Apparatus for GHz rate high duty cycle pulsing and manipulation of low and medium energy DC electron beams

Abstract

An ElectroMagnetic-Mechanical Pulser can generate electron pulses at rates up to 50 GHz, energies up to 1 MeV, duty cycles up to 10%, and pulse widths between 100 fs and 10 ps. A modulating Transverse Deflecting Cavity (TDC) imposes a transverse modulation on a continuous electron beam, which is then chopped into pulses by an adjustable Chopping Collimating Aperture. Pulse dispersion due to the modulating TDC is minimized by a suppressing section comprising a plurality of additional TDC's and/or magnetic quadrupoles. In embodiments the suppression section includes a magnetic quadrupole and a TDC followed by four additional magnetic quadrupoles. The TDC's can be single-cell or triple-cell. A fundamental frequency of at least one TDC can be tuned by literally or virtually adjusting its volume. TDC's can be filled with vacuum, air, or a dielectric or ferroelectric material. Embodiments are easily switchable between passive, continuous mode and active pulsed mode.

Claims

1. A combined ElectroMagnetic-Mechanical Pulser (EMMP) comprising: an input configured to accept input of a continuous electron beam; a first Transverse Deflecting Cavity (TDC) downstream of the input and configured to impose an oscillatory transverse deflection on the electron beam according to at least one of a time-varying electric field and a time-varying magnetic field generated within the first TDC; a Chopping Collimating Aperture (CCA) downstream of the first TDC and configured to block the electron beam when its deflection exceeds a threshold maximum or minimum, thereby chopping the electron beam into a stream of electron pulses having an electron pulse repetition rate; a dispersion suppressing section downstream of the CCA and configured to suppress a residual dispersion of the stream of electron pulses arising from the deflection imposed by the first TDC, the dispersion suppressing section including at least two elements selected from the group comprising TDC's and magnetic quadrupoles; and an output downstream of the dispersion suppressing section and configured to allow the stream of electron pulses to emerge from the EMMP.

2. The EMMP of claim 1, wherein the EMMP is able to produce streams of electron pulses having electron pulse repetition rates up to 50 GHz combined with pulse lengths in the range 100 fs to 10 ps, electron energies up to 1 MeV and duty cycles up to 10%.

3. The EMMP of claim 2, wherein the EMMP is able to produce streams of electron pulses having pulse repetition rates as low as 100 MHz.

4. The EMMP of claim 1, wherein the EMPP can be switched between an active mode that chops the electron beam and a passive mode that passes the electron beam through the EMMP without alteration.

5. The EMMP of claim 1, wherein the first TDC is a single-cell TDC.

6. The EMMP of claim 1, wherein the first TDC is a three-cell TDC.

7. The EMMP of claim 1, wherein at least one of the TDC's is empty or vacuum-filled.

8. The EMMP of claim 1, wherein at least one of the TDC's is a filled TDC that is at least partially filled with an axially symmetric dielectric material having permittivity greater than 1.

9. The EMMP of claim 1, wherein at least one of the TDC's is a filled TDC that is at least partially filled with an axially symmetric ferroelectric material having permittivity greater than 1.

10. The EMMP of claim 9, wherein a fundamental frequency of the filled TDC can be controlled by changing the permittivity of the ferroelectric material by at least one of adjusting its temperature and applying a dc electric potential difference across the ferroelectric material.

11. The EMMP of claim 1, wherein at least one TDC in the dispersion suppression section is a three-cell TDC.

12. The EMMP of claim 1, wherein a fundamental frequency of at least one TDC included in the EMMP can be tuned by adjusting a volume of the TDC.

13. The EMPP of claim 1, wherein the electron pulse repetition rate can be changed by replacing the first TDC with a replacement TDC having a primary resonance frequency that differs from the first TDC.

14. The EMMP of claim 1, wherein a fundamental frequency of the first TDC can be tuned by adjusting a volume of the first TDC, thereby adjusting the electron pulse repetition rate of the EMMP.

