Grid-controlled electron emission from a hollow

JOURNAL OF APPLIED PHYSICS

VOLUME 95, NUMBER 7

1 APRIL 2004

Grid-controlled electron emission from a hollow-anode electron source A. Krokhmal, J. Z. Gleizer, Ya. E. Krasik,a) V. Ts. Gurovich, and J. Felsteiner Physics Department, Technion, 32000 Haifa, Israel

共Received 20 October 2003; accepted 7 January 2004兲 We describe the operation of a hollow-anode electron source with a biased output grid in a diode powered by a 200 kV 400 ns pulse. The hollow anode had a ferroelectric plasma source incorporated in it. Three electrical schemes for the hollow-anode output grid bias were tested and compared. It is shown that the use of an autobias grid allows electron-beam generation with a current amplitude up to 1.2 kA in a plasma emission-limited mode and with insignificant plasma prefilling of the accelerating gap. The use of an externally biased output grid 共either with a positive or negative potential兲 showed the possibility to control the emission properties of the hollow-anode plasma without changing the amplitude of the discharge current. Electron beams with an amplitude up to 2 kA and insignificant plasma prefilling of the accelerating gap were obtained. It was found that the application of the accelerating pulse leads to a drastic increase in the potential of the plasma up to several kV. It is shown that, in spite of the large positive plasma potential, electron emission occurs due to the dynamics of ions inside the sheath near the hollow-anode grid. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1651340兴

I. INTRODUCTION

II. EXPERIMENTAL SETUP AND DIAGNOSTICS

There has been a continuous interest in pulsed electron beams with a current amplitude of few kA and an electron energy of several hundreds of keVs.1– 4 In previous articles, we described the design and operation of a hollow-anode 共HA兲 electron source with different igniters.5–7 It was shown that the HA with an incorporated ferroelectric plasma source is a powerful and the most compact electron source among the tested sources.7 This source was used in a diode for the generation of electron beams with an amplitude of I b ⭐1.5 kA, an electron energy of ⭐400 keV, and a pulse duration of ␶ p ⭐400 ns. One of the advantages of the HA plasma source in comparison with explosive emission plasma sources is the relatively easy control of the emission properties of the plasma. This control can be achieved either by the change of the amplitude of the HA discharge current I d or by the change of the output grid potential with respect to the plasma potential.3,8,9 The maximum of the plasma emission ability is reached when I d ⬇I b , where I b is the diode current. In the first case, the increase in the amplitude of I d leads to the increase in the plasma density n and temperature T e . 5–7 However, this method leads to the plasma expansion and a plasma prefilled mode of the diode operation2 that is unacceptable for many applications. Thus, it is preferable to use a bias-controlled grid3,8,9 while keeping I d ⫽constant. By adjustment of the geometrical transparency and the potential of the output grid, one can expect to reach I d ⬇I b . In this article, we report on the emission properties of the ferroelectric plasma source-assisted HA electron source with different schemes for the bias of the HA output grid.

