P. C. Efthimion

Recent results from the Paul Trap Simulator Experiment

The Paul Trap Simulator Experiment (PTSX) is a compact laboratory facility whose purpose is to simulate the nonlinear dynamics of intense charged particle beam propagation over large distances through an alternating-gradient transport system. The simulation is possible because the quadrupole electric fields of the cylindrical Paul trap exert radial forces on the charged particles that are analogous to the radial forces that a periodic focusing quadrupole magnetic field exert on the beam particles in the beam frame. Initial experiments clearly demonstrate the loss of confinement when the vacuum phase advance /spl sigma//sub v/ of the system exceeds 90/spl deg/. Recent experiments show that PTSX is able to successfully trap plasmas of moderate intensity for thousands of equivalent lattice periods.

Neutralized transport of high intensity beams

The NTX experiment at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final focus systems for high perveance heavy ion beams. A converging ion beam at the exit of the final focus magnetic system is injected into a neutralized drift section. The neutralization is provided by a metal arc source and an RF plasma source. Effects of a plasma plug, where electrons are extracted from a localized plasma in the upstream end of the drift section, and are then dragged along by the ion potential, as well as the volumetric plasma, where neutralization is provided by the plasma laid down along the ion path, are both studied and their relative effects on the beam spot size are compared. Comparisons with 3-D PIC code predictions will also be presented.

Plasma source development for the NDCX-I and NDCX-II neutralized drift compression experiments

In order to facilitate compression of a positive ion charge bunch longitudinally and transversely beyond the limit determined by the space-charge field of the bunch, a source of charge-neutralizing electrons must be provided. Plasma sources have been developed for the NDCX-I and NDCX-II experimental facilities, both for the 2-m-long, field-free drift regions and for the small-diameter interior of the multiTesla final focus solenoid. Barium titanate based cylinders with a high dielectric coefficient are used to line the wall of the 2-m-long drift region, and by applying a 9 kV pulse between the inner and outer surfaces of the cylinders, a plasma with a density in the 10 cm range is formed. A compact plasma source 2 long and 1.5 in diameter, also made using the barium titanate based material, has been developed for use in the bore of the final focus solenoid. Plasma generated near the wall of the plasma source will follow the fringing magnetic field lines of the solenoid and help to fill the bore of the magnet with plasma. Improved designs for the barium titanate plasma sources are being considered that use different inner-surface electrode materials and structures, and also use a modified electrical driver employing a spark gap crowbar switch. In addition, plasma source designs using so-called flashboard technology have been developed. In the flashboard plasma source, high density plasma is formed when the applied high voltage pulse causes a series of breakdowns between isolated copper patches aligned in rows along the surface of the 0.2 mm thick flashboard.

Ferroelectric Plasma Source for Heavy Ion Beam Charge Neutralization

Plasmas are employed as a source of unbound electrons for charge neutralizing heavy ion beams to allow them to focus to a small spot size. Calculations suggest that plasma at a density of 1-100 times the ion beam density and at a length ∼ 0.1-1 m would be suitable. To produce one-meter plasma, large-volume plasma sources based upon ferroelectric ceramics are being developed. These sources have the advantage of being able to increase the length of the plasma and operate at low neutral pressures. The source utilizes the ferroelectric ceramic BaTiO3to form metal plasma. The drift tube inner surface of the Neutralized Drift Compression Experiment (NDCX) will be covered with ceramic, and high voltage (∼ 1-5 kV) applied between the drift tube and the front surface of the ceramic by placing a wire grid on the front surface. A prototype ferroelectric source 20 cm long has produced plasma densities of 5×1011cm-3. The source was integrated into the previous Neutralized Transport Experiment (NTX), and successfully charge neutralized the K+ion beam. Presently, the one-meter source is being fabricated. The source is being characterized and will be integrated into NDCX for charge neutralization experiments.

A solenoid final focusing system with plasma neutralization for target heating experiments

Intense bunches of low-energy heavy ions have been suggested as means to heat targets to the warm dense matter regime (Temperature ~ 0.1 to 10 eV, solid density ~1% to 100%). In order to achieve the required intensity on target, a beam spot radius of approximately 0.5 mm, and pulse duration of 2 ns is required with an energy deposition of approximately 1 J/cm2. This translates to a peak beam current of 8 A for 0.4 MeV K+ ions. To increase the beam intensity on target, a plasma-filled high-field solenoid is being studied as a means to reduce the beam spot size from several mm to the sub-mm range. A prototype experiment to demonstrate the required beam dynamics has been built at Lawrence Berkeley National Laboratory. The operating magnetic field of the pulsed solenoid is 8 T. Challenges include suitable injection of the plasma into the solenoid so that the plasma density near the focus is sufficiently high to maintain space- charge neutralization of the ion beam pulse. Initial experimental results are presented.

