CN202473822U - Electronic impulse quasilinear symmetrical pulse width compression device - Google Patents

Abstract

The utility model relates to an electronic impulse quasilinear symmetrical pulse width compression device, establishing a Z axis along the transmission direction of the electronic impulse and setting a transmission-type photoelectric cathode, an alternating electric field resonant cavity and a direct-current power supply along the Z axis direction; setting an anode grid mesh at one side of the alternating electric field resonant cavity; and setting a modulation grid mesh at the other side of the alternating electric field resonant cavity. The device solves the problem that the existing production method for the super short electronic impulse with 100fs magnitude and the related systems have limitation in engineering application, and the utility model provides the electronic impulse quasilinear symmetrical pulse width compression device, which utilizes the alternating electric field to apply distinctiveness energy modulation action to the electronic impulse so as to achieve the aim for compressing the pulse width of the electronic impulse.

Description

Electronic impulse almost symmetric form pulsewidth compression set

Technical field

The utility model belongs to femtosecond Superfast time resolution technical field, relates in particular to a kind of electronic impulse almost symmetric form pulsewidth compression set.

Background technology

The time resolution ultrafast phenomena is just launched in many fields of basic research, research in new high-tech; Ultrafast property diagnostic tool, especially image converter tube streak camera and ultrafast electric diffraction appearance so that ultrashort electronic impulse quick control is the basis are being brought into play the effect that is difficult to substitute in the research of time resolution ultrafast phenomena.In order to improve its time resolution, produce (1fs=10 about 100fs ^-15S) even more short pulse duration and each pulse comprise 10 ³～10 ⁴The ultrashort electronic impulse technology of individual electronics is brought into schedule already.Yet; This type of technology still is in conceptual phase at present and fails to get in the practical applications; Its bottleneck mainly is a significant space charge effect in first energy dispersion of photocathode emission photoelectron and the high concentration electronic impulse, and these two factors can cause serious electronic impulse broadening.Also just therefore, existing relevant scholar has done a large amount of theories and experimental study to electron gun system electronic impulse broadening, and has proposed various electronic impulse pulsewidth compression thinkings and produced the ultrashort electronic impulse pulsewidth of 100fs magnitude method.

Up to now; The ultrashort electronic impulse production method that is seen in newpapers and periodicals roughly is divided into two types: one type be adopt electrostatic field with (or) magnetostatic field to electronic impulse carry out time-domain with (or) modulation in the spatial domain; As based on electronic impulse broadening effect analysis result, adopt the design of novel electron optical texture electron gun (as " S " shape and " returning " font) or introduce method such as electronic impulse broadening inhibition electrode to reach the purpose of compression of electronic pulse; Another kind of then is to adopt the transient state modulated electric fields; As based on electronic impulse broadening effect analysis result; Utilize the transient state modulated electric fields to modulate the purpose that produces ultrashort electronic impulse to reach, and utilize Intense Laser Field that the matter kinetics function of electronic impulse is isolated the electronic impulse of Ah's second-time from treat the modulation electric subpulse imposing distinctiveness along electronics before and after the electronic impulse.Though above method in theory in addition some in engineering research, be proved and had certain feasibility; But regrettably, these methods or because the feasibility of its engineering construction is less, or always exist the limitation in the application because it can not satisfy the related request of the ultrafast diagnosis research application that image converter tube streak camera and ultrafast electric diffraction appearance etc. are the basis with ultrashort electronic impulse quick control.Also just therefore, the exploration to ultrashort electronic impulse production method of this type of 100fs magnitude and related system is still continuing.

Summary of the invention

To the ultrashort electronic impulse production method of present 100fs magnitude and related system in the limitation aspect the practical applications; The utility model provides electronic impulse almost symmetric form pulsewidth compression set, utilize alternating electric field that electronic impulse is imposed distinctiveness energy modulating action to reach the purpose of compression of electronic pulse pulsewidth.