We used an experimental setup similar to that described in Ref. 7 关see Fig. 1共a兲兴. The ferroelectric sample was placed at the bottom of the anode and served as a cathode for the HA discharge. It was in a form of a disk, 14 cm in diameter, made of a BaTi with a rear solid electrode and a front strip electrode. The distance between the sample and the HA grid was 11 cm. The sample was ignited by a generator producing a positive voltage pulse 共⭐18 kV, ⬃500 A, ⬃200 ns兲 applied to the rear electrode. The HA discharge between the sample front surface and the anode was sustained by a pulsed generator 共10 kV, 5 ⍀, 10 ␮s兲. An accelerating pulse produced by high-voltage generator 共⭐300 kV, 80 ⍀, ⭐400 ns兲 was supplied to the diode at the maximum of I d . The diode voltage was measured by a voltage divider. Self-integrated Rogowsky coils were used for measurements of the discharge, diode, and grid currents. The potential of the plasma prior to and during the accelerating pulse was studied using the experimental setup that was shown in Fig. 1共a兲 but with a grounded HA 关see Fig. 1共b兲兴. The plasma potential ␸ pl was measured by a probe inserted radially inside the anode at distances of 5 cm and 1 cm from the ferroelectric sample and the anode axis, respectively. The probe was connected to the anode by a resistor R pr⫽11 k⍀ which determines the probe floating potential relative to the plasma of several electron temperatures.10 The plasma potential was calculated as ␸ pl ⫽R prI pr , where I pr is the probe current. The ferroelectric sample potential ␸ c was measured by an active voltage divider. Electron and ion flows from the HA plasma prior to and during the accelerating pulse were measured by a biased Faraday cup placed at a distance of 4 cm opposite to the 1 cm diameter hole made in the side surface of the anode. Inside the accelerating gap 共without accelerating pulse兲, these flows were measured by a biased collimated Faraday cup. The uniformity of the electron beams was checked by

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FIG. 2. Typical waveforms of the accelerating voltage ␸ d , diode current I b and space-charge-limited current I sc for the HA with an autobias control grid. Accelerating gap d⫽3 cm, HA discharge current I d ⫽1000 A.

FIG. 1. Experimental setups: 共a兲 HA electron source with grounded collector and 共b兲 HA electron source with grounded HA.

x-ray imaging of the beam obtained at a diode collector.5 The absence of explosive plasma spots at the HA grid was monitored by a fast framing camera 4Quik05A in each generator pulse. III. EXPERIMENTS WITH AN AUTOBIAS GRID

At first, we studied the HA with an autobias grid connected to the anode by a resistor R b . The HA grid current I gr is carried by electrons and ions emitted from the plasma. The larger electron current density as compared with the ion current density, determines the grid potential relative to the plasma ␸ gr⫽R b I gr⬍0. This ␸ gr causes formation of a double layer near the grid. The latter decreases the plasma penetration into the accelerating gap due to the decrease of the effective grid transparency. Measurements of the plasma ion j ip and electron j ep current densities inside the accelerating gap at a distance of 2 cm from the grid showed that at R b ⫽10 ⍀ ( 兩 ␸ gr兩 ⭓35 V) j ep becomes less than j ip . A further increase of R b up to 50 ⍀ leads to 兩 ␸ gr兩 ⬇60 V and to the increase in j ip up to ⬃5 mA/cm2.7 Taking into account the plasma expansion velocity ⬃106 cm/s, one obtains plasma density in the accelerating gap as ⭐1010 cm⫺3 . All further experiments with the autobias grid 共55% grid transparency and grid cell size of 235 ␮ m⫻235 ␮ m) were made with R b ⫽52 ⍀. In this case, it was found that in the range of I d ⫽0.6– 1.2 kA, the plasma parameters inside the anode cavity change in the range of n⫽(2 – 4)⫻1012 cm⫺3 and T e

⫽(2–5 eV). Thus, one can estimate the range of j ep as:10 j ep ⫽0.5n(kT e /m e ) 1/2⫽(7 – 21) A/cm2 . Here, k and m e are the Boltzmann constant and the electron mass, respectively. The potential difference between the ferroelectric sample and the anode, in the indicated range of I d , remained unchanged (80⫾10) V. Also, it was found that the plasma acquired a positive potential of ⬃10 V, ⬃80 V, and ⬃60 V with respect to the anode, ferroelectric sample, and grid, respectively. Experiments with an accelerating pulse were carried out with the accelerating gap 共the gap between the HA grid and collector兲 varied in the range of d⫽1.5– 3 cm. It was found that I b ⭐1200 A was always smaller than the space-chargelimited current I sc and that I b was almost constant during the accelerating pulse 共see Fig. 2兲. These data can be explained by the limited emission capability of the plasma. Indeed, one can estimate the electron current amplitude which can be emitted by HA electron source as I n ⫽ j ep S ␩ ⬇300 A, where ␩ and S are the grid geometrical transparency and the crosssectional area, respectively. The smaller obtained value of I b is related with the nonuniform radial plasma density distribution7 and the smaller value of the effective emission area as compared with the S ␩ value. In order to increase I b , experiments with a larger grid cell size were performed. However, these experiments showed plasma penetration inside the accelerating gap. The latter leads to an increase in I b at the beginning of the accelerating pulse due to the diode operation in the plasma prefilled mode.7 Let us note that in this mode of the diode operation, an increase of the discharge current ⌬I d above the value of I d was observed during the application of the accelerating pulse. IV. EXPERIMENTS WITH EXTERNALLY BIASED GRID