15. The EMMP of claim 1, wherein the dispersion suppressing section includes a second TDC and a third TDC.

16. The EMMP of claim 1, wherein the dispersion suppressing section includes a magnetic quadrupole followed by a second TDC.

17. The EMMP of claim 1, wherein the dispersion suppressing section includes two magnetic quadrupoles with a second TDC sandwiched in between them.

18. The EMMP of claim 1, wherein the dispersion suppressing section includes a first magnetic quadrupole and a second TDC, followed by four additional magnetic quadrupoles.

19. The EMPP of claim 1, wherein all TDC's included in the dispersion suppressing section are identical to the first TDC.

20. A device for producing a stream of electron pulses, the device comprising: a continuous electron beam generator, configured to emit a continuous electron beam; a first Transverse Deflecting Cavity (TDC) downstream of the continuous electron beam generator and configured to impose an oscillatory transverse deflection on the electron beam according to at least one of a time-varying electric field and a time-varying magnetic field generated within the first TDC; a Chopping Collimating Aperture (CCA) downstream of the first TDC and configured to block the electron beam when its deflection exceeds a threshold maximum or minimum, thereby chopping the electron beam into a stream of electron pulses having an electron pulse repetition rate; a dispersion suppressing section downstream of the CCA and configured to suppress a residual dispersion of the stream of electron pulses arising from the deflection imposed by the first TDC, the dispersion suppressing section including at least two elements selected from the group comprising TDC's and magnetic quadrupoles; and an output downstream of the dispersion suppressing section and configured to allow the stream of electron pulses to emerge from the EMMP.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual diagram that illustrates the fundamental concepts underlying embodiments of the present invention;

(2) FIG. 2A is a graphical illustration indicating the functioning and resulting beam dispersion of the first, modulating TDC in an embodiment where the first TDC is a single-cell TDC having a cavity with the magnetic field Hx of TM110 mode on-axis oriented transversely with respect to the beam propagation direction through the pipes on both ends of the cavity;

(3) FIG. 2B is a graphical illustration of the functioning and resulting beam dispersion in an embodiment similar to FIG. 2A, but where the first TDC is a three-cell TDC;

(4) FIG. 3 is a block diagram of an embodiment in which the dispersion suppressing section includes two TDC's;

(5) FIG. 4 is a block diagram of an embodiment in which the dispersion suppressing section includes a magnetic quadrupole followed by a TDC;

(6) FIG. 5 is a block diagram of an embodiment in which the dispersion suppressing section includes two magnetic quadrupoles with a TDC sandwiched in between; and

(7) FIG. 6 is a block diagram of an embodiment in which the dispersion suppressing section includes a magnetic quadrupole and a TDC, followed by four additional magnetic quadrupoles.

DETAILED DESCRIPTION

(8) Referring to FIG. 1, a conceptual diagram is shown that illustrates the fundamental concepts underlying embodiments of the present invention. In the illustrated embodiment, an initially continuous, dc electron beam 100 is transversely modulated into a sinusoid 110 by a pair of vacuum-filled TDC's 102, which are operated at a frequency within a range that extends from below 1 GHz to above 10 GHz. The amplitude of the sinusoid 110 grows as the modulated beam propagates, and then the beam 110 impinges upon a chopping, collimating aperture, or CCA 104, having an opening 106 that is adjustable between 10 and 200 m. The CCA chops the beam into pulses 108 that emerge from the CCA at an ultrahigh repetition rate that is twice the TDC modulation rate, because the pulses 108 are produced by cutting the sinusoid 110 of the beam modulation on both the up-swing and the down-swing. The aperture opening 106 and the modulating field of the TDC tune the pulse lengths to between 100 fs and 10 ps, resulting in duty cycles of the EMMP device of less than or equal to 20%.

