Experimental Study of Current Filamentation Instability

Current filamentation instability is observed and studied in a laboratory environment with a 60 MeV electron beam and a plasma capillary discharge. Multiple filaments are observed and imaged transversely at the plasma exit with optical transition radiation. By varying the plasma density the transition between single and multiple filaments is found to be $k_p{\sigma }_r\sim 2.2$. Scaling of the transverse filament size with the plasma skin depth is predicted in theory and observed over a range of plasma densities. Lowering the bunch charge, and thus the bunch density, suppresses the instability.

Figure 1

Experimental setup: electron bunch from a 60 MeV linac is focused to its waist at the entrance to the capillary. The plasma is generated by puffing ${\mathrm{H}}_2$ gas at 110 Torr into a 1 mm diameter by a 2 cm long capillary with an applied voltage of 15 kV. OTR is generated from the bunch or filaments electrons and the Au coating. The resulting light is collected and magnified by a microscope objective and relayed with turning mirrors out of the vacuum chamber to an electron multiplying–charged coupled device camera.

Figure 2

OTR images of the bunch at the capillary exit with arrows indicating filaments. Bunch parameters are ${\sigma }_{0x}\sim 80\mu \mathrm{m}$, ${\sigma }_{0y}\sim 50\mu \mathrm{m}$, and charge $Q\simeq 1.0\mathrm{nC}$. (a) Without plasma and (b) $n_e=1.6\times {10}^{16}{\mathrm{cm}}^{-3}$ ($k_p{\sigma }_{0y}=1.2$), (c) and (d) $n_e=1.2\times {10}^{17}{\mathrm{cm}}^{-3}$ ($k_p{\sigma }_{0y}=3.3$) and (e) and (f) $n_e=1.9\times {10}^{17}{\mathrm{cm}}^{-3}$ ($k_p{\sigma }_{0y}=4.1$). X-rays are seen in the images and are a few pixels in size. Color tables are the same for (b) through (f).

Figure 3

Number of filaments observed in single events as a function of the CFI parameter $k_p{\sigma }_{0y}$. For this measurement the charge $Q\simeq 1.0\mathrm{nC}$ and ${\sigma }_{0x}\sim 80\mu \mathrm{m}$, ${\sigma }_{0y}\sim 50\mu \mathrm{m}$. Similar events lead to only single filaments for $k_p{\sigma }_{0y}<2.2$ and multiple (one to five) filaments for $k_p{\sigma }_{0y}>2.2$. For $k_p{\sigma }_{0y}>4.5$ only one and two filaments are seen and could be due to merging of CFI generated filaments. In general single points represent multiple events at the same $k_p{\sigma }_{0y}$.

Figure 4

Transverse filament size (rms) as a function of $c/{\omega }_{pe}$: circular markers high charge scan where $Q\simeq 1.0\mathrm{nC}$ and ${\sigma }_{0x}=81\mu \mathrm{m}$, ${\sigma }_{0y}=53\mu \mathrm{m}$. Triangular markers low charge scan where $Q\simeq 0.54\mathrm{nC}$ and ${\sigma }_{0x}\simeq 89\mu \mathrm{m}$, ${\sigma }_{0y}\simeq 45\mu \mathrm{m}$. Both scans consist of six different plasma densities with each ten events recorded. The solid line represents the expected linear dependency between filament size and plasma skin depth.