The technical solution of the utility model is:

Electronic impulse almost symmetric form pulsewidth compression set; Set up the Z axle with the transmission direction of electronic impulse; Its special character is: comprise along Z-direction setting gradually transmission-mode photocathode 1, alternating electric field resonant cavity 3 and DC power supply; One side of said alternating electric field resonant cavity 3 is provided with anodic grid mesh 4, opposite side is provided with modulation aperture plate 5

The positive pole of said DC power supply connects anodic grid mesh 4; The negative pole of said DC power supply connects transmission-mode photocathode 1; It between said anodic grid mesh 4 and the transmission-mode photocathode 1 static evenly axial accelerating field district; Be electronic impulse energy modulator zone between anodic grid mesh 4 and the modulation aperture plate 5 in the said alternating electric field resonant cavity 3; Be the field-free drift space of electronic impulse between said modulation aperture plate 5 and the objective plane 7, along Z-direction the said electronic impulse energy modulator zone of uniform magnetic field be set and be provided with axial alternating electric field.

Above-mentioned triggering signal produces axial alternating electric field through signal trigger 6 in electronic impulse energy modulator zone.

Electronic impulse almost symmetric form pulsewidth compression method, its special character is: may further comprise the steps:

1] sets up the Z axle and uniform magnetic field is set with the electronic impulse transmission direction, and make that lateral cross section remains unchanged in the electronic impulse bundle transmission course along Z-direction;

2] in uniform magnetic field, set up static evenly axial accelerating field district, electronic impulse energy modulator zone and the field-free drift space of electronic impulse along Z-direction; Wherein: static evenly axially the accelerating field district is by transmission-mode photocathode 1 and anodic grid mesh 4 structures, and electronic impulse energy modulator zone is made up with modulation aperture plate 5 by the anodic grid mesh 4 of alternating electric field resonant cavity 3; The field-free drift space of electronic impulse is made up by system aperture plate 5 and objective plane 7;

3] modulation of almost symmetric form energy and pulsewidth compression:

3.1] the electronic impulse acceleration: for static evenly axial accelerating field district accelerating voltage U is provided through DC power supply, according to the primary power disperse Δ ε of electronic impulse ₀, the inceptive impulse width is τ ₀And the width d in static evenly axial accelerating field district ₁, the electronic impulse primitive axis is to length l ₀And forefront and last initial velocity difference Δ v along electronics ₀, following relation is arranged:

${\mathrm{\Δv}}_0=\sqrt{\frac{{2\mathrm{\Δ\ϵ}}_0}{m_e}}+\frac{\mathrm{\η}{\mathrm{U\τ}}_0}{d_1}.---\left(1\right)$

η=e/m wherein _eBe the charge-mass ratio of electronics,

Obtain: the axial energy of electronic impulse forefront electronics does

Electronic impulse is ε along the axial energy of electronics at last _Last=0,

Known initial axial energy is ε _iElectronic impulse evenly axially the transit time in accelerating field district is in static state:

$t_1\left({\mathrm{\ϵ}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\ϵ}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{{2\mathrm{\ϵ}}_i}{m_e}}],---\left(2\right)$

Can try to achieve the pulse duration τ=t of electronic impulse arrival anodic grid mesh in view of the above ₁(ε _Last)-t ₁(ε _First);

Get over that static evenly axially the time in accelerating field district is t if establish electronic impulse forefront place electronics _Min, following relation is then arranged:

$t_{\min }=t_{\mathrm{mid}}-\frac{\mathrm{\τ}}{2},---\left(2\right)$

In like manner, electronic impulse is last gets over the static even axially time t in accelerating field district along locating electronics _MaxAlso be tied to form upright just like ShiShimonoseki:

$t_{\max }=t_{\mathrm{mid}}+\frac{\mathrm{\τ}}{2}.---\left(4\right)$

Wherein the middle electronics of electronic impulse is getted over the time in static evenly axial accelerating field district:

t _mid＝(t _min+t _max)/2；

3.2] modulation:

3.2.1] apply axial alternating electric field for electronic impulse energy modulator zone, said axial alternating voltage is U _m(t)=-U ₀Sin (2 π t/T _m), wherein the positive direction of alternating voltage is a Z-direction, U ₀Be alternating voltage peak value, T _mBe the alternating voltage cycle, t=0 produces the moment of photoelectron pulse for the transmission-type photocathode;

Alternating voltage cycle T wherein _mMeet the following conditions:

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(5\right)$

$t_{\min }\≥(n+\frac{5}{12})T_m.---\left(6\right)$

Wherein n is a positive integer,

Obtain according to (5) and (6): $n\≤\frac{{6t}_{\mathrm{Min}}-{5t}_{\mathrm{Mid}}}{12(t_{\mathrm{Mid}}-t_{\mathrm{Min}})},$

