Fireballs are discharge phenomena in partially ionized low temperature plasmas. They may have been named this way because they appear as bright glows in an otherwise dark background plasma, and they are often spherical like ball lightning. Fireballs are usually formed on positively biased electrodes. One interesting aspect is the formation of a double layer at the outer boundary of the fireball. Fireballs have been studied by various research groups and the present work was done in collaboration with Prof. R. Schrittwieser's group at the University of Innsbruck, Austria. We have studied instabilities in fireballs which arise from the ionization phenomena, particle beams and force imbalances. Pulsating fireballs are a very common features.

Whistler Instabilities

Fig. 1. Images of fireballs. (a) Spherical fireball in Neon. (b) Spherical fireball in Argon. (c) Expanded sheath around a positively biased ball electrode just before triggering a fireball. (d) Fireball in a uniform axial magnetic field, forming a cylindrical discharge. (e) Fireball in a nonuniform magnetic field. (f) Fireball along a diverging magnetic field.

Some visual images of fireballs are shown in Fig. 1. The spherical glow arises from a positive bias (+ 50 V) to a 1 cm diam spherical electrode in a low density discharge background plasma at a few mTorr of Neon (red) or Argon (blue-white). In the absence of a magnetic field (a-c) the fireball forms on the side of the sphere. When the voltage is raised (> +10 V) a luminous sheath forms first, which is spherically symmetric around the electrode (c). With an abrupt onset the sheath expands and deforms into the fireball. In the presence of a uniform magnetic field the fireball assumes a cylindrical shape (d). In a non uniform magnetic field many shapes are possible such as an expanding and curved cylinder in a dipole magnetic field (e) or a transition from cylinder to sphere in a diverging field of a permanent magnet (f).

Wave time-of-flight diagram

Fig. 2. Start of a fireball with pulsation. (a) Electrode current at the start of a positive voltage pulse. The delay of the current onset increases when the ambient plasma density decreases due to a longer pulse repetition time. (b) Current and light of a pulsating fireball. (c) Voltage waveform and current spikes from an unstable fireball.

The formation of a fireball can be studied with a pulsed voltage as used in Fig. 2. The background plasma is formed by the afterglow of the preceding pulse. The repetition time determines the background density. Fig. 2a shows the electrode current for different background densities which affect the delay for the fireball formation. The fireball also exhibits current and light oscillations (Fig. 2b). The frequency and waveforms of these ionization instabilities can vary widely. The cause is an imbalance between plasma production in the fireball (causing its growth) and ion ejection from the fireball (causing its decay). In order to understand the source of the instability the wave topology has been investigated. The wave field topology is displayed below by vector fields of (By, Bz) and contours of Bx in a transverse y-z plane for waves emitted ahead of the spheromak. The phase fronts are slightly curved. On axis the magnetic field alternates between Bx and By, but has a negligible Bz component. This is fundamentally different from a whistler vortex topology, thus the wave is not excited by an oscillating toroidal current loop. Near the axis the wave magnetic field and currents are transverse to B0 and circularly polarized as shown in a hodogram (b). It displays the rotation of the wave magnetic field on axis at a fixed time. The polarization in space reverses sign when the wave propagates in opposite directions. The rotation in time at a fixed position remains the same and agrees with the cyclotron rotation of electrons. These are the characteristics of whistler waves.

Time-of-flight diagram

Fig. 3. Growth of a pulsed fireball which ejects ions. (a) Radial density profile at different times, showing the density increase and the expansion of the spherical fireball. (b) Density perturbations propagating away from a pulsed fireball. (c) Time-of-flight diagram of the propagating density pulses which are identified as supersonic ballistic ion signals.

Some of the transient processes occurring in the formation of the fireball are shown in Fig. 3. With a Langmuir probe the radial density profile has been measured at different times during the growth of a fireball and shows the increase in density and fireball half width Fig. 3a. An axial scan reveals the expulsion of several density perturbations (Fig. 3b). A time-of-flight diagram identifies the peaks as ballistic ions expelled from the fireball (Fig. 3c). The electric field set up by the positive electrode and the growing double layer attracts electrons and ejects ions from the fireball.