(9) After the beam 100 has been chopped into pulses 108, both the beam size and the divergence of the stream of pulses 108 will increase. As shown in FIG. 1, additional components 112, 114 are included in a divergence suppressing section downstream of the CCA 110 that reverses and suppresses this divergence. In the embodiment of FIG. 1, the divergence is partially suppressed by two additional components 112, 114 which can be two additional TDC's 112 and 114, or a magnetic quadrupole 112 followed by an additional TDC 114. This basic design removes energy spread and significantly reduces transverse-longitudinal correlations (i.e. x-z and y-z correlations) introduced by first TDC 102. But it does not restore the correct relation between the two transverse spatial components (i.e. x and y). In similar embodiments, as discussed in more detail below, various other combinations of TDC's and magnetic quadrupoles are utilized to more effectively demodulate the beam and reduce the spatial distortions, the emittance growth, and the energy spread. In embodiments, all TDC's included in the EMMP are identical, or at least have the same fundamental frequencies.

(10) At the last step, matching schemes after 114 included in the dispersion suppressing section bring the two transverse (with respect to the optical beam axis z) spatial components into the correct relation with each other. Namely, the two transverse beam components x and y of the pulsed beam 108 are made to be approximately equal, a state which is referred to herein as a round beam. Ideally, the continuous input beam 100 is round, but the electron pulses 108 emerging from the CCA 104 are not generally round anymore. The goal of having a matching scheme 114 is to make the pulsed beam 108 round again.

(11) For example, a vacuum TDC 102, externally driven by an RF source, operated in TM110 mode at f.sub.0=10 GHz (corresponding to a TDC diameter of 39 mm) and a CCA 104 can be used to form ultrahigh repetition rate pulse sequences having a repetition rate of 20 GHz (because pulses are produced by cutting both sides of the 10 GHz sinusoid). At the fixed fundamental TDC frequency of 10 GHz, the pulse length can be continuously changed between 100 fs and 10 ps by varying the CCA diameter and/or RF power in the TDC. The exact range of duty cycle depends on the ratio of the diameters of the TDC (determining f.sub.0) and the CCA, and the power fed by the RF source into the TDC. For the TM110 mode in a pillbox, a general relation between all the parameters involved is described as

(12) $\begin{array}{cc}P{B}^2=\frac{r{m}_e}{det}& \left(1\right)\end{array}$
where P and B are power and magnetic component of the electromagnetic field in the TDC 102, respectively; m.sub.e and e are the electron mass and charge, respectively; r is the radius of the CCA 104; d is the free-drifting distance between the TDC 102 and the CCA 104; and t is the electron pulse length. This leads to duty cycles of up to 210.sup.1 (or 20%).

(13) Note that the TDC technology is downwards compatible to sampling rates (or strobe rates) below 1 GHz by replacing vacuum in the TDC with a high permittivity dielectric. The general relation linking the TDC diameter (D), the fundamental TDC frequency (f.sub.0) and the permittivity () is

(14) $\begin{array}{cc}D\frac{1}{f_0\sqrt{\mathrm{.Math.}}}& \left(2\right)\end{array}$
With a high permittivity ferroelectric, the TDC can be continuously tunable too in a specific frequency range.

(15) With reference to FIGS. 2A, 2B, 3, and 4, additional magnetic quadrupoles and TDCs and/or TDC design modifications from a single-cell design to a three-cell design can further demodulate the beam 100 and reduce spatial distortions, the emittance growth and the energy spread. FIG. 2A illustrates the field distribution 200 within a single cell TDC 102, and the resulting dispersion 202 of the electron beam 100. FIG. 2B illustrates the field distribution 204 within a three cell TDC 102, and the resulting dispersion 206 of the electron beam 100.

(16) FIG. 3 is a simplified block diagram illustrating the embodiment of FIG. 1, whereas FIG. 4 is a simplified block diagram illustrating a similar embodiment in which the dispersion suppression section includes a magnetic quadrupole 400 in lieu of the second TDC's 112.