3.3] calculating of pulse compression amplitude:

Satisfying the modulation voltage cycle T _mCondition under, the electronic impulse forefront pulsewidth suppressed range of electronic impulse from electronic impulse energy modulator zone exit to objective plane is:

${\mathrm{\Δt}}_{\mathrm{first}}=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\ηU}}},---\left(7\right)$

The last suppressed range along electronics of the pulsed electron of electronic impulse from electronic impulse energy modulator zone exit to objective plane is:

${\mathrm{\Δt}}_{\mathrm{last}}=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\ηU}}},---\left(8\right)$

The reduced overall amplitude of electronic impulse from electronic impulse energy modulator zone exit to the pulsed electron of objective plane is:

Δt＝Δt _last-Δt _first.(9)

Δt＝2Δt _last. (10)

The utlity model has following advantage:

1, the almost of pulse compression: be mainly reflected in the linear relationship of the inner each several part electronics suppressed range of electronic impulse and its energy modulation voltage, and the electronic impulse compression is with modulation voltage almost variation relation in time;

2, the symmetric form of pulse compression: based on the almost characteristics of pulse compression, the pulse compression amplitude demonstrates the symmetry about its center time point;

3, compression set miniaturization: electronic impulse reduced overall amplitude is proportional to pulse compression modulation parameter, and this makes and can pass through to improve the axial dimension of energy modulation voltage with reduction pulse compression district, thereby finally realizes the miniaturization of this compression set.

Description of drawings

Fig. 1 alternating electric field electronic impulse pulsewidth compression set;

Fig. 2 is the signal of alternating electric field pulse compression principle;

Fig. 3 is that axial magnetic field is to the electron focusing effect of contraction;

Fig. 4 is an alternating electric field electronic impulse pulsewidth compression track;

Fig. 5 is for the pulse forefront and at last along electronic time～axial displacement relation;

Wherein Reference numeral is: the 1-transmission-mode photocathode; The 2-electronic impulse; 3-alternating electric field resonant cavity; The 4-anodic grid mesh; 5-modulates aperture plate; The 6-signal trigger; The 7-objective plane.

Embodiment

Electronic impulse almost symmetric form pulsewidth compression set is as shown in Figure 1; Adopt the cylindrical shaft symmetrical structure; Comprise along Z-direction and set gradually transmission-mode photocathode 1, alternating electric field resonant cavity 3 and power supply; One end face of said alternating electric field resonant cavity 3 is that double aperture plate 4, other end of doing anode is modulation aperture plate 5, and triggering signal produces the axial alternating electric field that satisfies set pulse compression requirement through signal trigger 6 between resonant cavity anodic grid mesh and modulation aperture plate.Axially uniform magnetic field is accomplished transverse focusing to electronic impulse to limit the lateral cross section of electronic impulse.The positive pole of said power supply connects anodic grid mesh 4, and the negative pole of said power supply connects transmission-mode photocathode 1.It between said anodic grid mesh 4 and the transmission-mode photocathode 1 static evenly axial accelerating field district; Be electronic impulse energy modulator zone between said anodic grid mesh 4 and the modulation aperture plate 5, be the field-free drift space of electronic impulse between said modulation aperture plate 5 and the objective plane 7.Alternating electric field district and field-free drift space are formed the electronic impulse compression stroke jointly.

Electronic impulse almost symmetric form pulsewidth compression method may further comprise the steps:

1] direction and the Z axle setting up the Z axle and the said magnetic field of uniform magnetic field

is set along Z-direction with the electronic impulse transmission direction are in the same way; It act as: through the transverse focusing constraint to electronic impulse, so that electronic impulse bundle lateral cross section remains unchanged;

2] in uniform magnetic field, set up electronic impulse almost symmetric form pulsewidth compression set; Comprise along Z-direction and set gradually transmission-mode photocathode 1, alternating electric field resonant cavity 3 and power supply; One side of said alternating electric field resonant cavity 3 is provided with anodic grid mesh 4, opposite side is provided with modulation aperture plate 5; The positive pole of said power supply connects anodic grid mesh, and the negative pole of said power supply connects transmission-mode photocathode 1, is static evenly axial accelerating field district between said anodic grid mesh 4 and the transmission-mode photocathode 1; Be electronic impulse energy modulator zone between said anodic grid mesh 4 and the negative electrode aperture plate 5, be the field-free drift space of electronic impulse between said modulation aperture plate 5 and the objective plane 7;

