CHAPTER 8. LOW DENSITY GAS DISCHARGE PLASMA CELL primarily for its low ionisation threshold (see Fig. 8.2). Furthermore, Argon is available at low cost and easy to handle due to its inert nature as a noble gas. It is also detectable with residual gas analysers with high sensitivity (to detect gas leakage to the accelerator beamline) and readily available in gas mixtures with hydrogen, which enables several diagnostics methods (see Sec. 8.4).
To allow fast ignition of the plasma arc discharge, a DC glow discharge is maintained in the discharge gas. This ensures that a sufficient number of ionised particles is present and further collisional ionisation by fast, ionised argon can be triggered without delay. A pulsed high voltage is then applied between the electrodes. The high current driven by this voltage locally heats the cathode and thus initiates the transition to an arc discharge. The dis-charge electronics circuit is discussed in detail below.
Figure 8.6: Polymer foil window glued over the 10 mm central aperture of a mod-ified DN40 Conflat vacuum flange.
The length of the discharge vessel of 100 mm was chosen such that bunch energy changes are well detectable with the PITZ bunch diagnostics for the expected wakefield amplitudes in the plasma. Due to the spatial con-straints in the PITZ beamline, the overall length of the setup was lim-ited to less than 480 mm. This max-imum overall length led to the deci-sion to use electron windows for sep-aration of the cell’s gas atmosphere from the accelerator beamline vac-uum instead of a differential pump-ing system. The inner diameter of the discharge vessel is 11 mm, which allowed a maximum electrode
aper-ture of 10 mm. This minimises beam particle losses at the end of the cell (where the transverse bunch size is big due to the typical beam focus point at the plasma entrance) while maintaining a direct line of sight (i.e. discharge path) between the electrodes. Vacuum sealing between glass and electrodes is realised with indium gaskets. The indium is pressed onto the discharge tube by compressing it between glass and adjacent metal flange surfaces. With this arrangement leakage rates compatible with ultra-high vacuum (UHV) requirements have been achieved (He leakage current <10−10 mbars ·l).
8.2. PLASMA CELL DESIGN Initially, a sealed-off operation of the cell (single gas fill prior to experiments) was pursued to minimise the amount of equipment that would have to remain in the accelerator tunnel during operation. Due to the reasons described in the next section, this was replaced by continuous gas exchange as shown in Fig.8.5(temperature controlled valve and pressure gauge on top and vacuum pumps on bottom of left hand side of cell).
Electron windows
Thin polymer foils, already applied in other setups at PITZ [202], were used as electron windows for separation of the beamline vacuum from the dis-charge gas atmosphere. They provide low gas permeation (permeation con-stant <10−13m2/s) with average scattering angles for 23 MeV electrons of 0.1 mrad – 0.2 mrad at 0.9µm – 1.9µm thicknesses [202]. For beamline instal-lation, the foils are glued onto a DN40 Conflat vacuum flange with a central aperture of 5 mm – 10 mm diameter, using a vacuum compatible epoxy resin.
An exemplary window is shown in Fig. 8.6. As the existing foil material showed significant quality variations between different samples, Al-coated polyethylene terephthalate (PET) capacitor foil material by Birkelbach Kon-densatortechnik GmbH was introduced in the course of this work. To prove the applicability of these windows, gas permeation through the capacitor foils was investigated for thicknesses of 0.9µm, 1.9µm and 4µm. Tests were
10-15 10-14 10-13 10-12 10-11 10-10 10-9 0
5 10 15 20 25
Foil counts
Permeation constant [m2/s]
PET coated with Al (Capacitor) PET coated with Al
Mylar Kapton
Figure 8.7: Measured gas permeation constants for different foil materials and specimens.
