C. A. Frost

Magnetic Bending of Laser Guided Electron Beams

Experiments have been performed to study magnetic turning of laser-guided electron beams. This technique is of interest for beam transport in circular high-current electron beam accelerators such as Sandias recirculating linac. A 1-MeV, 2-kA, 50-ns, electron beam was turned through a 45° angle with > 90% current transport efficiency. The 45° bend was accomplished by switching the beam between two laser ionized guide channels which intersected in the center of a 680 Gauss turning magnet. The beam radius was observed to increase as a result of turning by the uniform field in agreement with single particle simulations. These simulations predict much smaller emittance growth for optimized sector magnet bending.

Laser Generation and Transport of a Relativistic Electron Beam

A foilless diode usually requires an externally applied magnetic field to control expansion and transport of a relativistic electron beam. A new foilless diode has been developed that does not require an external magnetic field. A low pressure organic gas is introduced into the diode and the transport region. A UV laser beam is injected through the transport region and is terminated at the cathode. The laser photoionizes the low pressure gas forming an ionized channel that captures the electron beam near the cathode. The electron beam is focused and guided by electrostatic attraction to the ionized channel. A 1.5-MeV, 20-kA electron beam has been generated and transported 1 m using this technique. The laser was replaced by an 800-V, 250-ma, low-energy electron beam which was used to guide the relativistic electron beam 4 m through a 90° bend.

The IFA-2 Collective Ion Accelerator System

A second generation ionization front accelerator system (IFA-2) is being brought into operation. Results of IFA-2 IREB optimization experiments, laser characterization experiments, beam switch experiments, and laser deflector experiments are presented. The status of the IFA-2 system is summarized.

The IFA-2 Collective Accelerator

IFA proof-of-principle experiments (completed on the IFA-1 system) have already demonstrated accurately-controlled motion of the potential well at the head of an IREB, and IFA ion data sets (H+,D+,He++) imply that controlled accelerating fields of 50 MV/m have been achieved over 10 cm. A new system, IFA-2, is being developed to demonstrate controlled accelerating fields of 100 MV/m over 1 meter. For IFA-2, a new working gas (N,N dimethyl aniline) has been found that operates at room temperature and requires only one laser (XeCl-307 nm). This new working gas represents a major breakthrough for the IFA in regard to simplicity and ease of operation. Research on this new working gas (IREB-induced ionization cross section experiments, photoionization cross section experiments), and the status of the IFA-2 system, are discussed.

RADLAC-II upgrade experiments

The linear induction accelerator RADLAC-II (Radial Line Accelerator II) is being upgraded to produce a 20-MeV, 40-kA, annular electron beam. Prior to the upgrade, RADLAC II produced a 15-MeV, 15-kA electron beam. Modifications to the pulsed power, injector, and magnetic transport have resulted in a faster-rising flat-topped voltage pulse. A high-quality, 40-kA, 2.0-cm-diameter beam with a low perpendicular thermal velocity has been produced from the injector. The high-quality beam has been accelerated through two accelerating gaps. The final four accelerating stages are being added to RADLAC-II, and transport experiments through the full accelerator are beginning. Simulations show that the beam quality will be maintained through the entire accelerator.<<ETX>>

Production of tightly focused e-beams with high-current accelerators

Using numerical modeling, several approaches to the problem of designing an injector to produce a 3-30 kA, 2-4 mm-diameter electron beam in the 10-20 MeV energy range were studied. The cathode may be small in diameter and immersed in a strong magnetic field, producing an equilibrium beam for transport to a target (the immersed case). This approach appears to be the most promising for applications such as radiography. The alternative is the conventional nonimmersed cathode. For the production of small-diameter electron beams in the range of 10-20 MeV, the authors propose an inductive-voltage-adder, single-(diode)-gap approach, e.g. SABRE instead of the conventional multigap linac. For the injector, they propose the use of a small-diameter cathode in a field of 40-60 kG. Calculations predict that very high current densities can be expected, with relative insensitivity to parameter variations.<<ETX>>

IFA-2 Collective Ion Accelerator Experiments

Ion acceleration has now been demonstrated with the IFA-2 collective ion accelerator system. The IFA-2 system is described, photoionization experiments are summarized, and ion results are presented. Using a 1 MeV electron beam and a 30 cm acceleration length, IFA-2 has produced 5 MeV H+, 10 MeV D+, and 20 MeV He++. This means that accelerating fields of 33 MV/m over 30 cm have been achieved with a controlled collective accelerator for the first time.

Ionization front accelerator: High gradients, demonstrated particle acceleration, and a proposed relativistic accelerator

The Ionization Front Accelerator (IFA) is a collective ion accelerator for which high‐gradient particle acceleration has now been domenstrated. In the IFA, the space charge field at the front of an intense relativistic electron beam is controlled by a laser and used to accelerate an ion bunch. Two complete IFA systems have been built (IFA−1 and IFA−2). Here we present initial IFA−2 ion results that demonstrate that ions have been accelerated with controlled accelerating fields of 33 MV/m over 30 cm.Space charge fields of accelerators like the IFA and the plasma beat wave accelerator are compared, and both are shown to be capable of producing fields 1 GV/m and higher. The IFA systems are discussed and initial IFA‐2 ion results are presented. Lastly, a relativistic IFA is proposed that should, in principle, permit the attainment of virtually unlimited ion energies.

High voltage high brightness electron accelerators with MITL voltage adder coupled to foilless diodes

Summary form only given. It has recently been experimetnally and theoretically demonstrated that foilless diodes can be successfully coupled to self-magnetically insulated transmission line voltage adders to produce very small high-brightness, high-definition (no halo) electron beams. The RADLAC/SMILE experience opened the path to a new approach in high-brightness, high-energy induction accelerators. There is no beam drifting through the device. The voltage addition occurs in a center conductor, and the beam is created at the high voltage end in an applied magnetic field diode. This work was motivated by the remarkable success of the HERMES-III accelerator and the need to produce small-radius, high-energy, high-current electron beams for air propagation studies and flash X-ray radiography. Design examples of devices that can produce multikiloamp electron beams of as high as 100 MV energies and with radii as small as 1 mm have been considered.

RADLAC II/SMILE performance with a magnetically insulated voltage adder

A 12.5-m-long self-magnetically insulated transmission line (SMILE) that sums the voltages of eight, 2-MV pulse forming lines was installed in the RADLAC-II linear induction accelerator. The magnetic insulation criteria were calculated using parapotential flow theory and found to agree with MAGIC simulations. High-quality annular beams with beta perpendicular to <or=0.1 and a radius r/sub b/<2 cm were measured for currents of 50-100 kA extracted from a magnetic immersed foilless diode. These parameters were achieved with 11-15-MV accelerating voltages and 6-16-kG diode magnetic field. The experimental results exceeded design expectations and are in good agreement with code simulations.<<ETX>>