3] modulation of almost symmetric form energy and pulsewidth compression: disperse is Δ ε for the photoelectron primary power ₀, the inceptive impulse width is τ ₀The rectangle electronic impulse, its primary power disperse and discrete space-charge effect will cause significant pulse-spreading phenomenon jointly in the process of getting over the accelerating field district.If the electronic impulse width is τ during its arrival anodic grid mesh, then the shortest the and the longest transit time of electronic impulse internal electron also is pulse forefront and last edge t electron transit time of place _MinWith t _MaxBe respectively

$t_{\min }=t_{\mathrm{mid}}-\frac{\mathrm{\τ}}{2},---\left(1\right)$

$t_{\max }=t_{\mathrm{mid}}+\frac{\mathrm{\τ}}{2}.---\left(2\right)$

Wherein, t _Mid=(t _Min+ t _Max)/2.If the axial alternating voltage that produces in the electronic impulse energy modulator zone is U _m(t)=-U ₀Sin (2 π t/T _m), the voltage positive direction is propagated Z direction, U along electronic impulse here ₀Be modulation voltage peak value, T _mBe the modulation voltage cycle, t=0 produces the moment of photoelectron pulse corresponding to photocathode.Selected almost change in voltage zone is the zone between the positive and negative half-peak value of alternating voltage, and then the condition of electronic impulse almost symmetric form modulation pulsewidth compression does

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(3\right)$

$t_{\min }\≥(n+\frac{5}{12})T_m.---\left(4\right)$

Here n=0,1,2L.Simultaneous formula (3) can get with (4)

$n\≤\frac{{6t}_{\min }-{5t}_{\mathrm{mid}}}{12(t_{\mathrm{mid}}-t_{\min })}.---\left(5\right)$

Often get n in the reality for satisfying the maximum integer of formula (5).

The energy modulation voltage is U _gThe time get into the electronics of modulator zone through anodic grid mesh, it is at the axial velocity variation Δ v in energy modulator zone exit _zFor

${\mathrm{\Δv}}_z=\sqrt{2\mathrm{\ηU}}-\sqrt{2\mathrm{\η}(U+U_g)},---\left(6\right)$

The velocity variations of supposing to ignore the relative displacement of electronics between slot and only considering to cause by the energy modulation; Then passing through longitudinal length when whole electronic impulse is the drift space of L when arriving objective plane, and its relative position that is had of electronics of above-mentioned energy modulation is changed to

$\mathrm{\&Delta;z}=\frac{{\mathrm{L\&Delta;<figure-callout\; id="2"\; label="v\; z"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">v</figure-callout>}}_z}{\sqrt{2\mathrm{\&eta;U}}}.---\left(7\right)$

Therefore, the relative time changes delta t that it had does

$\mathrm{\&Delta;t}=-\frac{\mathrm{\&Delta;<figure-callout\; id="2"\; label="z"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">z</figure-callout>}}{\sqrt{2\mathrm{\&eta;}(U+U_g)}},---\left(8\right)$

Utilize condition U _g＜＜U can get

$\mathrm{\&Delta;t}\&ap;-\frac{\mathrm{\&Delta;<figure-callout\; id="2"\; label="z"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">z</figure-callout>}}{\sqrt{2\mathrm{\&eta;U}}}=\frac{{\mathrm{LU}}_g}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}}.---\left(9\right)$

Formula (9) is explained: at U _gUnder positive condition, the forward energy modulation makes electronics have the forward direction speed modulation, and this has finally shortened electron transit time; And U _gUnder negative condition, the modulation of negative sense energy makes electronics have the negative sense velocity modulation, and this has finally prolonged electron transit time.This means: if impose the modulation of negative sense energy to prolong its transit time for pulse forefront electronics; Impose the forward energy modulation to shorten its transit time along electronics at last for simultaneously pulse, then anodic grid mesh place electronic impulse width will be after it gets over electronic impulse energy modulator zone and field-free drift space in succession and obtain compression.In the formula (9), LU _gCan be referred to as pulse compression modulation parameter, this formula means that also pulse compression is proportional to the two/cube of modulating parameter and being inversely proportional to accelerating voltage simultaneously.