Initial Results on Neutralized Drift Compression Experiments (NDCX-IA) for High Intensity Ion Beam

Ion beam neutralization and compression experiments are designed to determine the feasibility of using compressed high intensity ion beams for high energy density physics (HEDP) experiments and for inertial fusion power. To quantitatively ascertain the various mechanisms and methods for beam compression, the Neutralized Drift Compression Experiment (NDCX) facility is being constructed at Lawrence Berkeley National Laboratory (LBNL). In the first neutralized drift compression experiment, a 280 KeV, 25 mA, K+ion beam is longitudinally 50-fold compressed using an induction core to produce a velocity tilt. This compression ratio is measured using various diagnostics.

RF plasma source for heavy ion beam charge neutralization

Highly ionized plasmas are being used as a medium for charge neutralizing heavy ion beams in order to focus the ion beam to a small spot size. A radio frequency (RF) plasma source has been built at the Princeton Plasma Physics Laboratory (PPPL) in support of the joint Neutralized Transport Experiment (NTX) at the Lawrence Berkeley National Laboratory (LBNL) to study ion beam neutralization with plasma. The goal is to operate the source at pressures /spl sim/10/sup -5/ Torr at full ionization. The initial operation of the source has been at pressures of 10/sup -4/-10/sup -5/ Torr and electron densities in the range of 10/sup 8/-10/sup 11/ cm/sup -3/. Recently, pulsed operation of the source has enabled operation at pressures in the 10/sup -6/ Torr range with densities of 10/sup 11/ cm/sup -3/. Near 100% ionization has been achieved. The source has been integrated with the NTX facility and experiments have begun.

Meter-long plasma source for heavy ion beam space charge neutralization

Plasmas are a source of unbound electrons for charge neutralizing intense heavy ion beams to allow them to focus to a small spot size and compress their axial pulse length. The plasma source should be able to operate at low neutral pressures and without strong externally- applied electric or magnetic fields. To produce one- meter-long plasma columns, sources based upon ferroelectric ceramics with large dielectric coefficients have been developed. The source utilizes the ferroelectric ceramic BaTiO3 to form metal plasma. The drift tube inner surface of the neutralized drift compression experiment (NDCX) is covered with ceramic material, and high voltage (~8 kV) is applied between the drift tube and the front surface of the ceramics. A lead- zirconium-titanate prototype ferroelectric plasma source (FEPS), 20 cm in length, has produced plasma densities of 5times1011 cm-3. It was integrated into the neutralized transport experiment (NTX), and successfully charge neutralized the K+ ion beam. A one-meter-long BaTiO3 source comprised of five 20-cm-long sources has been tested and characterized, producing relatively uniform plasma over the one-meter length of the source in the mid-1010 cm-3 density range. This source has been integrated into the NDCX device for charge neutralization and beam compression experiments. Initial beam compression experiments with this source yielded current compression ratios near 100. Future research will develop longer and higher plasma density sources to support beam compression experiments for high energy density physics applications.

Extreme compression of heavy-ion beam pulses: Experiments and modeling

Longitudinal bunch compression of intense ion beams for warm dense matter and heavy ion fusion applications occurs by imposing an axial velocity tilt onto an ion beam across the acceleration gap of a linear induction accelerator, and subsequently allowing the beam to drift through neutralizing plasma as the pulse compresses. The finite-size of the acceleration gap and time-dependent nature of the induction voltage waveform for longitudinal compression are demonstrated to increase the effective longitudinal temperature of the charge bunch, reduce the resulting fractional velocity tilt from its intended value, and transversely defocus the beam in a time-dependent manner. The over-focusing technique or a strong final- focus solenoid may be used to refocus the longitudinally compressing beam to the small spot size required (sub- mm to few mm) at a coincident focal plane. In the case of a final-focus solenoid, supersonic cathodic-arc plasma may be injected into the high-field region from the low- field end for beam neutralization experiments.

Development of Laser-Induced Fluorescence Diagnostic for the Paul Trap Simulator Experiment

The Paul Trap Simulator Experiment (PTSX) is a cylindrical Paul trap whose purpose is to simulate the nonlinear dynamics of intense charged particle beam propagation in alternating-gradient magnetic transport systems. For the insitu measurement of the transverse ion density profile in the PTSX device, which is essential for the study of beam mismatch and halo particle production, a laser-induced fluorescence diagnostic system is being developed. Instead of cesium, which has been used in the initial phase of the PTSX experiments, barium has been selected as the preferred ion for the laser-induced fluorescence diagnostic. The installation of the barium ion source and the characterization of the tunable dye laser system are discussed. The design of the collection optics with an intensified CCD camera system is also discussed. Finally, initial test results using the laser-induced fluorescence diagnostic are presented.