Leemans Micro-valve and nozzle Background Information

Wim Leemans, of the Center for Beam Physics (Group Leader, Experimental Beam Physics), is currently conducting research into utilizing pulsed lasers, rather than RF power, to accelerate electron beams.  Laser Wakefield acceleration in plasmas offers the potential of developing ultra-compact accelerators capable of producing high quality relativistic electron beams. Acceleration of electrons to energies as high as 100 MeV over mm-size distances has been demonstrated in several experiments. These energy gains correspond to accelerating electric fields in plasmas greater than 30 GV/m.

The accelerating technique that Wim’s group is developing utilizes a plasma channel to guide the electrons accelerated in the wake field of the pulsed laser.  The plasma channel is created by striking a gas plume with pulsed lasers that ionize and heat the gas to create a plasma with a cylindrical density distribution.  In current experiments, the gas plume is created by puffing gas (hydrogen, helium, or nitrogen) through a nozzle in 100 m-sec long bursts.  The conical plume is 1 to 2 mm in diameter.  The acceleration of the electrons is limited by the non-uniform density distribution in the gas and the short length of the plume.  Wim’s group is currently building a gas jet test stand to test nozzle and valve designs.  The current nozzle is conventionally machined with a bore diameter of 750 microns.  The gas bursts are created with a poppet-type solenoid valve (1000 psi backing pressure).  Wim hopes to test new nozzle designs as well as a piezoelectric valve with a faster opening and closing time (30-40 microseconds).

Wim views the gas jet technology as critical to the continued development of Laser Wakefield acceleration at LBNL.  He is particularly interested in the possibility of developing technologies to shape the density distribution of the gas plume and make it longer.  Ideally, the gas plume would have a length of several centimeters and a width of 100 to 200 microns.  If longer plumes can be produced, it would also be useful to adjust the density of the gas along the length of the plume.  The electrons gain velocity as they are accelerated in the plasma channel.  The speed of propagation of the laser in the gas medium decreases with gas density.  If the gas density is uniform throughout the length of the plume, the accelerated electrons will outpace the laser.  If the gas plume were created by a linear array of independently controlled nozzles, the density profile could be adjusted to balance the plasma density with the propagation speed of the laser, yielding optimal acceleration of electrons.

The micron scale nozzles and valves currently under development in the MEMS field may be applicable to the gas jet project.  In theory MEMS microfluidic devices are of an appropriate physical scale and could have the advantages of fast actuation (by virtue of the low inertia of moving parts) and independent control.  Assuming few components or existing designs would be readily available for a MEMS solution to this problem, significant engineering and testing may be required to create a working system.
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