(17) FIG. 5 illustrates an embodiment that is similar to FIG. 4 but includes a second identical magnetic quadrupole 402. This design is able to restore the ideal relation between the x and y components of the electron pulses 108, making the pulsed beam 108 round. While FIG. 5 illustrates a simplest matching solution, the alignment of the entire EMMP for this embodiment must be changed every time there is a change to the continuous input beam 100. If the continuous input beam is fixed, the EMMP alignment remains fixed once its best alignment has been found and set.

(18) FIG. 6 illustrates an embodiment similar to FIG. 5 that includes an additional quadrupole triplet 404, 406, 408 of magnetic quadrupoles. This quadrupole triplet 404, 406, 408 functions as a matching section that is able to preserve the roundness of the pulsed beam 108, regardless of any variations of the continuous input beam parameters. Hence, while this design of the matching scheme in FIG. 6 is more complex than for FIG. 5, the optimal EMMP alignment, once established and set, remains the same even if the parameters of the input beam 100 are changed.

(19) The design of specific solutions for removing the post-TDC distortions of the resulting electron pulses can be facilitated through the use of generalized matrix calculations in thin lens approximation. Matrix components depend on the type of the components, allowing the strength of various effects on electron dynamics in the phase space to be crudely predicted and evaluated. The matrix methodology disclosed herein relies on three basic assumptions: (1) electron optics elements are approximated as thin lenses; (2) a single particle/electron is considered; and (3) only linear matrix transformations are considered.

(20) These three assumptions are intertwined. When combined, they establish the basis for the geometrical optics framework in which the problem is solved. This idealized framework provides a good first-order model for rapid progress in the design, to be followed up with full ray-trace calculations including space charge effects to determine the effects of aberrations and undesired couplings on the electron phase space. An initial and a final state of an electron at input and at the output of the EMMP are linked in the momentum-coordinate phase space via a beam transport matrix as follows:

(21) $\begin{array}{cc}\left(\begin{array}{c}{x}_f\\ x_f^{}\\ y_f\\ y_f^{}\\ z_f\\ \frac{p_f}{p_0}\end{array}\right)=\left(\begin{array}{cccccc}{R}_{11}& R_{12}& R_{13}& R_{14}& R_{15}& R_{16}\\ R_{21}& R_{22}& R_{23}& R_{24}& R_{25}& R_{26}\\ R_{31}& R_{32}& R_{33}& R_{34}& R_{35}& R_{36}\\ R_{41}& R_{42}& R_{43}& R_{44}& R_{45}& R_{46}\\ R_{51}& R_{52}& R_{53}& R_{54}& R_{55}& R_{56}\\ R_{61}& R_{26}& R_{63}& R_{64}& R_{65}& R_{66}\end{array}\right)\left(\begin{array}{c}{x}_i\\ x_i^{}\\ y_i\\ y_i^{}\\ z_i\\ \frac{p_i}{p_0}\end{array}\right)& \left(3\right)\end{array}$
where x is the relative horizontal beam position, x is the horizontal divergence, y is the relative vertical beam position, y is the vertical divergence, z is the relative longitudinal position or time, and p/p.sub.0 is the relative longitudinal P.sub.0 momentum. In Eq. 3, the matrix R(66) is called the transport matrix. It is a result of multiplication of all matrices describing every single component of an EMMP design, including the drifting matrix, which describes empty gaps/pipes between hardware components. The perfect case is when the matrix R has only diagonal elements, indicating that an electron beam transformation took place, yet cross-correlations, described by off-diagonal elements resulting in pulse size change in transverse and longitudinal directions and energy spread, are absent.

(22) A number of combinations and designs were analyzed, and the main conclusion was that at least 2 active elements in the divergence suppression section must be present, after the CCA, to minimize the off-diagonal elements in the transport matrix. While the first element is always a TDC 102, second and third elements are shown as blank squares 112, 114 in FIG. 1, and should be determined from the matrix analysis. In general, these elements can be combinations of TDC's and/or magnetic quadrupoles (MQ's). Two such designs are illustrated in FIGS. 3 and 4. Thus, the resulting matrix is R(66) for the embodiments of FIG. 3 and FIG. 4, and the R-matrix is the product of 5-fold multiplication of 3 key matrices.