Wave time-of-flight diagram

Fig. 4. High frequency beam-plasma instability. (a) Waveforms of the electrode voltage and currents and a short-duration rf emission pulse detected with a tuned receiver. (b) Tuning the narrowband receiver frequency shifts the emission line in time due to density changes. (c) Density variation in time derived from the plasma frequency shift in Fig. 4b.

Electrons from the ambient plasma are accelerated into the fireball by the double layer potential drop (approx. equal to the ionization potential). The beam-plasma system inside the fireball excites electron plasma waves. Using an rf probe and a narrowband receiver the high frequency plasma waves are observed inside the fireball. Fig. 4a shows the rf emission line during the rise of the fireball. The emission forms a narrow line in time because the density rises and the instability excites waves near the electron plasma frequency. During the density decay the frequency drops in time (Fig. 4b). By tuning the receiver frequency the frequency decay can be measured and the density decay be determined (Fig. 4c).

Wave time-of-flight diagram

Fig. 5. Waveforms of the electrode voltage, current , light and rf emissions for a pulsating fireball. The rf modulation arises from the density oscillations and the narrow receiver bandwidth.

In a pulsating fireball the rf emission pulsates together with the current, density and light emission all shown in Fig. 5. If the rf emission is detected with a broadband receiver the frequency would shift with the time-dependent density. When the electrode voltage is turned off the fast electrons are no longer present and the rf signal and light emission drop rapidly.

Many other features of fireballs have been investigated and are described in separate sections. For example, the transition from a sheath to a fireball contains also ionization phenomena and high frequency instabilities. A relaxation instability arises when the sheath expands, electrons ionize gas in the sheath, causing excess ions to affect the sheath potential drop and resulting in a sheath collapse. Thus, ionization affects the sheath stability and associates rf instabilities. Strong sheath ionization triggers the formation of a fire ball.

A strong fireball does not only energize electrons and ions but also causes neutral gas heating. The heating is due to electron-neutral collisions. The heated gas inside the fireball expands radially which is identified from the motion of small objects in the gas flow. A pendulum is very sensitive to repeated gas puffs from a pulsed fireball in resonance with the pendulum frequency. Flow direction and gas pressure have been obtained. The neutral gas temperature has been estimated since objects in the gas flow become red hot.

Wave time-of-flight diagram

Fig. 6. Inverted Fireballs. (a) A coarse wire mesh with positive bias attracts electrons. Due to the transparency of the cage most electrons enter the sphere, gain energy and ionize. The interior forms an inverted fireball. (b) A strongly negative grid bias (- 500 V) attracts ions into the gridded sphere. Ion impact creates secondary electrons and a plasma is formed inside the cage. Electrons can escape through a small hole which acts like a positively biased electrode and forms a spherical fireball on the inside of the gridded sphere.

Finally, fireballs can arise in various other electrode configurations. For example in a gridded spherical electrode with positive bias the attracted electrons can pass into the sphere and form a fireball inside a cage. The properties of such "inverted" fireballs have been studied in detail. In a coarse grid few electrons are collected and most pass through the sphere. This results in transit time instabilities, similar to high frequency instabilities in electron rich sheaths.

When a gridded sphere is biased negatively it attracts ions and repels electrons, hence the interior would charge positively and free of a plasma. This is not the case for high negative bias (-500 V) because the impact of energetic ions on the wire mesh releases secondary electrons. This forms a plasma inside the sphere which is visible as a blue glow. The plasma potential is positive with respect to the grid, thus electrons are trapped. Steady state requires electron losses equal to electron production. A small hole has been provided for the electrons to escape. They exit as a beam whose energy is obtained from the cyclotron orbit in a weak ambient field. The interesting feature is a ball-shaped glow inside the sphere at the the grid opening. It is a fireball attached to the opening which acts like a positively biased electrode collecting electrons. The plasma potential inside the sphere is negative with respect to the outside plasma potential. A double layer separates the fireball from the more negative interior plasma. The size of the fireball controls the electron current which is much larger than the flux through the small hole.