87 research outputs found
Erratum: Collinear investigation of laser initiated reduced density channels [Appl. Phys. Lett. 43, 1010 (1983)]
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71132/2/APPLAB-45-7-808-1.pd
Collinear investigation of laser initiated reduced density channels
The characteristics of reduced density channels generated by laser initiated discharges have been investigated by means of collinear holographic interferometry and schlieren photography. We report the first direct measurements of the density profiles in the interior of such channels. Under unperturbed conditions these channels exhibit azimuthal asymmetries. Gas dynamics within the channel are also presented for the case of incident shock waves reflected from cylindrical and planar boundaries.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71306/2/APPLAB-43-11-1010-1.pd
Effect of xây coupling on the beam breakup instability
In solenoidal beam transport systems, motions in the x and y directions are coupled by the vĂB force. A twoâdimensional coupled mode description of the beam breakup (BBU) instability is presented; its dispersion relation is derived and compared with the oneâdimensional BBU dispersion relation. In the twoâdimensional description, instability growth is doubled and two additional wave modes are found in the regime of strong focusing. In the weak focusing regime, the twoâdimensional description gives the same dispersion relation as the oneâdimensional model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69723/2/APPLAB-58-7-699-1.pd
Copper vapor laser drilling of copper, iron, and titanium foils in atmospheric pressure air and argon
A copper vapor laser (511 and 578 nm) is used to drill submillimeter diameter holes in 0.025â0.127 mm thick foils of copper, iron, and titanium. Foils are machined in atmospheric pressure air and argon. The laser is repetitively pulsed at 10 kHz with a per pulse energy of 0.5 mJ giving an average power of 5 W at the sample surface for a pulse width of 40 ns. A pâiân photodiode and a photomultiplier tube detector are connected to a digitalâdisplay timing circuit that records the number of incident laser pulses used to drill through the sample. The number of pulses is converted to an average drilling time and can provide an estimate for the average laser energy used to drill the hole. Typical data for all three materials with a perâpulse fluence of 0.7 J/cm2 ranged from 0.1 to 500 s to produce holes of âŒ0.3 mm diameter. Drilling times decreased in some cases by an order of magnitude when machining in air. This is attributed to the increased laser absorption of the metalâoxide layer formed in air and was especially noticeable with titanium. A continuous wave thermal model is used to compare experimental data as well as verify the thermal machining mechanism.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69638/2/RSINAK-64-11-3308-1.pd
Dynamics of excimer laserâablated aluminum neutral atom plume measured by dye laser resonance absorption photography
We report the first dye laser resonance absorption photographs of a single species of aluminum groundâstate neutral atoms in the plume ablated from solid aluminum by KrF excimer laser radiation. Aluminum groundâstate neutral atoms were diagnosed by illuminating the ablated plume with a dye laser tuned to the 32P1/2â42S1/2 transition at 394.4 nm. Measurements have been performed in vacuum as well as in argon and air environments. Streaming velocities measured for neutral aluminum atoms in vacuum ranged from 0.5Ă106 cm/s at low excimer laser fluences of 1â2 J/cm2 to 3.4Ă106 cm/s at high fluences of 7 J/cm2. Dye laser resonance absorption photography measurements of ablated aluminum in argon and air showed slower expansion at 50 and 200 Torr, while observations at 760 Torr indicate turbulent mixing of aluminum neutrals near the surface. Differences between data in argon and air may be due to oxidation of neutral aluminum atoms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70489/2/APPLAB-58-15-1597-1.pd
Amorphous alloys formed by microsecond current pulses
We report the use of microsecond current pulses to transform layered crystalline nickelâzirconium films to amorphous alloy. Starting material was electron beam deposited multilayers with a composition modulation wavelength of 34 nm, an average composition of Ni63Zr37, and a total thickness of 680 nm. Electrical pulses were approximately rectangular and about 3 ÎŒs in duration, with an intensity of several hundred amperes, directly coupling 1.6 to 3 J of energy uniformly into the film. By monitoring current and voltage, the reaction and melting of the sample were observed, and the total energy of the pulse was easily computed. A sharp threshold in pulse energy for sample transformation was observed. A simple heat flow calculation demonstrated that the chemical energy released by the reaction, and the change in diffusion kinetics as the sample temperature exceeded the glass transition temperature of the amorphous alloy, are responsible for this sudden onset. The maximum temperature estimated from this calculation is below the melting point of the constituents, and the cooling rate is 107â108 K/s which is in agreement with the formation of amorphous alloy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69645/2/APPLAB-50-9-495-1.pd
Heating of Contaminants on Diamond Windows
This paper examines the RF heating of the contaminants on a diamond window. Both heating of an isolated internal graphite impurity, and of a thin film on the window surface, are discussed. Comparison with recent experiments is given. It is found that heating of isolated internal graphite impurities is unlikely to account for diamond window failure. Upon averaging over the entire window, the thin surface film in general absorbs approximately a fraction of one percent of the total incident power. Intense local heating on the surface contaminant is possible, however. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87504/2/234_1.pd
Extraction of ions from the matrix sheath in ablation-plasma ion implantation
A simple one-dimensional theory is presented to assess the implantation of ions from the ion matrix sheath (IMS) in an ablated plasma plume that is approaching a negatively biased substrate. Under the assumption that the plume geometry, the electron and ion density distributions, and the potential distribution are frozen during the IMS extraction, the implanted ion current is calculated as a function of time for various substrate-plume separations. This model accurately recovers Liebermanâs classic results when the plume front is initially in contact with the substrate. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70969/2/APPLAB-78-6-706-1.pd
Gyrotronâbackwardâwaveâoscillator experiments utilizing a high current, high voltage, microsecond electron accelerator
We report the first gyrotronâbackwardâwaveâoscillator experiments to produce high power (tube power of âŒ1â8 MW), longâpulse (0.3â1.2 ÎŒs) microwaves at high currents (0.1â2 kA) and high voltages (650â750 kV). Experiments were performed in the TE11 fundamental backwardâwave mode, with efficiencies of âŒ1%â2%. Mode competition was observed which is believed to originate from the TE21 absolute instability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70862/2/JAPIAU-72-4-1221-1.pd
Microwave absorption on a thin film
With the use of a simple model, it is shown that a thin film of contaminant on a microwave window may absorb up to 50% of the incident power, even if the film thickness is only a small fraction of its resistive skin depth. This unexpectedly large amount of absorption is conjectured to have played a significant role in window failure. The temperature rise in a thin film is estimated. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71216/2/APPLAB-82-9-1353-1.pd
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