4] pulse compression amplitude is calculated:

Under the prerequisite that satisfies electronic impulse almost symmetric form pulsewidth contractive condition, the pulse forefront is respectively with last suppressed range and the whole pulsewidth suppressed range of electronic impulse along electronics

${\mathrm{\&Delta;t}}_{\mathrm{first}}=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(10\right)$

${\mathrm{\&Delta;t}}_{\mathrm{last}}=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(11\right)$

Δt＝Δt _last-Δt _first. (12)

And the symmetric form contractive condition means U _m(t _MaxThe U of)=- _m(t _Min), thereby have again

Δt＝2Δt _last.(13)

The utility model embodiment mainly comprises: velocity modulation is selected with alternating electric field; The pulsewidth suppressed range calculates; The focusing effect of contraction of axial magnetic field.

1] the energy modulation is selected with alternating electric field:

In the electronic impulse transmission course, cause the principal element of its pulse stretcher to have: the difference of electronic impulse internal electron initial velocity aspect; Receive acceleration along the qualitative difference-frontier electron of electronic action power before and after its inner space charge effect paired pulses and then receive decelerating effect along electronics.This two species diversity makes the electronic impulse internal electron after the transmission equidistance, the disperse on the transit time occur, and this is the electronic impulse broadening.About the calculating of space charge effect electronic impulse broadening, still there are not parsing or comparatively simple and clear computational methods at present, thereby, will temporarily ignore in this partial pulse broadening argumentation below for more clearly setting forth the operation principle of the utility model.But can know to space Charge Effects that according to existing theory and experiment aspect its pulse stretching that causes presents good symmetry, also be along having almost equal broadening effect before and after the pulse.Even this makes under the situation that counts pulse stretching due to the space charge effect, below the argumentation of relevant the utility model operation principle still set up.

Under the focusing effect of contraction of axial magnetic field

, electron motion can adopt paraxonic approximate.Then electronical line shaft to the equation of motion does in the system

$\frac{d^2\mathrm{<figure-callout\; id="2"\; label="z\; dt"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">z</figure-callout>}}{{\mathrm{dt}}^{}}=\mathrm{\&eta;}{\mathrm{\&phi;}}^{\&prime;}\left(z\right),---\left(1\right)$

In the formula, the Z axle is the direction that initial point and sensing electronic impulse are advanced with transmission-type photocathode center, and z representes electronics in t axial coordinate constantly, and φ (z) expression axle is gone up the current potential that z is ordered, η=e/m _eCharge-mass ratio for electronics.Transform (1) and integration can get

${\left(\frac{\mathrm{dz}}{\mathrm{dt}}\right)}^2=2\mathrm{\&eta;}[\mathrm{\&phi;}\left(z\right)-\mathrm{\&phi;}\left(0\right)]+{\left(\frac{\mathrm{dz}}{\mathrm{dt}}\right)}^2{|}_{t=0},---\left(2\right)$

For the primary power disperse is Δ ε ₀, the pulse original width is τ ₀The rectangle electronic impulse, its pulse primitive axis is to length l ₀And forefront and last initial velocity difference Δ v along electronics ₀Respectively as follows:

$l_0={\mathrm{\&tau;}}_0\sqrt{\frac{2{\mathrm{\&Delta;\&epsiv;}}_0}{m_e}}+\frac{\mathrm{\&eta;U}{\mathrm{\&tau;}}_0^2}{{2d}_1},---\left(3\right)$

${\mathrm{\&Delta;v}}_0=\sqrt{\frac{2\mathrm{\&Delta;}{\mathrm{\&epsiv;}}_0}{m_e}}+\frac{\mathrm{\&eta;}{\mathrm{U\&tau;}}_0}{d_1}.---\left(4\right)$

Consider that current potential and structure are provided with the situation near practical engineering application, micron dimension l ₀Cause is much smaller than millimeter magnitude d ₁And can ignore, and the first energy of electronics is because of thinking that much smaller than acceleration area voltage the last edge of pulse and the initial axial energy of forefront electronics are respectively ε _Last=0,

, be ε then for initial axial energy by (2) Shi Kede _iThe photocathode emitting electrons, it in the transit time of quickening the place does