The experiments with the autobias grid showed the necessity to use an external bias in order to increase I b and to avoid diode operation in the plasma prefilled mode. We tested two electrical schemes for the HA with externally biased output grid 共see Fig. 3兲. In the first scheme 关see Fig. 3共a兲兴, a negatively charged capacitor (C⫽1 ␮ F, ␸ ⫽ ⫺2 kV) was connected between the grid and the anode. Prior to the application of the accelerating pulse a negative


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FIG. 3. Electrical schemes of the HA output grid bias: 共a兲 HA output grid with a negative bias and 共b兲 HA output grid with a positive bias.

bias voltage 共up to ⫺2 kV兲 was applied to the grid. This led to the appearance of the double layer between the grid and the plasma with the thickness determined by the plasma parameters and by the value of the bias voltage. The use of a large bias voltage prevents plasma prefilling when the double layer thickness exceeds the grid window cell size. However, in this case, extraction of electrons from the plasma becomes ineffective. To eliminate the influence of the bias voltage during the accelerating pulse, a triggered spark gap was used to short the grid and the anode prior to the application of the accelerating pulse. The control of the plasma emission boundary position was achieved by the change of the time delay ␶ between the spark gap switching and the beginning of the accelerating pulse. In this experiment, a grid with window cell size of 920 ␮ m⫻920 ␮ m was used. The plasma parameters inside the accelerating gap were studied at a distance of 2.5 cm from the grid. It was found that prior to the spark gap switching j ep is ⭐5 mA/cm2 and it increases up to a value ⭓25 mA/cm2 after the spark gap switching. Measurements of ion flow showed it negligible small value prior to the spark gap switching and j ip ⬃1 – 3 mA/cm2 after the spark gap switching. Thus, one can see the efficient suppression of the plasma penetration inside the accelerating gap. Typical wave forms of the diode ␸ d and I b at ␶ ⫽1.8 ␮ s, 4.3 ␮s, and 5 ␮s for HA operation at I d ⫽940 A are shown in Fig. 4. One can see that at ␶ ⫽1.8 ␮ s, electrons emission is significantly suppressed 关see Fig. 4共a兲兴. At ␶ ⭐1.8 ␮ s, a further decrease of I b was obtained down to I b ⬵250 A. An increase in ␶ causes an increase in I b and, at ␶ ⬇4 ␮ s, the diode operates with I b ⬇I sc 关see Fig. 4共b兲兴. The insignificant excess of I b above I sc at the beginning of the accelerating pulse is explained by a dilute plasma which prefills the accelerating gap after the spark gap switching. The further increase of ␶ ⭓5 ␮ s leads to a significant excess of I b above I sc 关see Fig. 4共c兲兴. This is consistent with the obtained continuous plasma flow through the grid after the spark gap switching. Similar to the case with the autobias grid, an increase in the discharge current ⌬I d was obtained during the accelerating pulse. The dependence of the discharge current during the accelerating pulse (I d ⫹⌬I d ) on ␶ for I d ⫽940 A is

