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Nonlinear Inverse Compton Scattering Experiment at Neptune Laboratory/UCLA

We are preparing an Inverse Compton Scattering (ICS) experiment, which will investigate nonlinear properties of the scattering utilizing a terawatt CO2 laser system with various polarizations in our Neptune Laboratory at UCLA. When the normalized amplitude of the vector potential a0 is larger than unity the scattering occurs in the nonlinear region; therefore, higher harmonics are also produced.

Since this experiment requires very small electron and laser beam sizes (~25 µm RMS) at the interaction point (IP), we have to use strong focusing for both beams. We employ an off axis parabolic (OAP) mirror with a focal length of 170 mm to focus the laser beam to the IP. We designed and manufactured permanent magnet quadrupoles (PMQ) with a 110 T/m gradient to focus the electron beam. Due to strong focusing requirements, we chose the 90 degree geometry.

Table 1: Electron and Laser beam design parameters.

Parameter

Value

Electron Beam Energy

14 MeV

Beam Emittance

5 mm-mrad

Electron beam spot size (RMS)

25 µm

Beam Charge

300 pC

Bunch Length (RMS)

4 ps

Laser beam size at IP (RMS)

25 µm

CO2 laser wavelength

10.6 µm

CO2 laser Rayleigh range

0.75 mm

CO2 laser power

500 GW

CO2 laser pulse length

200 ps

Table 2: Scattered photon parameters.

Parameter

Head-On

90 Deg

Photon wavelength

5.3 nm

10.7 nm

Photon energy

235.3 eV

117.7 eV

Pulse duration (FWHM)

10 ps

10 ps

Interaction time

5 ps

0.33 ps

# of periods that electrons see (No)

283

10

# of photons emitted per electron (N)

3.34

0.11

Total # od Photons

6.3x109

2x108

Half openning angle

2.7 mrad

15 mrad

Bandwidth

0.35%

10%

Permanent Magnet Quadrupoles (PMQ)

We used NdFeB permanent magnet 7/16" size cubes to manufacture a set of PMQs. They yield 1.21 T magnetization. We install 4 cubes in quadrupole mode in an octoganal iron yoke. The design of the PMQ's was done with Radia and the measured fields are in close agreement producing ~90 T/m and 110 T/m for single and double length PMQ's, respectively.

Permanent Magnet Dipole (PMD)

The large divergence angle of the Compton photons has necessitated the immediate dump of the electron beam following the scattering at the IP. This has led to the development of a permanent magnet dipole (PMD). The magnetization of the NdFeB pieces is 1.32 T. We designed the dipole to yield a 90 degree bend angle for a 12-14 MeV electron beam. The magnets are cut in the shape of an arc to maximize the aperture seen by the scattered photons (soft X-rays). Tuning of the electron exit trajectory is provided by moving the dipole along the direction of the bend.

Imaging the IP

Simultaneous imaging of the electron and laser beams at the IP is required to overlap the beams spatially and temporally. Because the final spot size of the electron beam is expected to be ~25 um RMS, traditional use of YAG or phosphor screens are not an option due to the "blooming" produced by secondary electron emission in these media at such a high incident electron density from the small spot. The electron beam size and position will instead be imaged using OTR produced off a metallic pyramid. The pyramid face the 10.6 um laser sees will be covered with a thin, uniform layer of graphite which will spark upon interaction with the laser. These two orthogonal faces are angled at 45 degrees such that light created at the pyramid is sent up to an intensified camera. The beams can be aligned along the edge which separates the two faces, thus allowing for their spatial overlap.

In addition to the spatial overlap of the two beams is the necessity of their simultaneous arrival at the IP. This will be accomplished using a Germanium crystal as a "gate". Beam electrons incident on the crystal will cause secondary emission and essentially cause the crystal to become opaque to 10.6 um radiation. Minimizing the 10.6 um transmission through the crystal corresponds to synchronization with the electron beam and the timing between both beams can be adjusted accordingly.

Scattered Photon Detection

Detection of the scattered photons will be accomplished using a soft x-ray camera with a thinned, back-illuminated CCD wafer. Because the detection area is only 12.3 x 12.3 mm and the source divergence is so large, a re-entrant housing was designed to get the x-ray camera as close to the source as possible. As higher harmonics of the scattered photons are expected to be produced off axis in the case of an initially circularly polarized laser beam, a micro-channel plate detector with a larger detection area (40 mm diameter) will be cross-calibrated against the soft x-ray CCD camera and used to image the higher harmonics.

Differentiating Between Higher Harmonics

Viewing the structure of the higher harmonics produced at various polarizations due to the nonlinear scattering between the electrons and laser photons at the IP is highly dependent on filtering and viewing individual harmonics. Given the parameters at Neptune, the 2nd harmonic is well transmitted (comparitively) by polypropylene (C3H6) such that a thin film sheet could be used to filter out the other harmonics to faint detection levels. Boron Nitride (BN) at sub-micron thick layers functions well (again, comparatively) as an attenuator of energies corresponding to the 1st, 2nd, and 4th harmonics. (Plots below from the Center for X-ray Optics website, www-cxro.lbl.gov)

ICS BOX

Beam Matching

Related Talks & Presentations

Selected References