(23) A free drift beam pipe of length d (empty space between either pair of optical components in the EMMP) [measured in meters] is described by the following

(24) $\begin{array}{cc}\left(\begin{array}{cccccc}1& d& 0& 0& 0& 0\\ 0& 1& 0& 0& 0& 0\\ 0& 0& 1& d& 0& 0\\ 0& 0& 0& 1& 0& 0\\ 0& 0& 0& 0& 1& \frac{d}{{}^2}\\ 0& 0& 0& 0& 0& 1\end{array}\right),& \left(4\right)\end{array}$
where is the Lorentz factor. Its value depends on the electron energy. The magnetic quadrupole with a focal length f [measured in meters] is described as

(25) $\begin{array}{cc}\left(\begin{array}{cccccc}1& 0& 0& 0& 0& 0\\ \frac{1}{f}& 1& 0& 0& 0& 0\\ 0& 0& 1& 0& 0& 0\\ 0& 0& -\frac{1}{f}& 1& 0& 0\\ 0& 0& 0& 0& 1& 0\\ 0& 0& 0& 0& 0& 1\end{array}\right).& \left(5\right)\end{array}$

(26) The TDC has a matrix

(27) $\begin{array}{cc}\left(\begin{array}{cccccc}1& 0& 0& 0& 0& 0\\ 0& 1& 0& 0& k& 0\\ 0& 0& 1& 0& 0& 0\\ 0& 0& 0& 1& 0& 0\\ 0& 0& 0& 0& 1& 0\\ k& 0& 0& 0& 0& 1\end{array}\right)& \left(6\right)\end{array}$
where k is the transverse momentum acquired by an electron in the TDC, measured in reciprocal meters or eV. In what follows, k will be referred to as the kick. Resulting transport matrices for the designs sketched in FIGS. 3 and 4 can be optimized in order to zero as many off-diagonal elements as possible.

(28) As an example, the following parameters of the continuous input beam can be considered: (1) beam energy (E.sub.0) 200 keV; (2) energy spread (E) 0.5 eV; (3) emittance 1.5 nmrad which is a product of a beam diameter of 10 m and a divergence angle of 0.15 mrad. For the 3TDC (FIG. 3) design, the R-matrix is

(29) $\begin{array}{cc}\left(\begin{array}{cccccc}1-\frac{d_1\left(d_1+d_2\right)k_1^2}{{}^2}& d_1+d_2& 0& 0& 0& -\frac{d_1\left(d_1+d_2\right)k_1}{{}^2}\\ -\frac{d_1\left(d_1+d_2\right)k_1^2}{d_2{}^2}& 1+\frac{d_1^2\left(d_1+d_2\right)k_1^2}{d_2\left(d_1\left(d_1+d_2\right)k_1^2-{}^2\right)}& 0& 0& 0& -\frac{{d_1^2\left(d_1+d_2\right)}^2{k}_1^3}{d_2\left(d_1^2{k}_1^2+d_1{d}_2{k}_1^2-{}^2\right)}\\ 0& 0& 1& d_1+d_2& 0& 0\\ 0& 0& 0& 1& 0& 0\\ 0& -\frac{d_1\left(d_1+d_2\right)k_1}{{}^2}& 0& 0& 1-\frac{d_1\left(d_1+d_2\right)k_1^2}{{}^2}& \frac{d_1+d_2}{{}^2}\\ 0& -\frac{{d_1^2\left(d_1+d_2\right)}^2{k}_1^3}{d_2\left(d_1^2{k}_1^2+d_1{d}_2{k}_1^2-{}^2\right)}& 0& 0& -\frac{d_1\left(d_1+d_2\right)k_1^2}{d_2}& 1+\frac{d_1^2\left(d_1+d_2\right)k_1^2}{d_2\left(d_1\left(d_1+d_2\right)k_1^2-{}^2\right)}\end{array}\right)& \left(7\right)\end{array}$
where k.sub.2=(d.sub.1+d.sub.2)/d.sub.2 and k.sub.3=.sup.2d.sub.1k.sub.1/(.sup.2d.sub.1(d.sub.1+d.sub.2)k.sub.1.sup.2) are found optimal for the overall system design, i.e. maximum off-diagonal elements are zeros.