FIG. 4. Typical waveforms of the accelerating voltage ␸ d , diode current I b , and space-charge-limited current I sc for the HA with the negatively biased output grid at different time delays ␶ between the spark gap switching and the beginning of the accelerating pulse: 共a兲 ␶ ⫽1.8 ␮ s, 共b兲 ␶ ⫽4.3 ␮ s, and 共c兲 ␶ ⫽5 ␮ s, d⫽3 cm, and I d ⫽940 A.

shown in Fig. 5. One can see that ⌬I d ⭓100 A for ␶ ⬎4 ␮ s. In addition, it was found that the increase of I d leads to the decrease in ⌬I d . The second scheme for the bias-controlled grid 关see Fig. 3共b兲兴 combines the autobias resistor R b ⫽52 ⍀ which connects the anode with the grid and a biased capacitor. The negative outlet of the capacitor is connected to the anode and the positive outlet is connected to the spark gap. This scheme was checked with a 55% transparency grid 共grid window cell size of 235 ␮ m⫻235 ␮ m). At the beginning of the HA discharge, the operation of this control grid was the same as the



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FIG. 5. Dependence of the HA discharge current (I d ⫹⌬I d ) during the accelerating pulse on the time delay ␶ between the moment of the spark gap switching and the beginning of the accelerating pulse for the HA with the negatively biased HA output grid. Prior to the accelerating pulse, I d ⫽940 A and d⫽3 cm.

HA source operation with the autobias grid. However, when the spark gap is switched a positive voltage of ⬃2 kV is applied to the grid. This leads to fast plasma expansion toward the accelerating gap. Thus, by adjusting ␶, one controls the plasma emission properties and plasma prefilling of the accelerating gap. Typical waveforms of ␸ d , I b , and the calculated I sc for ␶ ⫽1 ␮ s and I d ⫽940 A are shown in Fig. 6. Similar to the case with the first bias scheme, at the beginning of the accelerating pulse, there is some excess of I b above I sc . Also, it was found that at ␶ ⭓2 ␮ s, the amplitude of I b becomes larger than I sc and reaches 3 kA that is almost a short-circuit current for our generator. The x-ray image of the electron beam and the radial distribution of x-ray intensity for the HA with negatively biased grid is shown in Fig. 7. One can see a decrease in the x-ray intensity at already 5 cm radial distance as compared with the x-ray intensity at the central part of the image. Similar results for the radial x-ray intensity distribution were obtained for the HA electron source with the positively biased grid.

FIG. 6. Typical waveforms of the accelerating voltage ␸ d , diode current I b , and space-charge-limited current I sc for the HA with the positively biased output grid. d⫽3 cm and I d ⫽940 A. The time delay between the moment of the spark gap switching and the beginning of the accelerating pulse is ␶ ⫽1 ␮ s.

FIG. 7. 共a兲 Typical x-ray image of the electron beam obtained at the collector with the HA electron source having a negatively biased HA output grid. d⫽3 cm, I d ⫽940 A, and ␶ ⫽4.3 ␮ s. 共b兲 Radial intensity distribution of the x-ray image.

V. ELECTRON EMISSION FROM THE HA DURING THE ACCELERATING PULSE

The emission properties of the HA source depend on plasma parameters (n,T e ) which determine j ep and the effective area of the open plasma boundary. The latter is determined by the HA output grid parameters and by the thickness of the double layer at the vicinity of the grid. The application of the accelerating pulse causes extraction of electrons from the plasma which leads to an increase in the plasma potential. The latter causes an increase in the double layer thickness and might radically change the plasma emission properties. In addition, the increase in the plasma potential can cause plasma spots appearance at the surface of the anode due to the increased energy of plasma ions. Thus, the study of the plasma potential is important for understanding the HA operation under the application of the accelerating pulse. This study was carried out with a HA electron source having an autobias grid and grounded anode 关see Fig. 1共b兲兴. Typical waveforms of I b , plasma potential ␸ pl , ferroelectric sample potential ␸ c , and grid potential ␸ gr are shown in Fig. 8共a兲. One can see a drastic increase of ␸ pl , ␸ c , and ␸ gr relative to the anode during the accelerating pulse. The increase of ␸ pl was confirmed also by the data obtained by the biased Faraday cup 共see Sec. II兲. A large negative current of electrons toward the Faraday cup prior to the application of the accelerating pulse disappears during