$t_1\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{{2\mathrm{\&epsiv;}}_i}{m_e}}],---\left(5\right)$

Obviously, the pulse front edge electronics is compared the back along having the less flight time, also is that frontier electron arrives anodic grid mesh earlier.Can get the pulsewidth τ of this electronic impulse when passing through axial accelerating field district by formula (5) does

τ＝t ₁(ε _last)-t ₁(ε _first). (6)

Go up according to this to analyze and be prone to know: if can be at the anodic grid mesh place to imposing distinctiveness velocity modulation-frontier electron negatively-modulated, back along electronics positively-modulated along electronics before and after the electronic impulse; The electronic impulse of then quickening the place broadening must also be pulse compression because of the phenomenon that the pulse pulsewidth narrows down appears in this distinctiveness velocity modulation through necessarily getting over behind the space.

If the alternating voltage that produces in the resonant cavity is U _m(t)=-U ₀Sin (2 π t/T _m), the voltage positive direction is propagated Z direction, U along electronic impulse here ₀Be modulation voltage peak value, T _mBe the modulation voltage cycle, t=0 is corresponding to the moment of photocathode generation photoelectron pulse, and then initial axial energy is ε _iThe transit time of electronics in system shown in Figure 1 do

t(ε _i)＝t ₁(ε _i)+t _m(ε _i)，?(7.1)

$t_1\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{{2\mathrm{\&epsiv;}}_i}{m_e}}],---\left(7.2\right)$

t _m(ε _i)＝t ₂(ε _i)+t ₃(ε _i)，(7.3)

$t_2\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">d</figure-callout>}}_2}{{\mathrm{eU}}_m\left(t_1\left({\mathrm{\&epsiv;}}_i\right)\right)}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU}+e{U}_m\left(t_1\left({\mathrm{\&epsiv;}}_i\right)\right))}{m_e}}-\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}],---\left(7.4\right)$

$t_3\left({\mathrm{\&epsiv;}}_i\right)=\sqrt{\frac{m_e{L}^2}{2({\mathrm{\&epsiv;}}_i+\mathrm{eU}+{\mathrm{eU}}_m\left(t_1\left({\mathrm{\&epsiv;}}_i\right)\right))}}.---\left(7.5\right)$

Obviously, t ₁(ε _i), t ₂(ε _i) and t ₃(ε _i) be respectively transit time of acceleration place, energy modulator zone and the drift space of electronics, t _m(ε _i) be the transit time of electronics in the pulse compression space.Consider the satisfied ε that concerns of initial axial energy ₁＜ε ₂Two electronics, if its corresponding speed modulation voltage is satisfied

|U(ε ₁)|，|(ε ₂)|＞ε ₁，ε ₂，?(8.1)

U(ε ₁)＞0， (8.2)

U(ε ₂)＜0， (8.3)

Then can know that according to formula (7.1)～(7.4) perseverance has

t _m(ε _last)＜t _m(ε _first)， (9)

Pulse compression has also promptly appearred.

Can know according to Fig. 2:, must make electronic impulse energy modulation voltage be positioned at almost change in voltage zone and the pulse forefront is equated with the last energy modulation amplitude that receives along electronics in order electronic impulse to be carried out almost symmetry modulation compression.Here selected almost change in voltage zone is the zone (sin (x) ≈ x is arranged) between the positive and negative half-peak value of alternating voltage in this zone.Then the compression of almost symmetric form pulsewidth being carried out in electronic impulse needs satisfied condition to do

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(10\right)$

$t_1\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)\&GreaterEqual;(n+\frac{5}{12})T_m.---\left(11\right)$

Here t _MidFor quickening the center flight time of electronic impulse in the place, n=0,1,2 Λ.Simultaneous formula (10) can get with (11)

$n\&le;\frac{{6t}_1\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)-{5t}_{\mathrm{mid}}}{12[t_{\mathrm{mid}}-t_1\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)]}.---\left(12\right)$

Often get n in the reality for satisfying the maximum integer of formula (12).

