the generation of single (fast or slow) space-charge waves, the geometry factor due to perturbation in space-charge dominated beams, and the reflection of space-charge waves at bunched beam ends. [11-17]

2 On-going experimental programs\

There are three on-going experimental programs, namely, the experiment on the resistive-wall instability, the experiment on longitudinal energy spread, and the experiment on bending space-charge dominated beams. \

The resistive-wall instability experiment is performed in a transport channel made of a glass tube with resistive coating and a co-axial long solenoid for transverse focusing. Electron beams with localized perturbations are generated in a gridded electron gun and transported through the channel. The experiment has clearly demonstrated the growth of slow waves and the decay of fast waves. The growth/decay rates are measured in the long wavelength limit and are compared with theory. Good agreement has been found. [18,19] The experiment is still in progress to study the beam quality deterioration due to the instability; this entails the measurement of the longitudinal beam energy spread and transverse emittance associated with the perturbed particle distribution in both the fast and slow waves.\

The experiment on longitudinal energy spread has been performed to study the energy distribution along parabolic and rectangular bunches, and within localized space-charge waves. All of these measurements agree with theory. In addition, we are studying the longitudinal energy spread of a beam generated in a thermionic electron gun. Especially, we would like to see if and how relaxation of the temperature anisotropy () via Coulomb collisions (the Boersch effect) and collective mechanisms plays a role in increasing the energy spread. This research is in progress.\

In progress is also a new experiment to study the beam transport in a printed-circuit-quadruple channel with bending elements. This research is not only an essential part of our electron ring project described in the next section, but also an independent beam physics study. The goal of the experiment is to investigate the possible energy spread and emittance growth in bending of space-charge dominated beams. [20]

3 Design and development of an electron recirculator\

A major effort of our current research is the design and development of a small recirculator for the transport of a low-energy, highly space-charge dominated electron beam. [21] The ring will also provide a longer path for the study of beam dynamics than is available in our existing 5-m long transport channel. The major issues to be addressed with the ring include beam transport of currents considerably higher than the conventional tune-shift limit by fast resonance transversal in a small number of turns (100), equilibrium bunch profiles (parabolic, rectangular with ``ears''), bending and dispersion of space-charge dominated beams, behavior of localized space charge waves including bunch-end effects, longitudinal instability with induction modules, etc. An important goal of the electron ring experiments will be checking with theory and computer simulation codes.\

Figure shows the ring layout. A 10 keV, 100 mA, 30-75 ns electron beam from an electron gun will be injected into the ring (after transverse/longitudinal matching) with the aid of a pulsed, Panofsky-type quadruple system. The ring design is based on a ``wound-up'' linear FODO channel. Transverse focusing will be provided by 70 printed-circuit quadruples (plus two Panofsky quads of special design). Three induction/rf gaps will be used for longitudinal focusing. A pulsed extraction system similar to the injector will be included for beam analysis. Other diagnostics along the ring include fast beam position/current monitors, energy analyzers, and phosphor screen images. The ring main parameters are summarized in Table 1. The design of the ring is being developed with the collaboration of T. Godlove (FMT) and R. York (Michigan State University).

 
Figure 3.1: General layout of the electron recirculator 

The University of Maryland proposed electron ring represents an exciting opportunity to study the transport of highly space-charge dominated beams. Potential applications are for circular accelerators requiring intense pulses. These include high-energy physics booster synchrotrons, muon colliders, spallation sources, drivers for heavy-ion inertial fusion and free electron lasers.

References\

1.
M. Reiser, Theory and Design of Charged Particle Beams, John Wiley & Sons, Inc., New York, 1994.

2.
M. Reiser, ``Free Energy and Emittance Growth in Nonstationary Charged Particle Beams'', J. Appl. Phys. 70, 1919 (1991).

3.
M. Reiser and N. Brown, ``Thermal Distribution of Relativistic Particle Beams with Space Charge'', Phys. Rev. Lett., 71 (18), 2911 (1993) .

4.
M. Reiser and N. Brown, ``Proposed High-Current rf Linear Accelerators with Beams in Thermal Equilibrium'', Phys. Rev. Lett., 74 (7), 1111 (1995).

5.
N. Brown and M. Reiser, ``Thermal Equilibrium of Bunched Charged Particle Beams'', Physics of Plasmas 2 (3), 965 (1995).