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FIG. 9. Typical waveforms of the discharge and anode currents prior and during the accelerating pulse. ⌬I d is the jump in the discharge current during the accelerating pulse.

tional power supply for the HA discharge. The latter causes the increase in the discharge current ⌬I d 共Fig. 9兲 and ␸ c 关see Fig. 8共a兲兴. It was found that a change in I d in the range of 共300–1000兲A leads to the change of ␸ pl and ␸ gr in the range of ␸ pl⫽(6.6– 3) kV and ␸ gr⫽(3.6– 2) kV, respectively 共see Fig. 10兲. Finally, it was shown that during the accelerating pulse, the current balance is I b ⬇I a ⫹I d ⫹⌬I d . VI. DISCUSSION

FIG. 8. 共a兲 Typical waveforms of the diode current I b , plasma potential ␸ pl , ferroelectric plasma source front electrode potential ␸ c and grid potential ␸ gr during the accelerating pulse. 共b兲 Typical waveform of the current density measured with the 6 kV biased Faraday cup prior and during the accelerating pulse. I d ⫽800 A.

the accelerating pulse. Even a positive bias of 6 kV does not restore the electron flow 关see Fig. 8共b兲兴. At the end of the accelerating pulse, the electron flow restores its amplitude to the value which was obtained prior to the application of the accelerating pulse. In addition, significant changes were obtained in the anode current I a and in I d 共see Fig. 9兲. During the HA discharge, the anode current I a is carried by ions and electrons emitted from the plasma toward the anode and by secondary electrons emitted from the anode. One can see that during the accelerating pulse, I a is carried only by the ions emitted from the plasma and by secondary electrons11 and that I d is switched from the anode toward the collector. In the present scheme, generators which supply discharge and accelerating pulses are connected in series. Therefore, during the accelerating pulse, the generator, which supplies accelerating pulse, also serves as an addi-

During the HA discharge, a major part of I d flows between the ferroelectric sample and the anode and its output grid. Thus, one can conclude that 兩 I a 兩 ⬇ 兩 I d 兩 . One can consider two double layers 共DL兲 in the HA discharge. Namely, let us consider the DL1 between the dense plasma at the vicinity of the ferroelectric sample and the bulk plasma and the DL2 between the bulk plasma and the anode walls. In DL1, I d is carried mostly by electrons emitted from the surface plasma. The amplitude of the electron current, which is determined by the space-charge law, depends on the DL1 thickness and the potential difference between the surface and bulk plasmas. In DL2, I a is carried by electrons and ions emitted from the HA plasma toward the anode and by secondary electrons emitted from the anode walls. While the HA

FIG. 10. Dependencies of the HA plasma potential ␸ pl , ferroelectric plasma source potential ␸ c , and HA grid potential ␸ gr with respect to the grounded anode cavity on the amplitude of the discharge current during the accelerating pulse.


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J. Appl. Phys., Vol. 95, No. 7, 1 April 2004 TABLE I. Plasma density n, electron temperature T e , beam current I b , and calculated current I n for different amplitudes of the HA discharge current Id . Discharge current I d 共A兲

