990 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 4, APRIL 2012 Nuclear Radiation-Induced Atmospheric Air Breakdown in a Spark Gap Shaolin Liao, Nachappa Gopalsami, Senior Member, IEEE, Eugene R. Koehl, Thomas W. Elmer, II, Member, IEEE, Alexander Heifetz, Hual-Te Chien, and Apostolos C. Raptis, Life Member, IEEE Abstract—We have investigated the effect of pre-ionization by a radioactive 137 Cs γ -ray source on the atmospheric air breakdown conditions in a high-voltage spark gap. A standoff millimeter-wave (mmW) system was used to monitor the breakdown properties. A decrease in breakdown threshold was observed with an increase of radiation dose. We attribute this to a space charge-controlled elec- tron diffusion process in a cloud of radiation-induced ion species of both polarities. The space charge-dependent diffusion coefficient was determined from the measurement data. In addition, we found that the breakdown process shows random spikes with Poisson– like statistical feature. These findings portend the feasibility of remote detection of nuclear radiation using high-power mmWs. Index Terms—Air breakdown, breakdown voltage, millimeter wave (mmW), nuclear radiation, spark gap. I. I NTRODUCTION A TMOSPHERIC AIR breakdown has been of great interest in plasma physics [1], and its potential application to remote detection of nuclear radiation-induced ionized air has been explored recently [2]. Although the spark gap breakdown has been known [3] for plasma generation, the physics of breakdown parameters is not well understood in the presence of nuclear radiation, particularly at atmospheric pressure. In this paper, we present an analysis of the nuclear radiation-induced air breakdown process in a dc spark gap and its detection with a standoff millimeter-wave (mmW) system. The purpose of our investigation is twofold: 1) demonstrate feasibility of high- power mmW remote detection of nuclear radiation-induced ionized air from its equivalent dc air breakdown investigation and 2) shed light on the fundamental physics governing the nuclear radiation-induced air breakdown process. II. THEORETICAL BACKGROUND A. DC Spark Gap and High-Power Microwave Breakdown dc spark gap breakdown can be categorized into two types [4]: the Townsend process at low pressure (p)—spark gap separation (d) product, i.e., the pd value; and the streamer Manuscript received October 25, 2011; revised January 17, 2012; accepted January 29, 2012. Date of publication March 2, 2012; date of current version April 11, 2012. This work was supported by the Defense Threat Reduc- tion Agency under the U.S. Department of Energy Contract No. DE-AC02- 06CH11357. The authors are with the Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439 USA (e-mail: sliao@anl.gov; goplasami@anl. gov; dick.koehl@anl.gov; elmer@anl.gov; aheifetz@anl.gov; htchien@anl. gov; raptis@anl.gov). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2012.2187343 process at high pd value. The Townsend type is an electron avalanche process that involves both primary ionized electrons in the air and the secondary electrons emitted at the cathode due to impact of the positive ions [5], [6]. The breakdown voltage can be obtained by solving the generalized Townsend equation [7]. A streamer is a filament-like structure due to strong electric field around the rapidly advancing and highly concentrated avalanche front [8], [9]. The high-power microwave air breakdown can be explained in terms of the effective electric field strength, i.e., E eff = E rms /  1+(ω/ν c ) 2 , where E rms is the root-mean-square electric field and ω and ν c are the electromagnetic circular fre- quency and electron collision frequency in the air, respectively. High-power microwave experiments have also been studied in various frequency bands and resonant cavities [10]. In both cases, breakdown electric field is obtained when the electric field-dependent ionization rate ν i equals the attachment loss rate ν a plus the diffusion loss rate ν d , ν i (E b )= ν a (E b )+ ν d (E b ) ∼ ν a (E b ) - D Λ 2 eff (1) where E b is the breakdown dc or effective electric field strength and D and Λ eff are the diffusion coefficient and effective diffusion length, respectively. When ν i ≪ ν d , (1) gives, D ∼ ν i (E b )Λ 2 eff . (2) B. Preionization and Afterglow Effect Microwave breakdown under various background ionization concentrations ranging from 10 6 - 10 9 cm -3 has been tested [11] with a microwave cavity using Neon gas at pressures up to 100 torr. It has been found that the breakdown field strength could be reduced by more than 10% in the presence of pre- ionization. The authors related such remarkable observation to the transition from the free electron diffusion to the ambipolar diffusion, which means the diffusion coefficient is also a func- tion of ion density n i , D(n i ); however, its detailed form has not been obtained. The laser pre-ionization effect for an atmo- spheric spark gap based on resonance enhanced multi-photon ionization mechanism has also been investigated recently [12], where a reduction of breakdown voltage up to 50% has been achieved. However, the laser induced pre-ionization is along the path of the laser instead of the whole space in the spark gap. The afterglow in both dc spark gap and microwave air break- down has a similar transition from space charge-controlled 0093-3813/$31.00 © 2012 IEEE