(30) The TDC+MQ+TDC design (FIG. 4) has a transport matrix

(31) $\begin{array}{cc}\left(\begin{array}{cccccc}-\frac{d_2}{d_1}& 0& 0& 0& 0& 0\\ -\frac{\left({}^2-d_1^2{k}_1^2\right)\left(d_1+d_2\right)}{d_1{d}_2{}^2}& -\frac{d_1}{d_2}& 0& 0& 0& \frac{d_1{k}_1\left(d_1+d_2\right)}{d_2{}^2}\\ 0& 0& 2+\frac{d_1}{d_2}& 2\left(d_1+d_2\right)& 0& 0\\ 0& 0& \frac{d_1+d_2}{d_1{d}_2}& 2+\frac{d_1}{d_2}& 0& 0\\ \frac{k_1\left(d_1+d_2\right)}{{}^2}& 0& 0& 0& 1& \frac{d_1+d_2}{{}^2}\\ 0& 0& 0& 0& 0& 1\end{array}\right)& \left(8\right)\end{array}$
where d.sub.1 and d.sub.2 are the drift distances between the first TOC 102 and the MQ 400, and between the MQ 400 and the second TOC 114 respectively, and k.sub.1 is the kick strength of the first TOC 102. The focal length of the MQ 400 is f=d.sub.1d.sub.2/(d.sub.1+d.sub.2) and the kick strength of the second deflecting cavity is k.sub.2=d.sub.1/d.sub.2 k.sub.1.

(32) From the matrix (8) describing the TDC+MQ+TDC case, it can be seen that two block sub-matrices for the two transverse beam components x and y are different (namely, R.sub.11, R.sub.12, R.sub.21, R.sub.22 which are related to x and R.sub.33, R.sub.34, R.sub.43, R.sub.44 which are related to y). To make the beam round, one needs to make R.sub.11=R.sub.33, R.sub.12=R.sub.34, R.sub.21=R.sub.43, R.sub.22=R.sub.44. This is performed through additional MQ's (1 or 4) after the second TDC 114. Once these conditions are satisfied, x and y are equal at the output, meaning the pulsed beam is round (assuming that the continuous input beam is round).

(33) From the matrices presented in (7) and (8) above, it can be seen that in transverse directions both designs (FIG. 3 and FIG. 4) may lead to satisfactory results, such that sufficient spatial coherence in the beam is conserved upon EMMP installation. The main problem here is the matrix term R.sub.65 which is responsible for energy spread growth. In this idealized geometrical optics framework, R.sub.65 is zero for the TDC+MQ+TDC design of FIG. 4, but is finite for the 3 TDC design of FIG. 3. That is, in the 3 TDC case, the additional energy spread at E.sub.0=200 keV is higher than 1 eV on top of the default/intrinsic energy spread of 0.5 eV.

(34) The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application.

(35) This specification is not intended to be exhaustive. Although the present application is shown in a limited number of forms, the scope of the invention is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof. One or ordinary skill in the art should appreciate after learning the teachings related to the claimed subject matter contained in the foregoing description that many modifications and variations are possible in light of this disclosure. Accordingly, the claimed subject matter includes any combination of the above-described elements in all possible variations thereof, unless otherwise indicated herein or otherwise clearly contradicted by context. In particular, the limitations presented in dependent claims below can be combined with their corresponding independent claims in any number and in any order without departing from the scope of this disclosure, unless the dependent claims are logically incompatible with each other.