Electron temperature T e 共eV兲

Plasma density n 共cm⫺3兲

Beam current I b 共A兲

Calculated current I n 共A兲

600 800 1000

2 3 5

2⫻1012 3⫻1012 4⫻1012

940 1100 1200

470 870 1300

plasma is at a small potential of ⬃10 eV, plasma electrons with energy sufficient to overcome the plasma potential barrier, carry a major part of I a . When the accelerating pulse is applied, the fast extraction of electrons from the plasma leads to appearance of a noncompensated positive charge and a significant increase of the plasma, ferroelectric sample, and grid potentials. The plasma acquires a potential ␸ pl⬎ ␸ c 关see Fig. 9共a兲兴 that causes an increase in the DL1 potential difference ␸ pl⫺ ␸ c and, respectively, to the increase in the electron emission ⌬I d from the dense surface plasma. The power supply for this ⌬I d is the generator which supplies accelerating pulse. The increased ␸ pl cuts off the electron flow toward the anode and diverts this flow toward the collector. At that time, I a changes its polarity and it becomes equal to the sum of the currents of the plasma ions and secondary electrons. Thus, during the accelerating pulse, the current balance is I b ⬇I a ⫹I d ⫹⌬I d . In Table I, we present values of the currents I b and I n (I n ⫽ j ep S ␩ ) for different I d . One can see that for I d ⭐800 A, the value of I b is larger than I n and at I d ⬎800 A, I n ⬇I b . This result can be explained by an increase of T e during the accelerating pulse. Indeed, electrons extracted from the surface plasma during the accelerating pulse acquire an energy of several hundred eV in DL1. This energy could be transferred to the bulk plasma electrons by fast developing plasma instabilities. For instance, one can consider Langmuir oscillations12 which lead to a relaxation length of electrons in the plasma of L⬇2⫻103 n ⫺0.5E 3/2 ⭐10 cm for electrons with energy E⭐500 eV and plasma density n⭐1012 cm⫺3 . A rough estimate of the total amount of charged plasma particles inside the anode gives N ⬇1015. Thus, one can easily estimate dT e /dt⬇1 eV/ns. Let us note that we cannot exclude that part of the energetic electrons directly participates in I b . The increase of I d causes an increase in n, T e while ( ␸ pl⫺ ␸ c ) remains almost constant 共see Fig. 10兲. Accordingly, the rate of dT e /dt is increased which leads to an increase of the plasma emission ability. We understand that this explanation requires spectroscopic research of n and T e prior and during the accelerating pulse. Let us now consider the electron emission through the HA grid during the accelerating pulse. During the HA discharge, the boundary of the plasma with ␸ pl⬇10 V is at a distance of ⬃100 ␮m from the grid.7 The applied accelerating pulse penetrates through the grid cells and extracts electrons from the plasma boundary. Numerical calculations of the accelerating electric-field distribution showed that at a