Know that by formula (5) electronic impulse is the shortest to be respectively with the longest transit time:

t _min＝t ₁(ε _first)，(13)

t _max＝t ₁(ε _last)，(14)

Its flight time of electronics and the initial axial energy that then have the center flight time are:

$t_{\mathrm{mid}}=\frac{t_{\min }+{\mathrm{<figure-callout\; id="2"\; label="t\; max"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">t</figure-callout>}}_{\max }}{2},---\left(15\right)$

${\mathrm{\&epsiv;}}_{\mathrm{mid}}=\frac{m_e}{2}{(\frac{d_1}{t_{\mathrm{mid}}}-\frac{{\mathrm{eUt}}_{\mathrm{mid}}}{{2m}_e{d}_1})}^2.---\left(16\right)$

2] the pulsewidth suppressed range calculates

For the quantitatively variation of explanation electronic impulse pulsewidth, can try to achieve the transit time t in pulse compression space under the no velocity modulation situation ₀(ε _i) as follows,

$t_0\left({\mathrm{\&epsiv;}}_i\right)=\sqrt{\frac{m_e{(L+d_2)}^2}{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}},---\left(17\right)$

Then satisfying under the almost symmetry modulation contractive condition, the pulse forefront be changed to along electron transit time at last

$\mathrm{\&Delta;t}\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)=t_0\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)-t_m\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)$

$=\sqrt{\frac{m_e{(L+d_2)}^2}{2({\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU})}}-\frac{m_e{d}_2}{e{U}_m\left(t_{\min }\right)}[\sqrt{\frac{2[{\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU}+{\mathrm{eU}}_m\left(t_{\min }\right)]}{m_e}}-\sqrt{\frac{2({\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU})}{m_e}}]-,---\left(18\right)$

$\sqrt{\frac{m_e{L}^2}{2[{\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU}+{\mathrm{eU}}_m\left(t_{\min }\right)]}}$

$\mathrm{\&Delta;t}\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)=t_0\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)-t_m\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)$

$=\sqrt{\frac{m_e{(L+d_2)}^2}{2({\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU})}}-\frac{m_e{d}_2}{e{U}_m\left(t_{\max }\right)}[\sqrt{\frac{2[{\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU}+{\mathrm{eU}}_m\left(t_{\max }\right)]}{m_e}}-\sqrt{\frac{2({\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU})}{m_e}}]-.---\left(19\right)$

$\sqrt{\frac{m_e{L}^2}{2[{\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU}+e{U}_m\left(t_{\max }\right)]}}$

Then the whole pulsewidth suppressed range of electronic impulse does

Δt＝Δt(ε _last)-Δt(ε _first). (20)

In practical application, the whole relatively pulse compression of axial length district, velocity modulation district is very little, also is d ₂＜＜L; The same form of associating pulse compression condition (8.1) again, formula (18) can be approximately respectively with (19)

$\mathrm{\&Delta;t}\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(21\right)$

$\mathrm{\&Delta;t}\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(22\right)$

And the symmetric form contractive condition means U _m(t _MaxThe U of)=- _m(t _Min), formula (20) is

Δt＝2Δt(ε _last). (23)

The width τ ' that can get the electronic impulse of objective plane place in view of the above does

τ′＝τ-Δt. (24)

3] the focusing effect of contraction of axial magnetic field

This part is analyzed with electronic coordinate system is benchmark: electronic coordinate is that initial point is positioned at time the power on launch point of son of pole-face, and its reference axis X axle, Y axle and Z axle are parallel to the corresponding reference axis of system shown in Figure 1 respectively.In system axial magnetic field

Constraint under, the initial radial energy is ε _rThe radial motion of electronics in the XY plane as shown in Figure 3.Wherein, (0,0) is electronic launching point, and (x y) is the lateral attitude coordinate of electronics in electronic coordinate system.Cyclotron frequency, radius of gyration and the circle round cycle of electronics in this axial magnetic field is respectively

ω＝ηB， (25)

$R_i=\frac{1}{\mathrm{\&eta;<figure-callout\; id="2"\; label="B"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">B</figure-callout>}}\sqrt{\frac{{2\mathrm{\&epsiv;}}_r}{m_e}},---\left(26\right)$

$T=\frac{2\mathrm{\&pi;}}{\mathrm{\&eta;B}}.---\left(27\right)$

For the primary power disperse is Δ ε ₀, electronics emission effective radius is r ₀Initial cylindric electronic impulse, its lateral radius does under the radial constraint effect in magnetic field