6.
J. G. Wang and M. Reiser, ``Longitudinal Instability of Space Charge Dominated Beams in Transport Channels with Complex Wall Impedances'', Physics Fluids B, 5(7), 2286 (1993).

7.
C. K. Allen, N. Brown, and M. Reiser, ``Image Effects for Bunched Beams in Axisymmetric Systems'', Part. Accel. 45, 149 (1994).

8.
M. Reiser, C. R. Chang, D. Kehne, K. Low, T. Shea, H. Rudd, and I. Haber, ``Emittance Growth and Image Formation in a Nonuniform Space-Charge-Dominated Electron Beams'', Phys. Rev. Lett. 61, 2933 (1988).

9.
I. Haber, D. Kehne, M. Reiser and H. Rudd, ``Experimental, Theoretical and Numerical Investigation of the Homogenization of Density Nonuniformities in the Periodical Transport of a Space-Charge Dominated Beam'', Phys. Rev. A 44, 5194 (1991).

10.
D. Kehne, K. Low, M. Reiser, T. Shea, C. R. Chang, and Y. Chen, ``Experimental Studies of Electron Beam Transport in the Maryland Periodic Solenoid Channel'', Nucl. Inst. and Meth., A278, 194 (1989).

11.
J. G. Wang, D. X. Wang, and M. Reiser, ``Longitudinal Expansion of Space-Charge Dominated Drifting Beam Bunches with a Parabolic Line Charge Density Distribution'', Appl. Phys. Lett., 62(6), 645 (1993).
12.
D. X. Wang, J. G. Wang, D. Kehne, and M. Reiser, ``Experimental Verification of a Longitudinal Beam-Envelope Model for a Space-Charge Dominated Parabolic Bunch'', Appl. Phys. Lett., 62(25), 3232 (1993).

13.
D. X. Wang, J. G. Wang, D. Kehne, M. Reiser, and I. Haber, ``Longitudinal Expansion and Compression of Electron Bunches with Rectangular Line Charge Profiles'', Il Nuovo Cimento, Vol. 106 A, N.11, 1739 (1993).

14.
D. X. Wang, J. G. Wang, and M. Reiser, ``Restoration of Rectangular Pulse Shape after Edge Erosion for a Space-Charge Dominated Electron Bunch'', Phys. Rev. Lett., 73(1), 66 (1994).

15.
J. G. Wang, D. X. Wang, and M. Reiser, ``Generation of Space-Charge Waves due to Localized Perturbations in a Space-Charge Dominated Beam'', Phys. Rev. Lett., 71(12), 1836 (1993).

16.
J. G. Wang, H. Suk, D. X. Wang, and M. Reiser, ``Determination of the Geometry Factor for Longitudinal Perturbations in Space-Charge Dominated Coasting Beams'', Phys. Rev. Lett., 72(13), 2029 (1994).

17.
J. G. Wang, D. X. Wang, H, Suk, and M. Reiser, ``Reflection and Transmission of Space-Charge Waves at the Ends of a Space-Charge Dominated Electron Bunch'', Phys. Rev. Lett., 74(16), 3153 (1995).

18.
H. Suk, J. G. Wang, and M. Reiser, ``Resistive-Wall Instability Experiment in Space-Charge Dominated Electron Beams'', in the Proceedings of the 1995 Particle Accelerator Conference and International Conference on High-Energy Accelerators, Vol. 5, pp. 2974-2976, Dallas, Texas, May 1-5, 1995.

19.
J. G. Wang, H. Suk, and M. Reiser, ``Experimental Studies of Space-Charge Waves and Resistive-Wall Instability in Space-Charge Dominated Electron Beams'', to be published in Fusion Engineering and Design.

20.
S. Bernal, A. Dragt, M. Reiser, M. Venturini, J. G. Wang, and T. Godlove, ``Injector and Matching Section Design for a Model Electron Ring'', to be published in Fusion Engineering and Design.

21.
M. Reiser, S. Bernal, A. Dragt, M. Venturini, J. G. Wang, H. Onishi, and T. Godlove, ``Design Features of a Small Electron Ring for Study of Recirculating Space-Charge Dominated Beams'', to be published in Fusion Engineering and Design.

Table 1: Design parameters for the electron ring
5mm