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distance of 100 ␮m from the grid 共grid window cell size of 235 ␮ m⫻235 ␮ m), the potential is ⬃100 V inside the anode. The electron extraction leads to an increase of ␸ pl and, respectively, to the increase of the DL2 thickness. The latter was estimated in Ref. 7 by equating the ion saturation plasma current density10 and the space-charge-limited current density.13,14 In this case, numerical calculations showed that the electric field which penetrates through the grid is not enough to overcome the plasma potential barrier. Thus, the extraction of electrons from the plasma boundary is restricted. However, we obtained intense electron emission during the accelerating pulse. The above contradiction can be explained by the dynamics of ions in the HA grid region. Indeed, the accelerating electric field stops the ions emitted from the plasma from entering in the accelerating gap, and forces them back toward the grid, forming a virtual anode. Thus, the ions oscillate between the positively charged plasma and the virtual anode. Due to the curvature of the electric field near the grid wires, the ions acquire transverse velocities during the oscillations. The latter process leads to the increase in the ion density at the vicinity of the grid wires and, respectively, to the screening of the grid wire electric field by the ion positive space charge. Since the ions have longitudinal and transversal velocities, on the average, their motion can be considered as a spiral motion with decreasing radius r around the grid wires. Let us note that a similar model was considered in Ref. 15 where plasma electron trapping in spiral motion around the positive charged wire was used for generation microwave radiation by circular rotating electron bunches. One can write the equation of continuity: n 0 V 0 ⫽n(r)V(r), where n 0 and V 0 are the density and velocity of ions which have the largest ‘‘external orbit,’’ and n(r) and V(r) are the ion density and velocity at an arbitrary orbit, respectively. The ion velocity can be determined from the equality between the Coulomb and centrifugal forces: V(r) ⫽(qE r r/m i ) 1/2, where E r is the self-consistent radial electric field and q and m i are the ion charge and mass, respectively. Now, one can write the Poisson equation: d(rE r )/dr ⫺1/2 with the boundary condition at ⫽␧ 0 q 1/2n 0 rV 0 m 1/2 i (rE r ) the surface of the grid wire: E r (R)⫽⫺ ␸ gr /R, where R is the grid wire radius. The electric-field distribution can be presented as: E r ⫽( ␸ gr /r) 兵 1⫺K 关 (r/R) 2 ⫺1 兴其 2/3, where K 3/2 and A is the ion atomic num⫽5.6⫻10⫺12n 0 V 0 A 1/2R 2 / ␸ gr ber. The calculated distribution of the electric field for R ⫽50 ␮ m, ␸ gr⫽1 kV, A⫽14, n 0 ⫽1013 cm⫺3 , and V 0 ⫽5 ⫻106 cm/s shows that already at r⫽2R, the electric field E r is almost screened by ions. This allows extraction of plasma electrons through the internal area of the grid cells. One can estimate the ratio between the linear ion ‘‘circular’’ current density j il ⫽en 0 V 0 (r⫺R)⬇4 A/m and the measured linear ion current density toward the grid, j igr ⭐1 A/m. This estimate shows a vortex of the ions trajectories. The latter allows one to consider the Boltzmann distribution of ions with some effective temperature T ief . Now, one can write the Poisson equation d 关 ␰ (d ␺ /d ␰ ) 兴 /d ␰ ⫽⫺ 关 n 0 eR 2 /(␧ 0 ␸ 0 ) 兴 ␰ exp(⫺␺/␪) with the conditions ␸ ⬇ ␸ 0 and d ␸ /dr⫽0 at the plasma boundary. Here, ␰ ⫽r/R,


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␺ ⫽ ␸ / ␸ 0 , and ␪ ⫽kT i /(e ␸ 0 ) are dimensionless variables. Numerical calculations showed that ions with T ief⬃1 keV create a satisfactory screening of the grid potential at a distance of ⬃2R. VII. CONCLUSION

It was shown that HA electron sources with a biased HA output grid allow generation of electron beams with a current density up 20 A/cm2 and significantly reduced plasma prefilling of the accelerating gap. In the case of the autobias grid, the decrease in plasma prefilling was achieved by the increased double layer thickness around the grid wires due to j ep ⬎ j ip . Also, it was shown that the amplitude of I b is limited by the emission capabilities of the plasma. In order to increase the plasma emission capability, a scheme which combines the autobias and an external positive bias of the grid was tested. A positive bias was applied to the autobiased grid with some time ␶ prior to the application of the accelerating pulse. By adjusting ␶, a controllable emission capability of the plasma was demonstrated. However, it was found that because of the fast plasma expansion, the range of ␶ when the plasma prefilling can be neglected, is very narrow. The last tested scheme of the HA utilizes the large negative grid bias potential which prevents the plasma penetration into the acceleration gap even for the grid with a large window cell size. This negative bias is turned off with some time ␶ prior to the beginning of the accelerating pulse. The generation of the electron beam with I b ⬇1700 A at ␸ d ⫽150 kV with a weak plasma prefilling of the accelerating gap was demonstrated.

It was found that the application of the accelerating pulse leads to the increase in the plasma potential ␸ pl up to several kV. It was shown that the increased ␸ pl switches the emission of plasma electrons toward the collector. The emission of plasma electrons through the HA grid, which is at negative potential with respect to the plasma, is explained by the screening of the grid potential by the space charge of ions oscillating in the sheath near the grid.

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