CHAPTER 8. LOW DENSITY GAS DISCHARGE PLASMA CELL
Figure 8.8: Overlay of optical and laser microscope pictures of a single-sided metalised polymer foil window of 1.9µm thickness, which was exposed to 2 mC±0.5 mC of ∼23 MeV electrons. The foil is viewed from the non-metalised side. The zoom shows an alleged position of beam passage.
conducted by separating an evacuated test volume from a gas volume filled with 130 mbar of helium gas by the foil specimen. A leak tester was used to monitor the helium gas current on the evacuated side. Gas permeation constants Kp were calculated from these measurements by
Kp = Q˙ ·d
∆p·A , (8.6)
where ˙Q is the measured gas current, d is the foil thickness, A the perme-ation area and ∆pthe pressure difference between the two test volumes. The results are plotted in Fig. 8.7. Due to the much better reproducibility of the permeation, beam measurements presented in this work were conducted with 1.9µm-thick single-sided metalised PET foil windows mounted to the gas discharge cell, which corresponds to a mean scattering angle of the beam electrons of (0.19±0.05) mrad at 23 MeV [195].
After exposure to the PITZ electron beam with an integrated charge of
∼(2±0.5) mC, an increased gas permeation to the accelerator beamline was detected and alterations to the foils were found after dismounting. These al-terations are documented in Fig.8.8. Several spots are visible by a tempering-like colour change as well as by their elevation of severalµm, which is revealed by the laser microscope height scan. These changes are addressed to local heating of the foil due to energy deposition by scattered beam electrons.
Thermal conduction through the foil material was calculated to be negligibly
8.2. PLASMA CELL DESIGN
Figure 8.9: Calculated temperature rise in the polymer foil during continuous bunch passage. (a) shows the temperature distribution directly before passage of the 30th bunch within a 10 Hz train, (b) the temperature distribution directly after passage of the 30th bunch in a 10 Hz train. The development of the maximum foil temperature during a 10 Hz train of bunches (red dots) is shown in (c). The maximum foil temperature between two bunches is plotted in (d).
small. Also convective heat transfer does not contribute significantly to the foil temperature, as the discharge gas on the pressurised side of the foil has a small density. The radiative heat transfer was estimated by numerically cal-culating the temperature rise at a position where a 23 MeV, 0.2 mm RMS size transverse Gaussian bunch with a charge of 1 nC passes the foil. Heat depo-sition due to the scattering of bunch electrons was calculated to be∼10−5W at 10 Hz bunch repetition rate. The results of this calculation are shown in Fig. 8.9. A quasi-equilibrium between radiated heat and foil temperature is reached after passage of ca. 15 bunches (t = 1.4 s), as shown in Fig. 8.9 (c).
The temperature distributions [Fig. 8.9 (a,b)] correspond to the transverse Gaussian profile of the electron bunches. Neither is the melting temperature (∼250◦C) reached, nor is the temperature drop between two bunches such high that it may cause mechanical stress in the foil [Fig. 8.9 (d)]. Neverthe-less, the maximum temperatures after the passage of a bunch exceed the glass transition temperature of PET (∼70◦C) [203]. This might cause a change
CHAPTER 8. LOW DENSITY GAS DISCHARGE PLASMA CELL
0.0 0.5 1.0 1.5 2.0 2.5 3.0
600 800 1000 1200 1400 1600
U (V)
p (mbar)
Ar pos.
Ar neg.
N neg.
H neg.
Figure 8.10: Measured breakdown voltages for different gas species. Positive and negative polarity high voltage measurements are shown for argon.
of mechanical stability at the point of bunch passage, which is supported by the fact that the foil has gained elevation towards the evacuated beamline volume. An expansion towards one direction forced by the pressure difference over the foil might spread the PET in glass phase thin, which increases gas permeation and can lead to microscopic and ultimately macroscopic window breaking.
Discharge electronics
A negative polarity was initially chosen for the discharge voltage to exploit hollow cathode effects during the transition from glow to arc discharge [204].
Even though the hollow electrodes were finally replaced by a less complex, plane electrode design, the negative voltage was kept, as it provides mini-mum breakdown voltages in argon, as shown in Fig. 8.10.
To supply the low DC current for maintaining a glow discharge and the pulsed, high voltage and high current for the arc discharge, a pulse electron-ics circuit as sketched in Figs. 8.11 and 8.5, bottom right, was set up, which is connected to the discharge cell via coaxial transmission cables. The inner conductor of the coaxial transmission cables is connected to one electrode, the outer conductor to the other discharge electrode via the outer metal rods.
Capacitor C in Fig. 8.11, which has a capacitance of several µF, is charged via a charging resistor R1 to a voltage well above the breakdown voltage