$r_c=r_0+\frac{2}{\mathrm{\&eta;<figure-callout\; id="2"\; label="B"\; filenames="HDA0000122256090000011.png"\; state="\{\{state\}\}">B</figure-callout>}}\sqrt{\frac{2{\mathrm{\&Delta;\&epsiv;}}_0}{m_e}}.---\left(28\right)$

Surface analysis can be known on the certificate, and electron cyclotron cycle and electronics initial radial energy are irrelevant, and electronics has the radial distance that equates with initial position again in the moment through the cycle integral multiple that circles round, and this focussing force defines the horizontal cross section of electron beam and remains unchanged

Instance is following:

Consider that is approached the used pulsed electron beam of actual ultrafast diagnostic system: quicken place accelerating voltage 30kV; Initiating electron pulse pulsewidth 150fs; Photoelectronic initial energy disperses Δ ε ₀=0.2eV; Electron beam initial radium 0.5mm; Anodic grid mesh and photocathode axial distance are 3mm; Energy modulator zone axial length is 1mm; Electronic impulse drift space axial length is 10mm; Axially uniform magnetic field is 300Gauss.Be provided with down in such structure and current potential, expection pulse compression amplitude minimum is 200fs.

Can get electronic impulse by parameter settings such as said structure, current potentials in the parameter of quickening the place is: t _Min=5.8107 * 10 ^-11S, t _Max=5.8407 * 10 ^-11S, t _Mid=5.8257 * 10 ^-11S.The condition that pulse compression is satisfied according to the almost symmetric form also is that relational expression (10)～(15) in this " embodiment " part can get n=31, T _m=1.8494 * 10 ^-12S.According to relational expression (18), (19) and (20) in " embodiment " part, the pulse compression amplitude of minimum 200fs means that the device electric parameter must satisfy the following relationship formula

Δt≥200fs. (29)

Can get t by above gained time parameter _Max=31.5811T _m, t _Max=31.4189T _m, thereby U is arranged _m(t _MaxThe U of)=- ₀Sin (2 π t _Max/ T _m)=0.50U ₀, U _m(t _MinThe U of)=- ₀Sin (2 π t _Min/ T _mThe 0.50U of)=- ₀Separating conditional relationship formula (29) can get

U ₀≥120V. (30)

Can get the transverse focusing effect of contraction characterization parameter r of axial magnetic field by formula (28) to electronic impulse _c=0.6mm.

It is as shown in Figure 5 that above-mentioned current potential and structure are provided with under the condition Fig. 1 shown device internal electron pulse pulsewidth compression track.Can be known by figure: because the first energy dispersion of photoelectron, its pulsewidth of 150fs initiating electron pulse is at accelerating field offset rapid about 300fs of broadening in the scope of the about 0.1mm of photocathode; And in the pulse compression space of velocity modulation district and drift region composition, the modulated 100fs that is compressed to of electronic impulse, suppressed range are 200fs.The pulse forefront is as shown in Figure 5 along electronic time～axial displacement relation with at last, and wherein (a) and (b) quicken place, velocity modulation district and drift space with (c) laying respectively at.By scheming to be prone to knowledge: quickening the place, edge, pulse front and back distance to axial increases along with the increase of transit time, thereby pulse stretching occurred; In the velocity modulation district, more weak mudulation effect has caused the pulse compression phenomenon, but not obvious with respect to quickening the place pulse width variation; At drift space, significant pulse compression effect occurs.

Claims (1)

1. electronic impulse almost symmetric form pulsewidth compression set; Set up the Z axle with the transmission direction of electronic impulse; It is characterized in that: comprise along Z-direction setting gradually transmission-mode photocathode (1), alternating electric field resonant cavity (3) and DC power supply; One side of said alternating electric field resonant cavity (3) is provided with anodic grid mesh (4), opposite side is provided with modulation aperture plate (5)

The positive pole of said DC power supply connects anodic grid mesh (4); The negative pole of said DC power supply connects transmission-mode photocathode (1); It between said anodic grid mesh (4) and the transmission-mode photocathode (1) static evenly axial accelerating field district; Be electronic impulse energy modulator zone in the said alternating electric field resonant cavity (3) between anodic grid mesh (4) and the modulation aperture plate (5); Be the field-free drift space of electronic impulse between said modulation aperture plate (5) and the objective plane (7), along Z-direction the said electronic impulse energy modulator zone of uniform magnetic field be set and be provided with axial alternating electric field.

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